Unit 11 ThermodynamicsUnit 11 Thermodynamics

Chapter 16Chapter 16

ThermodynamicsThermodynamics

DefinitionDefinitionA study of heat transfer that accompanies A study of heat transfer that accompanies

chemical changeschemical changesConcerned with overall chemical changesConcerned with overall chemical changes

Chemical Change involves:Chemical Change involves:A change in energyA change in energyA degree of disorderA degree of disorder

In Thermodynamics…In Thermodynamics…

System refers to the reaction itselfSystem refers to the reaction itselfSurroundings refers to everything elseSurroundings refers to everything elseStandard Standard ConditionsConditions

25°C (298 K)25°C (298 K)1 atmosphere1 atmosphere1 molar solution1 molar solution

EnthalpyEnthalpy

Symbol:Symbol: HHMeasure of heat content (energy) of a Measure of heat content (energy) of a

system at constant pressuresystem at constant pressureCan’t be measure directly, can only Can’t be measure directly, can only

measure the measure the changechange in enthalpy. in enthalpy.We call this the We call this the Heat of Reaction, Heat of Reaction, ΔΔHH

Measure of the heat released or absorbed in a Measure of the heat released or absorbed in a chemical reaction!chemical reaction!

ΔΔHHrxnrxn = = ΔΔHHproductsproducts - - ΔΔHHreactantsreactants

Hess’s LawHess’s Law This summer your parents This summer your parents

decide they are going to take decide they are going to take you on a road trip to CA. You you on a road trip to CA. You drive 400 mi day 1, 350 mi day drive 400 mi day 1, 350 mi day 2, 275 mi day 3, and 100 mi day 2, 275 mi day 3, and 100 mi day 4. What was the total mileage 4. What was the total mileage of the trip?of the trip?

You add your daily totals, You add your daily totals, 400 mi + 350 mi + 275 mi + 100 400 mi + 350 mi + 275 mi + 100

mi = 1125 mi totalmi = 1125 mi total In the same way, if the reaction In the same way, if the reaction

we want requires two or more we want requires two or more equations we can sum the equations we can sum the enthalpy changes to get the enthalpy changes to get the total energy change!total energy change!

Hess’s LawHess’s Law

Say we want to find the Say we want to find the ΔΔHH of the reaction: of the reaction: A + D → EA + D → E

We know the following:We know the following:

A + B → CA + B → C ΔΔH H = 27 kJ= 27 kJ

C + D → B + EC + D → B + E ΔΔHH = -15 kJ = -15 kJ Using Hess’s Law we can calculate the Using Hess’s Law we can calculate the ΔΔHH

we want by taking the sum of the reaction: we want by taking the sum of the reaction: (27 kJ) + (-15 kJ) = 12 kJ(27 kJ) + (-15 kJ) = 12 kJ

Hess's law states that the change in Hess's law states that the change in enthalpy of the reaction equals the sum enthalpy of the reaction equals the sum of the enthalpy change for the of the enthalpy change for the intermediate steps of the reaction. Hess's intermediate steps of the reaction. Hess's law could also be stated " as the heat law could also be stated " as the heat evolved or absorbed in a chemical evolved or absorbed in a chemical process is the same whether the process process is the same whether the process takes place in one or several steps. takes place in one or several steps. Hess's law is also noted as the law of Hess's law is also noted as the law of constant heat summation. constant heat summation.

Standard Heat of FormationStandard Heat of Formation

Change in enthalpy from the formation of 1 mol of Change in enthalpy from the formation of 1 mol of a compound, in its standard state, from its a compound, in its standard state, from its elements.elements.

Symbol:Symbol: “ “ ” ” refers to standard conditionsrefers to standard conditions

Units: kJ/molUnits: kJ/mol Example:Example:

SS(s)(s) + O + O2(g)2(g) → SO → SO2(g)2(g) -297 kJ/mol-297 kJ/mol

* Table 16-7 on page 510 lists Standard * Table 16-7 on page 510 lists Standard HHff

0fH

0fH

Chemical ReactionsChemical Reactions

Compare and contrast the following graphs:Compare and contrast the following graphs:

What did you notice? What did you notice? How would you write these reactions in standard format?How would you write these reactions in standard format?

Standard Heat of FormationStandard Heat of Formation

What can we tell from values?What can we tell from values?Positive value means?Positive value means?Negative value means?Negative value means?

Thermochemical equationsThermochemical equations

4Fe4Fe(s)(s) + 3O + 3O2(g)2(g) → 2Fe → 2Fe22OO3(s)3(s) + 1625 kJ + 1625 kJ

NHNH44NONO3(s) 3(s) + 27 kJ → NH+ 27 kJ → NH44++

(aq)(aq) + NO + NO33--(aq)(aq)

0fH

When you state the height of a mountain, it is relative When you state the height of a mountain, it is relative to another point (usually sea level).to another point (usually sea level).

In the same way enthalpies of formation are stated In the same way enthalpies of formation are stated based on the following arbitrary standard:based on the following arbitrary standard: Every free element in its standard state has a value of Every free element in its standard state has a value of

exactly 0.0 kJ.exactly 0.0 kJ. That way when the heat of formation is negative the That way when the heat of formation is negative the

system has lost heat, when positive the system has gained system has lost heat, when positive the system has gained

heatheat!!

Where do Standard Heats of Where do Standard Heats of formation come from?formation come from?

0fH

Check for understanding!Check for understanding!

Do elements in their Do elements in their standard states standard states possess zero energy?possess zero energy?

Why are elements in Why are elements in their standard states their standard states assigned enthalpies assigned enthalpies of zero?of zero?

What does the +33.2 What does the +33.2 on the graph tell you?on the graph tell you?

What does the -396 What does the -396 on the graph tell you?on the graph tell you?

Enthalpy change from Standard Enthalpy change from Standard Heat of FormationHeat of Formation

Use standard heat of formation to calculate Use standard heat of formation to calculate ΔΔHHrxnrxn for the combustion of methane for the combustion of methane

CHCH4(g)4(g) + 2O + 2O2(g)2(g) → CO → CO2(g)2(g) + 2H + 2H22OO(l)(l)

We can summarize Hess’s Law into the We can summarize Hess’s Law into the following equation: following equation:

ΔΔHHrxnrxn = = ΣΔΣΔHHff(products) - (products) - ΣΔΣΔHHff(reactants)(reactants) The symbol The symbol ΣΣ means “to take the sum of the means “to take the sum of the

terms.”terms.”

Enthalpy change from Standard Enthalpy change from Standard Heat of FormationHeat of Formation

Use standard heat of formation to calculate Use standard heat of formation to calculate ΔΔHHrxnrxn for the combustion of methane for the combustion of methane

CHCH4(g)4(g) + 2O + 2O2(g)2(g) → CO → CO2(g)2(g) + 2H + 2H22OO(l)(l)

First: look up First: look up ΔΔHHff values values Second: Use the formula and multiply each Second: Use the formula and multiply each

term by the coefficient of the substance in the term by the coefficient of the substance in the balanced chemical equationbalanced chemical equation

Third: Do the mathThird: Do the math

Enthalpy change from Standard Enthalpy change from Standard Heat of FormationHeat of Formation

Use standard heat of formation to calculate Use standard heat of formation to calculate ΔΔHHrxnrxn for for

the combustion of methanethe combustion of methane

CHCH4(g)4(g) + 2O + 2O2(g)2(g) → CO → CO2(g)2(g) + 2H + 2H22OO(l)(l)

ΔΔHHff(CO(CO22) = -394 kJ) = -394 kJ ΔΔHHff(H(H22O) = -286 kJ O) = -286 kJ

ΔΔHHff(CH(CH44) = -75 kJ) = -75 kJ ΔΔHHff(O(O22) = 0.0 kJ) = 0.0 kJ

ΔΔHHrxnrxn = [(-394 kJ) + (2)(-286 kJ)] – [(-75 kJ) + (2)(0.0 = [(-394 kJ) + (2)(-286 kJ)] – [(-75 kJ) + (2)(0.0

kJ)]kJ)]

productsproducts - reactants - reactants ΔΔHHrxnrxn = [-966 kJ] – [-75 kJ] = -891 kJ = [-966 kJ] – [-75 kJ] = -891 kJ Is this reaction endothermic or exothermic?Is this reaction endothermic or exothermic?

Exothermic ReactionsExothermic Reactions

Exothermic ReactionsExothermic Reactions

Potential Energy converts to Kinetic Potential Energy converts to Kinetic Energy as you release the energy to the Energy as you release the energy to the surroundings.surroundings.

Where is the Potential energy stored?Where is the Potential energy stored?How do we know there is a shift to Kinetic How do we know there is a shift to Kinetic

energy?energy?Most reactions are exothermic and Most reactions are exothermic and

spontaneousspontaneous

Endothermic ReactionsEndothermic Reactions

Endothermic ReactionsEndothermic Reactions

Energy content of products is greater than Energy content of products is greater than reactantsreactants

If the products have more energy why are If the products have more energy why are the surroundings cold? (Hint: PE↑)the surroundings cold? (Hint: PE↑)

The surroundings feel cold because the The surroundings feel cold because the bonds absorb the heat energy from the bonds absorb the heat energy from the surroundings, sosurroundings, so

Kinetic energy converts to Potential Kinetic energy converts to Potential energyenergy

Reaction SpontaneityReaction Spontaneity

What happens when you leave a iron nail What happens when you leave a iron nail outside for a few months?outside for a few months?

What happens when you light a gas stove?What happens when you light a gas stove? Do these reactions take place spontaneously? Do these reactions take place spontaneously?

(without outside intervention)(without outside intervention) Will the reverse of these reactions take place Will the reverse of these reactions take place

spontaneously?spontaneously? What about ice melting at room temperature?What about ice melting at room temperature? There is something more than There is something more than ΔΔHH determining determining

spontaneity!spontaneity!

EntropyEntropy

Symbol: Symbol: SSMeasure of the disorder or randomness of Measure of the disorder or randomness of

the particles that make up the systemthe particles that make up the systemMolecules are more likely to exist in a high Molecules are more likely to exist in a high

state of disorder than in a low state of state of disorder than in a low state of disorder.disorder.

Change in entropy is similar to change in Change in entropy is similar to change in enthalpyenthalpy

ΔΔSSsystemsystem ==SSproducts products - - SSreactantsreactants

EntropyEntropy

Do you expect the Do you expect the ΔΔSS of the phase change of the phase change shown below to be positive or negative?shown below to be positive or negative?

SSproducts products > > SSreactants reactants ΔΔSSsystemsystem positivepositive

SSproducts products < < SSreactants reactants ΔΔSSsystemsystem negativenegative

EntropyEntropy

What happens to molecules when you What happens to molecules when you increase their temperature? increase their temperature?

Do you think this will increase or decrease Do you think this will increase or decrease their entropy?their entropy?

ΔΔSS positive positive = more = more entropy, ie more disorderentropy, ie more disorder ΔΔSS negative = negative = lessless entropy, ie less disorder entropy, ie less disorder Reactions tend to go spontaneous towards Reactions tend to go spontaneous towards

increased entropy.increased entropy.

Entropy PracticeEntropy Practice

Predict the Predict the ΔΔSS for the following for the following changes:changes:

1.1. HH22O(l) → HO(l) → H22O(g)O(g)

2.2. COCO22(g) → CO(g) → CO22(aq) (aq)

3.3. 2SO2SO33(g) → 2SO(g) → 2SO22(g) + O(g) + O22(g) (g)

4.4. NaCl(s) → NaNaCl(s) → Na++(aq) + Cl(aq) + Cl--(aq)(aq)

5.5. CHCH33OH(l) → CHOH(l) → CH33OH(aq) OH(aq)

Gibbs Free EnergyGibbs Free Energy

By calculating free energy (energy that is By calculating free energy (energy that is available to do work) we can determine if a available to do work) we can determine if a reaction is spontaneous.reaction is spontaneous.

Just like Enthalpy and Entropy we can only Just like Enthalpy and Entropy we can only measure the free energy as a change.measure the free energy as a change.

ΔΔGGsystemsystem = = ΔΔHHsystemsystem – T – TΔΔSSsystemsystem

The sign of The sign of ΔΔGGsystemsystem tells you if the reaction is tells you if the reaction is spontaneous:spontaneous:Negative = spontaneous, will occurNegative = spontaneous, will occurPostive = nonspontaneous, will NOT occurPostive = nonspontaneous, will NOT occur

Gibbs Free EnergyGibbs Free Energy

How do the enthalpies and entropies affect How do the enthalpies and entropies affect reaction spontaneity?reaction spontaneity?

What is happening if What is happening if ΔΔGGsystemsystem = 0? = 0?

--ΔΔHHsystemsystem ++ΔΔHHsystemsystem

++ΔΔSSsystemsystem

Always spontaneous,Always spontaneous,

--ΔΔGGsystemsystem

Spontaneous only atSpontaneous only at highhigh temperatures,temperatures,

+ or - + or - ΔΔGGsystemsystem

--ΔΔSSsystemsystem

Spontaneous only at Spontaneous only at lowlow temperatures,temperatures,

+ or - + or - ΔΔGGsystemsystem

Never spontaneous Never spontaneous

++ΔΔGGsystemsystem

Review of SymbolsReview of Symbols ΔΔHH – Tells us if a reaction is endothermic or – Tells us if a reaction is endothermic or

exothermic (measure of change in energy)exothermic (measure of change in energy) Postive = endothermicPostive = endothermic Negative = exothermicNegative = exothermic

ΔΔSS – Tells us if the reaction is more or less – Tells us if the reaction is more or less ordered (randomness of particles)ordered (randomness of particles) Positive = more disorderedPositive = more disordered Negative = more orderedNegative = more ordered

ΔΔG – G – Tells us if the reaction is spontaneous by Tells us if the reaction is spontaneous by determining the amount of energy available to determining the amount of energy available to do work.do work. Positive = nonspontaneousPositive = nonspontaneous Negative = spontaneousNegative = spontaneous