Thermochemistry The study of energy and its transformations

ThermochemistryThermochemistry

The study of energy and The study of energy and its transformationsits transformations

DefinitionsDefinitions

When examining chemical systems or When examining chemical systems or reactions, in order to keep track of reactions, in order to keep track of energy changes, we consider the energy changes, 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 Typically, it is the reactants and products.products.

The surroundings include everything The surroundings include everything else in the universe. Often, we just else in the universe. Often, we just consider the immediate surroundings, consider the immediate surroundings, such as the reaction vessel.such as the reaction vessel.

Energy ChangesEnergy Changes

Chemical reactions and Chemical reactions and physical changes typically physical changes typically involve a transfer of energy. involve a transfer of energy. If a process such as melting If a process such as melting ice requires energy, the ice requires energy, the reverse process of freezing reverse process of freezing water releases the same amount water releases the same amount of energy.of energy.


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

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


If a reaction requires heat, If a reaction requires heat, energy flows from the energy flows from the surroundings into the system. surroundings into the system. These reactions are These reactions are endothermic.endothermic.

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

Physical Changes and Physical Changes and EnergyEnergy

Changing the state of a Changing the state of a substance involves an energy substance involves an energy change. In melting or boiling a change. In melting or boiling a substance, the attractive forces substance, the attractive forces between the atoms or molecules must between the atoms or molecules must be overcome, and heat is required. be overcome, and heat is required. This process is endothermic.This process is endothermic.

When a substance cools and When a substance cools and condenses or freezes, heat is given condenses or freezes, heat is given off, and the process is exothermic.off, and the process is exothermic.

Chemical Reactions and Chemical Reactions and Energy Energy

In chemical reactions, the In chemical reactions, the energy changes result from the energy changes result from the breaking and the formation of breaking and the formation of chemical bonds.chemical bonds.

Bond breaking Bond breaking alwaysalways requires requires energy.energy.

Bond making Bond making alwaysalways releases energy. releases energy.

Chemical Reactions and Chemical Reactions and Energy Energy

Bond breaking Bond breaking alwaysalways requires requires energy.energy.

Bond making Bond making alwaysalways releases energy. releases energy.

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

Types of SystemsTypes of Systems

Systems can be open (both energy and Systems can be open (both energy and matter are exchanged, closed (only matter are exchanged, closed (only energy is exchanged) and isolated energy is exchanged) and isolated (neither energy nor matter is (neither energy nor matter is exchanged with the surroundings.)exchanged with the surroundings.)

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

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

Definitions - EnergyDefinitions - Energy

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

Heat and work are ways that Heat and work are ways that objects can exchange energy.objects can exchange energy.

Definitions - EnergyDefinitions - Energy

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

Definitions- EnergyDefinitions- Energy

Potential 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 the potential to produce energy if they react.if they react.

Internal EnergyInternal Energy

The The internal energyinternal energy (E, or U) of (E, or U) of a system is the sum of the kinetic a system is the sum of the kinetic and potential energies of all of the and potential energies of all of the particles of the system. particles of the system.

It is generally not possible to It is generally not possible to determine the internal energy of a determine the internal energy of a system, but we can measure changes system, but we can measure changes in internal energy.in internal energy.

Internal energy is changed by Internal energy is changed by the flow of work and/or heat.the flow of work and/or heat.


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

Internal EnergyInternal EnergyInternal energy (E or U) is a Internal energy (E or U) is a

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

Internal Energy & the Internal Energy & the 11stst Law Law

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

Energy can be converted from Energy can be converted from one form to another, but cannot be one form to another, but cannot be created or destroyed.created or destroyed.

It is not possible to measure It is not possible to measure the total energy of a system, but the total energy of a system, but it is possible to determine changes it is possible to determine changes in energy.in energy.


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

ΔE = EΔE = Efinalfinal – E – Einitialinitial

(some texts use the symbol U for (some texts use the symbol U for internal energy)internal energy)

The 1The 1stst Law of Law of ThermodynamicsThermodynamics

Since the energy change of Since the energy change of the system is equal and the system is equal and opposite to the energy change opposite to the energy change of the surroundings,of the surroundings,

ΔEΔEsystemsystem= –ΔE= –ΔEsurroundingssurroundings

oror

ΔUΔUsystemsystem= –ΔU= –ΔUsurroundingssurroundings

The 1The 1stst Law of Law of ThermodynamicsThermodynamics

The implication of the The implication of the first law of thermodynamics is first law of thermodynamics is that:that:

The energy of the universe is The energy of the universe is constant.constant.

Work and HeatWork and Heat

Energy is the capacity to do Energy is the capacity to do work or transfer heat. The work or transfer heat. The internal energy of a system changes internal energy of a system changes when heat is exchanged, or when the when heat is exchanged, or when the system does work on its system does work on its surroundings, or when the surroundings, or when the surroundings do work on the system.surroundings do work on the system.

ΔE = q + wΔE = q + w

oror

ΔU = q + wΔU = q + w

Energy, Work and HeatEnergy, Work and Heat

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

ΔΔE = heat + workE = heat + work

ΔΔE = q + wE = q + w

WorkWork

Chemical reactions can be Chemical reactions can be harnessed to do electrical work harnessed to do electrical work (as with batteries), or (as with batteries), or expansion work (such as expansion work (such as expanding gases in an internal expanding gases in an internal combustion engine).combustion engine).

Our focus will be on Our focus will be on expansion work.expansion work.

WorkWork

Expansion 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 pushes back the atmosphere as the pushes back the atmosphere as the reaction proceeds.reaction proceeds.

If the volume of the system If the volume of the system contracts during reaction (more contracts during reaction (more gaseous reactants than products), gaseous reactants than products), work is done by the surroundings on work is done by the surroundings on the system.the system.

Expansion WorkExpansion Work

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


work = 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 Distance

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

work = Pressure (length x width) work = Pressure (length x width) x ∆hx ∆h

work = P ∆Vwork = P ∆V


work = P ∆Vwork = P ∆V

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

work = - P ∆Vwork = - P ∆V


ΔΔE = q –PΔVE = q –PΔV

Solving for q, the heat change,Solving for q, the heat change,

q = q = ΔΔE + PΔVE + PΔV

The heat change, q, will vary The heat change, q, will vary with the reaction conditions. with the reaction conditions. Reactions in an open vessel are Reactions in an open vessel are performed at constant pressure, performed at constant pressure, those in a sealed vessel are those in a sealed vessel are performed at constant volume.performed at constant volume.

Reactions at Constant Reactions at Constant VolumeVolume


At constant volume, expansion At constant volume, expansion work isnwork isn’’t possible, and the heat t possible, and the heat change, qchange, qvv, equals the change in , equals the change in internal energy.internal energy.

qqvv = = ΔΔEE

oror

qqvv = = ΔΔUU

indicates constant volume



At constant pressure, q At constant pressure, q becomes qbecomes qpp, and , and

qqp p = = ΔΔE + PΔV E + PΔV

oror

qqp p = = ΔΔU + PΔVU + PΔVdenotes constant pressure



At constant pressure, q At constant pressure, q becomes qbecomes qpp, and , and

qqp p = = ΔΔE + PΔV E + PΔV

oror

qqp p = = ΔΔU + PΔVU + PΔVdenotes constant pressure

Energy and EnthalpyEnergy and Enthalpy

qqpp = ΔE + PΔV = ΔE + PΔV

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

qqpp = ΔE + PΔV = ΔH = ΔE + PΔV = ΔH

EnthalpyEnthalpy

Enthalpy is a state Enthalpy is a state function, and is independent of function, and is independent of reaction pathway.reaction pathway.

ΔΔH = HH = Hfinalfinal-H-Hinitialinitial

ΔΔH = HH = Hproductsproducts-H-Hreactantsreactants

Standard ConditionsStandard Conditions

Many reactions are categorized Many reactions are categorized by their by their standard enthalpy changestandard enthalpy change, , ΔHΔHoo. The degree sign indicates . The degree sign indicates standard conditions.standard conditions.

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


In a given experiment, the In a given experiment, the quantities of reactants and quantities of reactants and enthalpy change will vary, but the enthalpy change will vary, but the standard enthalpy changestandard enthalpy change is is reported based on molar reported based on molar quantities.quantities.

The enthalpy of a reaction will The enthalpy of a reaction will also vary with the physical states also vary with the physical states of reactants or products as well of reactants or products as well as temperature and pressure.as temperature and pressure.


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

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

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


For example, since oxygen For example, since oxygen is a diatomic gas at 25is a diatomic gas at 25ooC, the C, the standard state of oxygen is standard state of oxygen is OO22(g) at a pressure of 1 atm.(g) at a pressure of 1 atm.

Thermochemical Thermochemical EquationsEquations

Chemical reactions (or changes of Chemical reactions (or changes of state) may be written with their state) may be written with their enthalpy of reactionenthalpy of reaction. For example, . For example,

CHCH44((gg) + 2 O) + 2 O22((gg) ) CO CO22((gg) + 2 H) + 2 H22O(O(ll) )

ΔH=–890.4 kJ/molΔH=–890.4 kJ/mol

The enthalpy change is for the The enthalpy change is for the reaction written, reaction written, assuming molar assuming molar quantitiesquantities. 890.4 kJ of heat is . 890.4 kJ of heat is produced when a mole of CHproduced when a mole of CH44((gg) is ) is burned.burned.

Thermochemical Thermochemical EquationsEquations

If the reaction is reversed, 890.4 If the reaction is reversed, 890.4 kJ is required to produce each mole of kJ is required to produce each mole of CHCH44((gg).).

COCO22((gg) + 2 H) + 2 H22O(O(ll) ) CHCH44((gg) + 2 O) + 2 O22((gg))

ΔH=+890.4 kJ/molΔH=+890.4 kJ/mol

If the coefficients in a balanced 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.

CalorimetryCalorimetry

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

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

Heat Capacity (C)Heat Capacity (C)

The The heat capacityheat capacity of a substance of a substance is the amount of heat absorbed, is the amount of heat absorbed, usually in joules, per 1 degree (C usually in joules, per 1 degree (C or K) increase in temperature. The or K) increase in temperature. The amount (mass) of the substance also amount (mass) of the substance also determines the amount of heat lost determines the amount of heat lost or gained.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 a is for a gramgram of a substance. of a substance. It has the units J/It has the units J/ooC-g or J/K-C-g or J/K-g.g.

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

J/K-mol.J/K-mol.

Heat CapacityHeat Capacity

Calorimeter ConstantCalorimeter Constant

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

Coffee Cup CalorimetryCoffee Cup Calorimetry

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


The heat change for the The heat change for the reaction, qreaction, qpp, is equal to the , is equal to the enthalpy change for the reaction.enthalpy change for the reaction.

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


qqreactionreaction = q = qcontentscontents + q + qcalcal

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

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


qqreactionreaction = q = qcontentscontents + q + qcalcal

qqcal cal = C= Ccal cal ((ΔΔT)T)

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

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

Obtaining Obtaining ΔΔH of H of ReactionReaction

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

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

If the temperature decreased If the temperature decreased during the reaction, the reaction during the reaction, the reaction is endothermic, and q is positive.is 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 is for a specific quantity of reactants. Typically, quantity of reactants. Typically, ΔΔHHrxnrxn is for is for molarmolar quantities of quantities of reactants. To calculate reactants. To calculate ΔΔHHrxnrxn from from qqpp, you must calculate the heat , you must calculate the heat change change per moleper mole of reactant. of reactant.

Problem: CalorimetryProblem: Calorimetry

A coffee cup calorimeter contains A coffee cup calorimeter contains 125. grams of water at 24.2125. grams of water at 24.2ooC. A 10.5 C. A 10.5 g sample of KBr, also at 24.2g sample of KBr, also at 24.2ooC, is C, is added. After dissolving, the mixture added. After dissolving, the mixture reaches a final temperature of 21.1reaches a final temperature of 21.1ooC. C. Calculate ∆H Calculate ∆Hsolnsoln in joules/gram and in joules/gram and kJ/mol. Assume the specific heat of kJ/mol. Assume the specific heat of the solution is 4.18 J/g-the solution is 4.18 J/g-ooC, and no C, and no heat is transferred to or from the heat is transferred to or from the calorimeter or surroundings.calorimeter or surroundings.

Constant Volume Constant Volume CalorimetryCalorimetry

Certain reactions, notably combustion Certain reactions, notably combustion reactions, do not lend themselves to reactions, do not lend themselves to open vessels. These reactions are open vessels. These reactions are usually carried out in a sealed usually carried out in a sealed reaction vessel called a reaction vessel called a bomb bomb calorimetercalorimeter. .

The bomb calorimeter is a rigid The bomb calorimeter is a rigid steel container that is sealed after steel container that is sealed after the reactants have been added. The the reactants have been added. The reaction takes place once an reaction takes place once an electrical current is sent through an electrical current is sent through an ignition wire to the reaction mixture.ignition wire to the reaction mixture.

Bomb CalorimetryBomb Calorimetry

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


Ignition wire

Reaction vessel

gaskets

O2 inlet


Once 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 water or oil bath until it reaches a constant temperature.a constant temperature.

A current is sent through the A current is sent through the ignition wire, and the combustion ignition wire, and the combustion reaction takes place. The heat reaction takes place. The heat given off by the reaction is given off by the reaction is evident from the increase in evident from the increase in temperature of the water/oil bath.temperature of the water/oil bath.


