thermochemistry: chemical energy chapter 8. energy is the capacity to do work thermal energy is the...
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Thermochemistry: Chemical Energy
Chapter 8
Energy is the capacity to do work
• Thermal energy is the energy associated with the random motion of atoms and molecules
• Chemical energy is the energy stored within the bonds of chemical substances
• Nuclear energy is the energy stored within the collection of neutrons and protons in the atom
• Electrical energy is the energy associated with the flow of electrons
• Potential energy is the energy available by virtue of an object’s position, or stored energy
• Kinetic energy is moving energy.
E = qrxn + w
Heat is the transfer of thermal energy between two bodies that are at different temperatures.
Energy Changes in Chemical Reactions
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
900C400C
greater thermal energy
Thermochemistry is the study of heat change in chemical reactions.
The system is the specific part of the universe that is of interest in the study.
open
mass & energyExchange:
closed
energy
isolated
nothing
SYSTEMSURROUNDINGS
Calorimetry and Heat CapacityMeasure the heat flow at constant pressure (H).
Calorimetry and Heat CapacityMeasure the heat flow at constant volume (E).
First Law of Thermodynamics
• Energy is neither created nor destroyed.• 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. 5.5
Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.
Endothermic process is any process in which heat has to be supplied to the system from the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
H2O (g) H2O (l) + energy
energy + 2HgO (s) 2Hg (l) + O2 (g)
energy + H2O (s) H2O (l)
Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.
H = H (products) – H (reactants)
H = heat given off or absorbed during a reaction at constant pressure
Hproducts < Hreactants
H < 0Hproducts > Hreactants
H > 0
Enthalpy Changes 01
• Enthalpies of Physical Change:
Enthalpies of Physical and Chemical Change
Enthalpy of Fusion (Hfusion): The amount of heat necessary to melt a substance without changing its temperature.
Enthalpy of Vaporization (Hvap): The amount of heat required to vaporize a substance without changing its temperature.
Enthalpy of Sublimation (Hsubl): The amount of heat required to convert a substance from a solid to a gas without going through a liquid phase.
Thermochemical Equations
H2O (s) H2O (l) H = 6.01 kJ
Is H negative or positive?
System absorbs heat
Endothermic
H > 0
6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.
Thermochemical Equations
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H = -890.4 kJ
Is H negative or positive?
System gives off heat
Exothermic
H < 0
890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.
H2O (s) H2O (l) H = 6.01 kJ
• The stoichiometric coefficients always refer to the number of moles of a substance
Thermochemical Equations
• If you reverse a reaction, the sign of H changes
H2O (l) H2O (s) H = -6.01 kJ
• If you multiply both sides of the equation by a factor n, then H must change by the same factor n.
2H2O (s) 2H2O (l) H = 2 x 6.01 = 12.0 kJ
H2O (s) H2O (l) H = 6.01 kJ
• The physical states of all reactants and products must be specified in thermochemical equations.
Thermochemical Equations
H2O (l) H2O (g) H = 44.0 kJ
How much heat is transfered when 266 g of white phosphorus (P4) burn in air?
P4 (s) + 5O2 (g) P4O10 (s) H = -3013 kJ
266 g P4
1 mol P4
123.9 g P4
x-3013 kJ1 mol P4
x = -6470 kJ
The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.
The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius.
C = ms
Heat (q) absorbed or released:
q = mst
q = Ct
t = tfinal - tinitial
Calorimetry and Heat CapacityAssuming that a can of soda has the same specific heat as water, calculate the amount of heat (in kilojoules) transferred when one can (about 350 g) is cooled from 25 °C to 3 °C.
Specific heat (Water) = 4.18g °C
J
Temperature change = 3 C-25 °C = - 22 °C
Mass = 350 g
q = (specific heat) x (mass of substance) x T
Calorimetry and Heat CapacityCalculate the amount of heat transferred.
= -32 000 J -22 °C
1000 J
1 kJ
x 350 gxHeat evolved =
= -32 kJ
g °C
4.18 J
x-32 000 J
Calorimetry and Heat Capacity
Bomb Calorimetry
• Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H.
E = qrxn + w
• W = VΔP
• For most reactions, the difference is very small.
• ΔE almost = qrxn
Constant-Volume Calorimetry
Constant-Volume Calorimetry
No heat enters or leaves!
qsys = qwater + qCal + qrxn
qsys = 0
qrxn = - (qwater + qCal)
qwater = mst
qCal = CCalt
Constant-Pressure Calorimetry
No heat enters or leaves!
qsys = qwater + qcal + qrxn
qsys = 0
qrxn = - (qwater + qcal)
qwater = mst
qcal = Ccalt
Reaction at Constant PH = qrxn
Hess’s Law
Haber Process:
Multiple-Step Process
N2H4(g)2H2(g) + N2(g)
Hess’s Law: The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.
2NH3(g)3H2(g) + N2(g) H° = -92.2 kJ
H°1 = ?
2NH3(g)N2H4(g) + H2(g)
H°1+2 = -92.2 kJ2NH3(g)3H2(g) + N2(g)
H°2 = -187.6 kJ
H°1 = H°1+2 - H°2
Hess’s Law
= -92.2 kJ - (-187.6 kJ) = 95.4 kJ
H°1 + H°2 = H°1+2
Hess’s Law 01
• Hess’s Law: The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.
• 3 H2(g) + N2(g) 2 NH3(g) H° = –92.2 kJ
Hess’s Law• The industrial degreasing solvent methylene
chloride (CH2Cl2, dichloromethane) is prepared from methane by reaction with chlorine:
• CH4(g) + 2 Cl2(g) CH2Cl2(g) + 2 HCl(g)
• Use the following data to calculate H° (in kilojoules) for the above reaction:
• CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) H° = –98.3 kJ
• CH3Cl(g) + Cl2(g) CH2Cl2(g) + HCl(g) H° = –104 kJ
Standard Heats of Formation
H2(g) + 1/2 O2(g) H2O(l) H°f = –286 kJ/mol
3/2 H2(g) + 1/2 N2(g) NH3(g) H°f = –46 kJ/mol
2 C(s) + H2(g) C2H2(g) H°f = +227 kJ/mol
*Standard state (°):
P = 1atm, t = 25°C,
Concentration = 1 mol/Lit
Standard Heats of Formation
• Standard Heats of Formation (H°f): The enthalpy
change for the formation of 1 mole of substance in its standard state from its constituent elements in their standard states.
• The standard heat of formation for any element in its standard state is defined as being ZERO.
H°f = 0 for an element in its standard state
Standard Heats of Formation
• Calculating H° for a reaction:
H° = H°f (Products) – H°f (Reactants)
• For a balanced equation, each heat of formation must be multiplied by the stoichiometric coefficient.
aA + bB cC + dD
H° = [cH°f (C) + dH°f (D)] – [aH°f (A) + bH°f (B)]
Standard Heats of Formation
-1131Na2CO3(s)49C6H6(l)-92HCl(g)
-127AgCl(s)-235C2H5OH(g)95.4N2H4(g)
-167Cl-(aq)-201CH3OH(g)-46NH3(g)
-207NO3-(aq)-85C2H6(g)-286H2O(l)
-240Na+(aq)52C2H4(g)-394CO2(g)
106Ag+(aq)227C2H2(g)-111CO(g)
Some Heats of Formation, Some Heats of Formation, HHff° ° (kJ/mol)(kJ/mol)
Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49.04 kJ/mol.
2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)
H0rxn 6H0 (H2O)f12H0 (CO2)f= [ + ] - 2H0 (C6H6)f[ ]
H0rxn = [ 12x(–393.5) + 6x( -286 ) – [ 2x(49.04) ] = -6542.0
kJ-6542.0
2 mol= - 3271.0 kJ/mol C6H6
See Page 292 for Standard Heat of Formation
Standard Heats of FormationUsing standard heats of formation, calculate the standard enthalpy of reaction for the photosynthesis of glucose (C6H12O6) and O2 from CO2 and liquid H2O.
C6H12O6(s) + 6O2(g)6CO2(g) + 6H2O(l) H° = ?
H° = [H°f (C6H12O6(s))] - [6 H°f (CO2(g)) + 6 H°f (H2O(l))]
= 2816 kJ
[(6 mol)(-393.5 kJ/mol) + (6 mol)(-285.8 kJ/mol)]
H° = [(1 mol)(-1260 kJ/mol)] -
Bond Dissociation Energy• Bond Dissociation Energy: Can be used to determine an
approximate value for H°f .
H°f = D (Bonds Broken) – D (Bonds Formed)
ΔHorxn = D (Reactant bonds) - D (Product bonds)
• For the reaction: H-H (g) + Cl-Cl (g) 2H-Cl (g)
H°rxn = (D(H–H) + D(Cl-Cl))-(2 X D(H–Cl) ) H°rxn = -185 kJ
H°f = -92.5 KJ/mol
Bond Dissociation Energy 02
Calculate an approximate H° (in kilojoules) for the synthesis of
ethyl alcohol from ethylene:C2H4(g) + H2O(g) C2H5OH(g)
HH
HH HH
HH
CC CC
O-HO-H
CC HH
HH
+ H-O-H+ H-O-H
HH
CC HH
HH
ΔHorxn = D (Reactant bonds) - D (Product bonds)
ΔHorxn = (DC=C + 4 DC H + 2 DO H) - (DC C + DC O + 5 DC H + DO H)
ΔHorxn = [(1 mol)(611 kJ/mol) + (4 mol)(410 kJ/mol)
+(2 mol)(460 kJ/mol)] - [(1 mol)(350 kJ/mol) + (1 mol)( 350 kJ/mol) + (5 mol)(410 kJ/mol) + (1 mol)(460 kJ/mol)]
ΔHorxn = 39 kJ
ΔHorxn = (DC=C + 4 DC H + 2 DO H) - (DC C + DC O + 5 DC H + DO H)
Thermodynamics
State functions are properties that are determined by the state of the system, regardless of how that condition was achieved.
Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.
energy, pressure, volume, temperature
Predicting spontaneity of a chemical Reaction
• Second Law of Thermodynamics: Reactions proceed in the direction that increases the entropy of the system plus surroundings.
• A spontaneous process is one that proceeds on its own without any continuous external influence.
• A nonspontaneous process takes place only in the presence of a continuous external influence.
Introduction to Entropy 02
• The measure of molecular disorder in a system is called the system’s entropy; this is denoted S.
• Entropy has units of J/K (Joules per Kelvin).
S = Sfinal – Sinitial
– Positive value of S indicates increased disorder.
– Negative value of S indicates decreased disorder.
Introduction to Entropy 03
Introduction to Entropy 05
• Predict whether S° is likely to be positive or negative for each of the following reactions.
• a. 2 CO(g) + O2(g) 2 CO2(g)
b. 2 NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g)
c. C2H4(g) + Br2(g) CH2BrCH2Br(l)
d. 2 C2H6(g) + 7 O2(g) 4 CO2(g) + 6 H2O(g)
Introduction to Entropy 04
• To decide whether a process is spontaneous, both enthalpy and entropy changes must be considered:
• Spontaneous process:
Decrease in enthalpy (–H), Increase in entropy (+S).
• Nonspontaneous process:
Increase in enthalpy(+H),Decrease in entropy (–S).
Introduction to Free Energy 01
• Gibbs Free Energy Change (G): Weighs the relative contributions of enthalpy and entropy to the overall spontaneity of a process.
G = H – TS
G < 0 Process is spontaneous
G = 0 Process is at equilibrium
G > 0 Process is nonspontaneous
Which of the following reactions are spontaneous under
standard conditions at 25°C?
•AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) G° = –55.7 kJ
2 C(s) + 2 H2(g) C2H4(g)
G° = 68.1 kJ
N2(g) + 3 H2(g) 2 NH3(g)
H° =-92.2 kJ S° = -199 J/K
ΔGo = ΔHo -TΔSo = ( -92.2 kJ) - (298 K)(- 0.199 kJ/K) = -32.9 kJBecause ΔGo is negative, the reaction is spontaneous.
Introduction to Free Energy 04
• Equilibrium (G° = 0): Estimate the temperature at which the following reaction will be at equilibrium. Is the reaction spontaneous at room temperature?
– N2(g) + 3 H2(g) 2 NH3(g)
H° = –92.0 kJ S° = –199 J/K
– Equilibrium is the point where G° = H° – TS° = 0
– T = 462 K