Thermochemistry
Have you ever wondered how your body regulates its own temperature
The human body has the ability to maintain a constant temperature within the range 965-99degF (358-372degC)
This is controlled by your own thermostat the hypothalamus gland
This gland controls the rate at which the body metabolizes glucose and other heat related functions
C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol
Thermochemistry
C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol
bull For every mole of glucose you burn you are burning _____ calories
bull This process releases energy therefore it is exothermic
bull The hypothalamus gland controls other mechanisms in the body to help retain or release heat
bull Thermochemistry at work
Thermochemistry
Chapter 5Thermochemistry
John D Bookstaver
St Charles Community College
St Peters MO
2006 Prentice Hall Inc
Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr
and Bruce E Bursten
Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103
Thermochemistry
Energy
bull The ability to do work or transfer heatWork Energy used to cause an object that
has mass to moveHeat Energy used to cause the
temperature of an object to rise
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its position or chemical composition
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol
bull For every mole of glucose you burn you are burning _____ calories
bull This process releases energy therefore it is exothermic
bull The hypothalamus gland controls other mechanisms in the body to help retain or release heat
bull Thermochemistry at work
Thermochemistry
Chapter 5Thermochemistry
John D Bookstaver
St Charles Community College
St Peters MO
2006 Prentice Hall Inc
Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr
and Bruce E Bursten
Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103
Thermochemistry
Energy
bull The ability to do work or transfer heatWork Energy used to cause an object that
has mass to moveHeat Energy used to cause the
temperature of an object to rise
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its position or chemical composition
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Chapter 5Thermochemistry
John D Bookstaver
St Charles Community College
St Peters MO
2006 Prentice Hall Inc
Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr
and Bruce E Bursten
Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103
Thermochemistry
Energy
bull The ability to do work or transfer heatWork Energy used to cause an object that
has mass to moveHeat Energy used to cause the
temperature of an object to rise
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its position or chemical composition
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Energy
bull The ability to do work or transfer heatWork Energy used to cause an object that
has mass to moveHeat Energy used to cause the
temperature of an object to rise
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its position or chemical composition
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Potential Energy
Energy an object possesses by virtue of its position or chemical composition
(Chemicals have potential within the bonds)
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Kinetic Energy
Energy an object possesses by virtue of its motion
12
KE = mv2
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Units of Energy
bull The SI unit of energy is the joule (J)
bull An older non-SI unit is still in widespread use The calorie (cal)
1 cal = 4184 Jbull Calories that we eat are actually kilocalories are
abbreviated as Cal
1 J = 1 kg m2
s2
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
System and Surroundings
bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)
bull The surroundings are everything else (here the cylinder and piston)
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Work
bull Energy used to move an object over some distance
bull w = F d
where w is work F is the force and d is the distance over which the force is exerted
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Heat
bull Energy can also be transferred as heat
bull Heat flows from warmer objects to cooler objects
bull Recall cold is simply a lack of heat
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Transferal of Energy
a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall
b) As the ball falls its potential energy is converted to kinetic energy
c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is
a constant if the system loses energy it must be gained by the surroundings and vice versa
Use Fig 55
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E
Use Fig 55
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system
E = Efinal minus Einitial
Use Fig 55
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Changes in Internal Energy
bull If E gt 0 Efinal gt Einitial
Therefore the system absorbed energy from the surroundings
This energy change is called endergonic
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Changes in Internal Energy
bull If E lt 0 Efinal lt Einitial
Therefore the system released energy to the surroundings
This energy change is called exergonic
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Changes in Internal Energy
bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)
bull That is E = q + w
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
E q w and Their Signs
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Exchange of Heat between System and Surroundings
bull When heat is absorbed by the system from the surroundings the process is endothermic
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Exchange of Heat between System and Surroundings
When heat is released by the system to the surroundings the process is exothermic
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
State Functions
Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
State Functionsbull However we do know that the internal energy
of a system is independent of the path by which the system achieved that state In the system below the water could have reached
room temperature from either direction
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the
system not on the path by which the system arrived at that state
bull And so E depends only on Einitial and Efinal
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
State Functions
bull However q and w are not state functions
bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different
in the two cases
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
State Functions
Yes
Your bank account
(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)
No
The distance traveled from Penncrest to
K o P mall The distance will depend on the route taken
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Work
When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston
w = minusPV
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpy
bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system
bull Enthalpy is the internal energy plus the product of pressure and volume
H = E + PV
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpy
bull When the system changes at constant pressure the change in enthalpy H is
H = (E + PV)
bull This can be written
H = E + PV
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpy
bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression
H = E + PV
H = (q+w) minus w
H = q
bull So at constant pressure the change in enthalpy is the heat gained or lost
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic then when H is positive
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Endothermicity and Exothermicity
bull A process is endothermic when H is positive
bull A process is exothermic when H is negative
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Endo or Exo State whether ΔH is positive or negative
bull The combustion of fuel in a car engine
bull Salting winter roads
bull Making snow on the ski trails
bull Skiing down the trails
bull Snow-covered trails melt in the spring time
bull Placing an ice pack on an injury
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
bull The combustion of fuel in a car engine
Heat is leaving ΔH is negative exothermicbull Salting winter roads
Energy between NaCl molecules will leave ΔH is negative exothermic
bull Making snow on the ski trails
Heat is leaving ΔH is negative exothermicbull Skiing down the trails
Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic
bull Snow covered trails melt in the spring time
Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury
Heat released by injury is absorbed ΔH is positive endothermic
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Demonstration
Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpies of Reaction
The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants
H = Hproducts minus Hreactants
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpies of Reaction
This quantity H is called the enthalpy of reaction or the heat of reaction
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
The Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpy Calculation
Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ
What is the enthalpy change for the oxidation of 500 g of sugar
q = -825 kJ
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Calorimetry
Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Heat Capacity and Specific Heat
bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity
bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Heat Capacity and Specific Heat
Specific heat then is
Specific heat =heat transferred
mass temperature change
s =q
m T
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Constant Pressure Calorimetry
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Constant Pressure Calorimetry
Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation
q = m s T
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Using Specific Heat Capacity
A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC
Tfinal = 62degC
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Same Type of Problemhellip
A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC
Tfinal = 21degC
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Another Example
A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)
smetal = 044 JgmiddotK
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system
qrxn = -Ccal x ΔT
Ccal is the heat capacity of the bomb calorimeter
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Bomb Calorimetry
bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H
bull For most reactions the difference is very small
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Using a Bomb Calorimeter
Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows
2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a
bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic
-130 x103 kJmol CH6N2
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Hessrsquos Law
H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested
bull However we can estimate H using H values that are published and the properties of enthalpy
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
RecallhellipThe Truth about Enthalpy
1 Enthalpy is an extensive property
2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction
3 H for a reaction depends on the state of the products and the state of the reactants
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Hessrsquos Law
Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Hessrsquos Law
Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Example
Calculate ΔH for this reaction
2C (s) + H2 (g) rarr C2H2 (g)
Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ
H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ
2268 kJ
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
You Try This One
Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas
C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be
measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane
C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ
CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ
Use these energies to obtain ΔH for the formation of methane
ΔH = -748 kJ
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Calculation of H
bull Imagine this as occurringin 3 steps
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
C3H8 (g) 3 C(graphite) + 4 H2 (g)
3 C(graphite) + 3 O2 (g) 3 CO2 (g)
4 H2 (g) + 2 O2 (g) 4 H2O (l)
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
bull The sum of these equations is
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Enthalpies of Formation
An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)
Calculation of H
H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]
= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]
= (-23237 kJ) - (-10385 kJ)
= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Calculation of H
We can use Hessrsquos law in this way
H = nHf(products) - mHf(reactants)
where n and m are the stoichiometric coefficients
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Standard Enthalpies of Formation
Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas
C(graphite) + H2(g) rarr C2H2 (g)
Write the equation corresponding to the standard enthalpy of formation of solid silver chloride
Ag(s) + frac12Cl2(g) rarr AgCl(s)
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Use the Formula
Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)
CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Fuels
The vast majority of the energy consumed in this country comes from fossil fuels
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Thermochemistry is Clearly Important to Our Everyday Lives
bull Personal Nutrition ndash nutritional value labels
bull Creation of alternative fuel sources
bull Addressing Global Warming
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions
Thermochemistry
Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction
or out of the reactionbull Transformed energy in a chemical reaction comes from or forms
chemical bonds and is exchanged with the surroundings as heat andor work
bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy
bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction
bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions