Thermochemistry

ThermochemistryThermochemistryThe study of energy and its The study of energy and its

transformationstransformations

Definitions - EnergyDefinitions - Energy Chemical systems contain both Chemical systems contain both kinetic kinetic

energyenergy and and potential energypotential energy. .

Energy is the capacity to do work or to Energy is the capacity to do work or to produce heat. produce heat.

An example of both is the combustion of An example of both is the combustion of gasoline. The gaseous products expand gasoline. The gaseous products expand and do work (moving the pistons of an and do work (moving the pistons of an engine) and the reaction also produces engine) and the reaction also produces heat.heat.

Definitions - EnergyDefinitions - EnergyKinetic energyKinetic energy is the energy of is the energy of

motion, and it depends upon the motion, and it depends upon the mass of the object and its velocity. mass of the object and its velocity. Since molecules, especially those of Since molecules, especially those of gases, are in motion, they possess gases, are in motion, they possess kinetic energy.kinetic energy.

Definitions- EnergyDefinitions- EnergyPotential energyPotential energy is energy due is energy due

to position to position or compositionor composition. .

Chemical energyChemical energy is potential is potential energy due to composition. For energy due to composition. For example, gasoline and oxygen have example, gasoline and oxygen have the potential to produce energy if the potential to produce energy if they react.they react.

DefinitionsDefinitionsWhen examining chemical systems or When examining chemical systems or

reactions, we consider the reactions, we consider the systemsystem and its and its surroundingssurroundings..

The system is where we put our focus. The system is where we put our focus. Typically, it is the reactants and products.Typically, it is the reactants and products.

The surroundings include everything else The surroundings include everything else in the universe.in the universe.

DefinitionsDefinitionsIf a reaction results in the evolution of If a reaction results in the evolution of heat, energy flows out of the system heat, energy flows out of the system and into the surroundings. These and into the surroundings. These reactions are reactions are exothermic.exothermic.

The energy lost by the system must be The energy lost by the system must be equal to the energy gained by the equal to the energy gained by the surroundings. surroundings.

Internal EnergyInternal EnergyThe The internal energyinternal energy (E) of a system is the (E) of a system is the sum of the kinetic and potential energies sum of the kinetic and potential energies of all of the particles of the system. of all of the particles of the system. It is generally not possible to determine It is generally not possible to determine the internal energy of a system, but we the internal energy of a system, but we can measure changes in internal energy.can measure changes in internal energy.Internal energy is changed by the flow of Internal energy is changed by the flow of work and/or heat.work and/or heat.

Internal Energy & the 1Internal Energy & the 1stst LawLaw

The First Law of The First Law of Thermodynamics states that:Thermodynamics states that:

The total internal energy of an The total internal energy of an isolated system is constant.isolated system is constant.

However, it is not possible to However, it is not possible to completely isolate a system from its completely isolate a system from its surroundings.surroundings.

Internal EnergyInternal EnergySince energy may flow to or Since energy may flow to or

from the surroundings and the from the surroundings and the system, we are concerned with system, we are concerned with energy changesenergy changes rather than the rather than the absolute value of the internal absolute value of the internal energy.energy.

ΔE = EΔE = Efinalfinal - E - Einitialinitial

Internal EnergyInternal EnergyInternal Energy is a Internal Energy is a state state

functionfunction. That is, it depends solely . That is, it depends solely on the present state of the system, on the present state of the system, and not how it may have gotten to a and not how it may have gotten to a particular state. A state function is particular state. A state function is independent of pathway.independent of pathway.

Internal EnergyInternal EnergyInternal energy (E) is a Internal energy (E) is a state state

functionfunction, and depends only on the , and depends only on the state of the system, and not how it state of the system, and not how it got to that state.got to that state.

Chemical Reactions and Chemical Reactions and Energy Energy

Bond breaking Bond breaking alwaysalways requires energy. requires energy.

Bond making Bond making alwaysalways releases energy. releases energy.

In exothermic reactions, more energy In exothermic reactions, more energy is released in forming the products is released in forming the products than is used in breaking apart the than is used in breaking apart the reactants.reactants.

Exothermic ReactionsExothermic Reactions The heat that is released comes from the potential The heat that is released comes from the potential

energy stored in the bonds of the reactants.energy stored in the bonds of the reactants.

DefinitionsDefinitionsIf a reaction involves the absorption If a reaction involves the absorption of heat into the system it is an of heat into the system it is an endothermicendothermic reaction. reaction.

More energy is required to break the More energy is required to break the bonds in the reactants than is bonds in the reactants than is released in forming the products.released in forming the products.

Endothermic ReactionsEndothermic Reactions

Chemical Reactions & Chemical Reactions & WorkWork

In addition to the release or taking in In addition to the release or taking in of energy (heat) when bonds are of energy (heat) when bonds are broken and formed, chemical broken and formed, chemical reactions can do expansion work.reactions can do expansion work.Expansion work is done when the Expansion work is done when the volume of reactants differs volume of reactants differs significantly from the volume of significantly from the volume of reactants.reactants.

WorkWorkExpansion work results when a Expansion work results when a reaction produces more gaseous reaction produces more gaseous products than reactants, and thus products than reactants, and thus pushed back the atmosphere as the pushed back the atmosphere as the reaction proceeds.reaction proceeds.If the volume of the system contracts If the volume of the system contracts during reaction (more gaseous during reaction (more gaseous reactants than products), work is done reactants than products), work is done by the surroundings on the systemby the surroundings on the system

Expansion WorkExpansion WorkUsually we just consider the volumes of Usually we just consider the volumes of gases in a chemical reaction. gases in a chemical reaction.

2 H2 H22O(g) O(g) 2 H 2 H22(g) + O(g) + O22(g) (g)

Since 2 moles of gaseous reactants Since 2 moles of gaseous reactants produce 3 moles of gaseous products, produce 3 moles of gaseous products, the system expands, and does work in the system expands, and does work in pushing back the atmosphere.pushing back the atmosphere.

Expansion WorkExpansion Work

Expansion WorkExpansion Workwork = Force x Distancework = Force x Distance

Pressure = Force/Area, or Pressure = Force/Area, or Force = Pressure(Area)Force = Pressure(Area)

work = Pressure(Area) x Distancework = Pressure(Area) x Distancework = Pressure(Area) x ∆hwork = Pressure(Area) x ∆h

work = Pressure (length x width) x ∆hwork = Pressure (length x width) x ∆hwork = P ∆Vwork = P ∆V

Expansion WorkExpansion Workwork = P ∆Vwork = P ∆V

If gases are produced by a reaction If gases are produced by a reaction and the volume expands, the system and the volume expands, the system is doing work on the surroundings. is doing work on the surroundings. The sign, when considering The sign, when considering the the systemsystem, must be negative. So,, must be negative. So,

work = - P ∆Vwork = - P ∆V

Energy, Work and HeatEnergy, Work and HeatAs the As the systemsystem loses energy to loses energy to

the surroundings, it can do so by the surroundings, it can do so by losing heat (q), and/or doing work losing heat (q), and/or doing work (w). As a result, (w). As a result,

ΔΔE = heat + workE = heat + workΔΔE = q + wE = q + wΔΔE = q -PΔVE = q -PΔV

Energy, Work and HeatEnergy, Work and HeatΔΔE = q –PΔVE = q –PΔV

Solving for q, the heat change,Solving for q, the heat change,q = q = ΔΔE + PΔVE + PΔV

Energy and EnthalpyEnergy and EnthalpyChemical reactions are Chemical reactions are

sometimes carried out in a sealed sometimes carried out in a sealed vessel with a fixed volume, a vessel with a fixed volume, a bomb bomb calorimetercalorimeter. In this apparatus, the . In this apparatus, the volume of the reaction mixture volume of the reaction mixture (system) cannot change. As a result, (system) cannot change. As a result, at constant volume,at constant volume,

qqvv = ΔE = ΔE

Energy and EnthalpyEnergy and EnthalpyMany chemical reactions are carried Many chemical reactions are carried out in an open vessel. In this case, the out in an open vessel. In this case, the reaction is performed at constant reaction is performed at constant pressure, and the volume of the system pressure, and the volume of the system is free to change during the course of is free to change during the course of the reaction. The heat transferred at the reaction. The heat transferred at constant pressure, qconstant pressure, qpp, is defined as:, is defined as:

qqpp = ΔE + PΔV = ΔE + PΔV

Energy and EnthalpyEnergy and Enthalpyqqpp = ΔE + PΔV = ΔE + PΔV

Since an open vessel is such a common Since an open vessel is such a common apparatus, the heat transferred at apparatus, the heat transferred at constant pressure is given its own constant pressure is given its own name, the name, the enthalpy changeenthalpy change, ΔH., ΔH.

qqpp = ΔE + PΔV = ΔH = ΔE + PΔV = ΔH

EnthalpyEnthalpyEnthalpy is a state function, and Enthalpy is a state function, and

is independent of reaction pathway.is independent of reaction pathway.ΔΔH = HH = Hfinalfinal-H-Hinitialinitial

ΔΔH = HH = Hproductsproducts-H-Hreactantsreactants

Sign ConventionsSign ConventionsOur focus will always be Our focus will always be on the systemon the system. If . If energy flows out of the system into the energy flows out of the system into the surroundings, it will have a negative (-) surroundings, it will have a negative (-) sign.sign.If energy flows from the surroundings into If energy flows from the surroundings into the system, it will have a positive (+) sign.the system, it will have a positive (+) sign.

Energy, Heat & WorkEnergy, Heat & Work

Standard ConditionsStandard ConditionsMany reactions are categorized by their Many reactions are categorized by their standard enthalpy changestandard enthalpy change, ΔH, ΔHoo. The . The degree sign indicates standard conditions.degree sign indicates standard conditions.

Standard conditions specify that the Standard conditions specify that the reactants and products are in the same reactants and products are in the same molar amounts represented by the molar amounts represented by the coefficients in the balanced chemical coefficients in the balanced chemical reaction.reaction.

Standard ConditionsStandard ConditionsIn a given experiment, the quantities In a given experiment, the quantities of reactants and enthalpy change will of reactants and enthalpy change will vary, but the vary, but the standard enthalpy standard enthalpy changechange is reported based on molar is reported based on molar quantities.quantities.The enthalpy of a reaction will also The enthalpy of a reaction will also vary with the physical states of vary with the physical states of reactants or products as well as reactants or products as well as temperature and pressure.temperature and pressure.

Standard ConditionsStandard ConditionsA A thermodynamic standard state thermodynamic standard state

refers to a specific set of conditions. refers to a specific set of conditions. The standard is used so that values The standard is used so that values of enthalpy changes can be directly of enthalpy changes can be directly compared.compared.

The standard state is the most The standard state is the most stable form of a substance at 1 atm stable form of a substance at 1 atm pressure and 25pressure and 25ooC.C.

Standard ConditionsStandard Conditions Standard conditions are indicated using a Standard conditions are indicated using a

degree symbol ( degree symbol ( o o ). Standard conditions for ). Standard conditions for thermochemical data differ from the thermochemical data differ from the standard conditions used in the gas laws.standard conditions used in the gas laws.1. All gases have a pressure of exactly 1 1. All gases have a pressure of exactly 1 atm.atm.2. Pure substances are in the form that 2. Pure substances are in the form that they they normally exist in at 25normally exist in at 25ooC and 1 atm C and 1 atm pressure.pressure.3. All solutions have a concentration of 3. All solutions have a concentration of exactly exactly 1M.1M.

Standard ConditionsStandard ConditionsFor example, since oxygen is a For example, since oxygen is a

diatomic gas at 25diatomic gas at 25ooC, the standard C, the standard state of oxygen is Ostate of oxygen is O22(g) at a pressure (g) at a pressure of 1 atm.of 1 atm.

CalorimetryCalorimetryCalorimetryCalorimetry is the science of measuring is the science of measuring heat. It typically involves measuring heat. It typically involves measuring temperature changes as a substance loses temperature changes as a substance loses or gains heat.or gains heat.

Since substances vary in how much their Since substances vary in how much their temperature changes as heat is lost or temperature changes as heat is lost or gained, it is important to know the gained, it is important to know the heat heat capacitycapacity (C) of substances involved in the (C) of substances involved in the reaction.reaction.

Heat Capacity (C)Heat Capacity (C)The The heat capacityheat capacity of a substance is the of a substance is the amount of heat absorbed, usually in amount of heat absorbed, usually in joules, per 1 degree (C or K) increase joules, per 1 degree (C or K) increase in temperature. The amount (mass) of in temperature. The amount (mass) of the substance also determines the the substance also determines the amount of heat lost or gained.amount of heat lost or gained.

C = C = heat absorbed heat absorbed = _q_ = _q_ increase in temp. ΔTincrease in temp. ΔT

The The specific heat capacityspecific heat capacity is for is for a a gramgram of a substance. It has the of a substance. It has the units J/units J/ooC-g or J/K-g.C-g or J/K-g.

The The molar heat capacitymolar heat capacity is for a is for a molemole of a given substance. It has of a given substance. It has the units J/the units J/ooC-mol or C-mol or J/K-mol.J/K-mol.

Heat CapacityHeat Capacity

Calorimeter ConstantCalorimeter ConstantIn measuring heat changes during a In measuring heat changes during a reaction, any heat absorbed or lost be reaction, any heat absorbed or lost be the calorimeter (the apparatus itself) the calorimeter (the apparatus itself) must be considered. If this amount of must be considered. If this amount of heat is significant, theheat is significant, the calorimeter calorimeter constantconstant may be provided or measured. may be provided or measured. This is the heat capacity of the specific This is the heat capacity of the specific apparatus used, and is expressed in J or apparatus used, and is expressed in J or kJ per degree change in temperature (K kJ per degree change in temperature (K or or ooC).C).

Coffee Cup CalorimetryCoffee Cup CalorimetryA simple device for A simple device for determining heat determining heat changes of aqueous changes of aqueous reactions at constant reactions at constant pressure is a coffee pressure is a coffee cup calorimeter. cup calorimeter. Since the contents Since the contents are open to the are open to the atmosphere, the atmosphere, the pressure, atmospheric pressure, atmospheric pressure, remains pressure, remains constant during the constant during the reaction.reaction.

Coffee Cup CalorimetryCoffee Cup CalorimetryThe heat change for the reaction, qThe heat change for the reaction, qpp, is , is equal to the enthalpy change for the equal to the enthalpy change for the reaction.reaction.

If heat is given off, it goes towards If heat is given off, it goes towards warming up the contents of the warming up the contents of the calorimeter and toward warming up calorimeter and toward warming up the calorimeter walls, thermometer, the calorimeter walls, thermometer, stirrer, etc.stirrer, etc.

Coffee Cup CalorimetryCoffee Cup Calorimetryqqreactionreaction = q = qcontentscontents + q + qcalcal

qqcontents contents = (mass of solution) (= (mass of solution) (ΔΔTTsolnsoln)C)Csolnsoln

CCsoln soln is the specific heat capacity of the is the specific heat capacity of the reaction mixture. If solutions are reaction mixture. If solutions are aqueous and fairly dilute, the specific aqueous and fairly dilute, the specific heat capacity of water, 4.18J/heat capacity of water, 4.18J/ooC-g, may C-g, may be used.be used.

Coffee Cup CalorimetryCoffee Cup Calorimetryqqreactionreaction = q = qcontentscontents + q + qcalcal

qqcal cal = C= Ccal cal ((ΔΔT)T)

CCcalcal is the calorimeter heat capacity. It is the calorimeter heat capacity. It includes the heat needed to warm up the includes the heat needed to warm up the walls, thermometer and stirrer of the walls, thermometer and stirrer of the calorimeter, along with any heat loss due calorimeter, along with any heat loss due to leaks.to leaks.

In many simple calculations, CIn many simple calculations, Ccal cal is assumed is assumed to be negligible, and may be ignored.to be negligible, and may be ignored.

Obtaining Obtaining ΔΔH of H of ReactionReaction

The enthalpy change of a reaction, The enthalpy change of a reaction, ΔΔH, H, can be obtained from qcan be obtained from qpp. .

First, a sign must be assigned. If the First, a sign must be assigned. If the temperature increased during the temperature increased during the reaction, the reaction is exothermic, reaction, the reaction is exothermic, and q is negative.and q is negative.If the temperature decreased during If the temperature decreased during the reaction, the reaction is the reaction, the reaction is endothermic, and q is positive.endothermic, and q is positive.

Obtaining Obtaining ΔΔH of H of ReactionReaction

The enthalpy change of a reaction, The enthalpy change of a reaction, ΔΔH, can be obtained from qH, can be obtained from qpp. .

Also, qAlso, qpp is for a specific quantity of is for a specific quantity of reactants. Typically, reactants. Typically, ΔΔHHrxnrxn is for is for molarmolar quantities of reactants. To calculate quantities of reactants. To calculate ΔΔHHrxnrxn from q from qpp, you must calculate the , you must calculate the heat change heat change per moleper mole of reactant. of reactant.

Problem: CalorimetryProblem: Calorimetry A coffee cup calorimeter contains 125. A coffee cup calorimeter contains 125.

grams of water at 24.2grams of water at 24.2ooC. A 10.5 g C. A 10.5 g sample of KBr, also at 24.2sample of KBr, also at 24.2ooC, is added. C, is added. After dissolving, the mixture reaches a After dissolving, the mixture reaches a final temperature of 21.1final temperature of 21.1ooC. Calculate C. Calculate ∆H∆Hsolnsoln in joules/gram and kJ/mol. Assume in joules/gram and kJ/mol. Assume the specific heat of the solution is 4.18 the specific heat of the solution is 4.18 J/g-J/g-ooC, and no heat is transferred to or C, and no heat is transferred to or from the calorimeter or surroundings.from the calorimeter or surroundings.

Constant Volume Constant Volume CalorimetryCalorimetry

Certain reactions, notably combustion Certain reactions, notably combustion reactions, do not lend themselves to open reactions, do not lend themselves to open vessels. These reactions are usually vessels. These reactions are usually carried out in a sealed reaction vessel carried out in a sealed reaction vessel called a called a bomb calorimeterbomb calorimeter. . The bomb calorimeter is a rigid steel The bomb calorimeter is a rigid steel container that is sealed after the container that is sealed after the reactants have been added. The reaction reactants have been added. The reaction takes place once an electrical current is takes place once an electrical current is sent through an ignition wire to the sent through an ignition wire to the reaction mixture.reaction mixture.

Bomb CalorimetryBomb CalorimetryThe steel bomb is The steel bomb is immersed in an immersed in an insulated bath insulated bath containing either containing either water or mineral water or mineral oil. As the oil. As the combustion combustion reaction releases reaction releases heat, the heat is heat, the heat is transferred to the transferred to the bath.bath.

Bomb CalorimetryBomb Calorimetry

Ignition wire

Reaction vessel

gaskets

O2 inlet

Bomb CalorimetryBomb CalorimetryOnce the bomb has been charged Once the bomb has been charged with reactants, it is placed in the with reactants, it is placed in the water or oil bath until it reaches a water or oil bath until it reaches a constant temperature.constant temperature.A current is sent through the ignition A current is sent through the ignition wire, and the combustion reaction wire, and the combustion reaction takes place. The heat given off by the takes place. The heat given off by the reaction is evident from the increase reaction is evident from the increase in temperature of the water/oil bath.in temperature of the water/oil bath.

Bomb CalorimetryBomb CalorimetryThe heat generated by the reaction The heat generated by the reaction warms up the contents of the calorimeter warms up the contents of the calorimeter (the bomb, thermometer, container (the bomb, thermometer, container walls, stirrer) and the water (or oil) bath.walls, stirrer) and the water (or oil) bath.Usually, the calorimeter constant, which Usually, the calorimeter constant, which is the heat capacity of the entire is the heat capacity of the entire apparatus, is provided, or determined by apparatus, is provided, or determined by combusting a substance with a known combusting a substance with a known energy of combustion.energy of combustion.

Problem: Bomb Problem: Bomb CalorimetryCalorimetry

The energy released by combustion The energy released by combustion of benzoic acid is 26.42 kJ/g. The of benzoic acid is 26.42 kJ/g. The combustion of .1584g of benzoic acid combustion of .1584g of benzoic acid increases the temperature of a bomb increases the temperature of a bomb calorimeter by 2.54 calorimeter by 2.54 ooC. C. a) Calculate the calorimeter a) Calculate the calorimeter constant.constant.

Problem: Bomb Problem: Bomb CalorimetryCalorimetry

b) 0.2130 g of vanillin (C8H8P3) is b) 0.2130 g of vanillin (C8H8P3) is burned in the same calorimeter with burned in the same calorimeter with a temperature increase of 3.25a temperature increase of 3.25ooC. C. Calculate the energy of combustion Calculate the energy of combustion of vanillin in kJ/g and kJ/mol.of vanillin in kJ/g and kJ/mol.

Hess’s LawHess’s LawEnthalpy is a Enthalpy is a state functionstate function. .

This means that a change in This means that a change in enthalpy depends solely on the enthalpy depends solely on the initial and final states (products and initial and final states (products and reactants), and is independent of the reactants), and is independent of the reaction pathway.reaction pathway.

Hess’s LawHess’s LawHess’s Law is a method that Hess’s Law is a method that

combines related chemical reactions combines related chemical reactions and their enthalpy changes. Since and their enthalpy changes. Since enthalpy changes are independent of enthalpy changes are independent of pathway, as long as the net reaction pathway, as long as the net reaction matches the reaction of interest, the matches the reaction of interest, the sum of the enthalpy changes will sum of the enthalpy changes will yield yield ΔΔH for the desired reaction.H for the desired reaction.

Hess’s LawHess’s Law

Hess’s LawHess’s LawThere are a few basic rules in applying There are a few basic rules in applying

Hess’s Law:Hess’s Law:

1. If a reaction is reversed, the sign of 1. If a reaction is reversed, the sign of ΔΔH is also reversed.H is also reversed.

2. If the coefficients in a balanced 2. If the coefficients in a balanced reaction are multiplied by an integer, reaction are multiplied by an integer, the value of ∆H is multiplied by the the value of ∆H is multiplied by the same integer.same integer.

Problem: Hess’s LawProblem: Hess’s LawCalculate ∆HCalculate ∆Hoo for the reaction: for the reaction: CC66HH44(OH)(OH)22((aqaq)   +  H)   +  H22OO22((aqaq)) C C66HH44OO22((aqaq)  +  2 H)  +  2 H22O(O(ll))

Using: Using: 1)C1)C66HH44(OH)(OH)22((aqaq) ) CC66HH44OO22((aqaq) + H) + H22((gg) ∆H) ∆Hoo = +177.4 = +177.4

kJkJ2)  H2)  H22 ( (gg)  +   O)  +   O22((gg)) H H22OO22((aqaq)               ∆H)               ∆Hoo = –191.2 = –191.2

kJkJ3)  H3)  H22 ( (gg)  + 1/2  O)  + 1/2  O22((gg) ) H H22O(O(gg)            ∆H)            ∆Hoo = –241.8 = –241.8

kJ kJ 4)  H4)  H22O(O(gg) ) H H22O(O(ll)                                ∆H)                                ∆Hoo = –43.8 kJ = –43.8 kJ

Standard Enthalpies of Standard Enthalpies of FormationFormation

A A formation reactionformation reaction involves combining involves combining elements, in their standard states, to elements, in their standard states, to form form one moleone mole of a compound. of a compound.

A table of standard enthalpies of A table of standard enthalpies of formation (∆Hformation (∆Hff

oo) is in appendix of the ) is in appendix of the text. The ∆Htext. The ∆Hff

oo values of most common values of most common compounds have been determined and compounds have been determined and tabulated.tabulated.

Standard Enthalpies of Standard Enthalpies of FormationFormation

CaCOCaCO33((ss) has a ∆H) has a ∆Hffoo of -1207 kJ/mol. This of -1207 kJ/mol. This

is the enthalpy change for the reaction:is the enthalpy change for the reaction:

Ca(Ca(ss) + C() + C(graphitegraphite) + 3/2 O) + 3/2 O22((gg) ) CaCO CaCO33((ss) )

Fractional coefficients are acceptable since Fractional coefficients are acceptable since all quantities are molar, and a formation all quantities are molar, and a formation reaction produces reaction produces one moleone mole of a of a compound.compound.

Standard Enthalpies of Standard Enthalpies of FormationFormation

∆∆HHffoo values can be used to calculate the values can be used to calculate the

standard enthalpy changes for many standard enthalpy changes for many reactions.reactions.In an application of Hess’s Law, it is as if the In an application of Hess’s Law, it is as if the reactants are decomposed into their reactants are decomposed into their elements, and then the elements are elements, and then the elements are recombined into the desired products. Since recombined into the desired products. Since enthalpies of reaction are independent of enthalpies of reaction are independent of pathway, this provides an accurate way to pathway, this provides an accurate way to calculate enthalpies of reactions.calculate enthalpies of reactions.

Standard Enthalpies of Standard Enthalpies of FormationFormation

CH4(g) + 2O2(g) CO2(g) + 2 H2O(l)

Hess’s Law and ∆HHess’s Law and ∆Hffoo

∆∆HHrxnrxnoo = ∑ ∆H = ∑ ∆Hff

oo(products) - ∑ (products) - ∑ ∆H∆Hff

oo(reactants) (reactants)

Problem: Use standard heats of Problem: Use standard heats of formation to calculate ∆Hformation to calculate ∆Hoo

rxnrxn for: for:

KClOKClO33(s) (s) KCl(s) + 3/2 O KCl(s) + 3/2 O22(g)(g)

Bond Dissociation Bond Dissociation EnergiesEnergies

Bond dissociation energies can Bond dissociation energies can be used to estimate the enthalpy of a be used to estimate the enthalpy of a reaction. The enthalpy change will reaction. The enthalpy change will approximately equal the energy of approximately equal the energy of bonds broken – energy of bonds bonds broken – energy of bonds formed.formed.

ΔHΔHoo ≈D ≈D(bonds broken) (bonds broken) –D–D(bonds formed)(bonds formed)

Bond EnergiesBond EnergiesSince bond energies are Since bond energies are averageaverage values for a variety of molecules, they values for a variety of molecules, they will only provide an estimate of the will only provide an estimate of the enthalpy change for a specific enthalpy change for a specific reaction. The following relationship reaction. The following relationship can also be used to estimate enthalpies can also be used to estimate enthalpies of reaction.of reaction.

ΔHΔHoo ≈D ≈D(reactant bonds) (reactant bonds) –D–D(product bonds)(product bonds)

Predicting SpontaneityPredicting SpontaneityA process is A process is spontaneousspontaneous if it occurs with if it occurs with no outside intervention. An example is the no outside intervention. An example is the melting of ice above a temperature of 0melting of ice above a temperature of 0ooC. C. Although the melting of ice is endothermic, Although the melting of ice is endothermic, it will still occur on its own if the it will still occur on its own if the temperature is high enough.temperature is high enough.The spontaneity of a process will depend The spontaneity of a process will depend on the enthalpy change, the temperature on the enthalpy change, the temperature and the and the entropyentropy change. change.

EntropyEntropyEntropyEntropy is a measure of randomness or is a measure of randomness or disorder. Spontaneous reactions or disorder. Spontaneous reactions or processes may involve an increase in processes may involve an increase in entropy.entropy.In the example of melting ice, the liquid In the example of melting ice, the liquid water is more random in structure than the water is more random in structure than the solid. As a result, ΔS, the entropy change, solid. As a result, ΔS, the entropy change, is positive.is positive.

ΔS = SΔS = Sfinalfinal-S-Sinitialinitial

EntropyEntropyΔS = SΔS = Sfinalfinal-S-Sinitialinitial

Entropy values for pure substances can Entropy values for pure substances can be calculated, and typically have the be calculated, and typically have the units J/mol-K.units J/mol-K.

Gases have higher entropy values than Gases have higher entropy values than liquids, and mixtures have greater liquids, and mixtures have greater entropy than pure substances.entropy than pure substances.

SpontaneitySpontaneityA process will A process will alwaysalways be spontaneous if it be spontaneous if it releases heat and increases in entropy.releases heat and increases in entropy.

A process will A process will nevernever be spontaneous if it be spontaneous if it absorbs heat and involves a decrease in absorbs heat and involves a decrease in entropy.entropy.

For other combinations of enthalpy and For other combinations of enthalpy and entropy changes, temperature will play a entropy changes, temperature will play a role.role.

Free EnergyFree EnergyThe free energy change, ΔG, is The free energy change, ΔG, is

used to predict if a process is used to predict if a process is spontaneous. It considers the spontaneous. It considers the enthalpy change, entropy change enthalpy change, entropy change and temperature.and temperature.

ΔG = ΔH - TΔSΔG = ΔH - TΔS

Free EnergyFree EnergyIf ΔG is negative, the process is If ΔG is negative, the process is

spontaneous at the specified spontaneous at the specified temperature. If positive, the process temperature. If positive, the process is not spontaneous, and if ΔG = 0, is not spontaneous, and if ΔG = 0, the system is the system is at equilibriumat equilibrium..

ΔG = ΔH - TΔSΔG = ΔH - TΔS

Free EnergyFree EnergyAt At equilibriumequilibrium, the forward , the forward

process occurs at the same rate as process occurs at the same rate as the reverse process. Neither the reverse process. Neither direction is favored.direction is favored.