The heat generated by the The heat generated by the reaction warms up the contents of the reaction warms up the contents of the calorimeter (the bomb, thermometer, calorimeter (the bomb, thermometer, container walls, stirrer) and the container walls, stirrer) and the water (or oil) bath.water (or oil) bath.

Usually, the calorimeter Usually, the calorimeter constant, which is the heat capacity constant, which is the heat capacity of the entire apparatus, is provided, of the entire apparatus, is provided, or determined by combusting a or determined by combusting a substance with a known energy of substance with a known energy of combustion.combustion.

Problem: Bomb Problem: Bomb CalorimetryCalorimetry

The energy released by The energy released by combustion of benzoic acid is combustion of benzoic acid is 26.42 kJ/g. The combustion 26.42 kJ/g. The combustion of .1584g of benzoic acid of .1584g of benzoic acid increases the temperature of a increases the temperature of a bomb calorimeter by 2.54 bomb 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 b) 0.2130 g of vanillin (C(C8HH8OO3) is burned in the same ) is burned in the same calorimeter with a temperature calorimeter with a temperature increase of 3.25increase of 3.25ooC. Calculate C. Calculate the energy of combustion of the energy of combustion of vanillin in kJ/g and kJ/mol.vanillin in kJ/g and kJ/mol.

HessHess’’s Laws Law

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


HessHess’’s Law is a method that s Law is a method that combines related chemical combines related chemical reactions and their enthalpy reactions and their enthalpy changes. Since enthalpy changes changes. Since enthalpy changes are independent of pathway, as are independent of pathway, as long as the net reaction matches long as the net reaction matches the reaction of interest, the sum the reaction of interest, the sum of the enthalpy changes will of the enthalpy changes will yield yield ΔΔH for the desired H for the desired reaction.reaction.



There are a few basic rules in There are a few basic rules in applying Hessapplying Hess’’s Law:s Law:

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

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

Problem: HessProblem: Hess’’s Laws Law

Calculate ∆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 kJ = +177.4 kJ

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

3) H3) H22 ( (gg) + 1/2 O) + 1/2 O22((gg) ) H H22O(O(gg) ∆H) ∆Hoo = = –241.8 kJ –241.8 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 involves combining elements, in their combining elements, in their standard states, to form standard states, to form one moleone mole of of a compound.a compound.

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

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

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


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

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

Ca(Ca(ss) + C() + C(graphitegraphite) + 3/2 O) + 3/2 O22((gg) ) CaCOCaCO33((ss) )

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


∆∆HHffoo values can be used to values can be used to

calculate the standard enthalpy calculate the standard enthalpy changes for many reactions.changes for many reactions.

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


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

HessHess’’s Law and ∆Hs 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 the for the combustion of methane (CHcombustion of methane (CH44) :) :

CHCH44(g) + 2 O(g) + 2 O22(g) (g) 2 H 2 H22O(g) + O(g) + COCO22(g)(g)

Bond Dissociation Bond Dissociation EnergiesEnergies

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

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

Bond EnergiesBond Energies

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

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

Bond Energies - ProblemBond Energies - Problem

Use average bond enthalpies Use average bond enthalpies to estimate the enthalpy of to estimate the enthalpy of reaction for:reaction for:

NN22((gg) + 3 H) + 3 H22((gg) ) 2 NH 2 NH33((gg))

Energetics of Ionic Energetics of Ionic BondsBonds

The The lattice energylattice energy is a is a measure of the strength of measure of the strength of ionic bonds within a specific ionic bonds within a specific crystal structure. It is crystal structure. It is usually defined as the energy usually defined as the energy change when a mole of a change when a mole of a crystalline solid is formed crystalline solid is formed from its gaseous ions.from its gaseous ions.

MM++((gg) + X) + X--((gg) ) MX(s) MX(s)

Lattice EnergyLattice Energy

Your text uses a different Your text uses a different convention, defining the lattice convention, defining the lattice energy as the energy change when a energy as the energy change when a mole of a crystalline solid is mole of a crystalline solid is converted to its gaseous ions.converted to its gaseous ions.

MX(MX(ss) ) M M++((gg) + X) + X--((gg))

This approach results in positive This approach results in positive values for lattice energy, and it values for lattice energy, and it requires energy to break apart and requires energy to break apart and vaporize the ions in the solid.vaporize the ions in the solid.

Lattice EnergyLattice Energy

Lattice energies cannot be measured Lattice energies cannot be measured directly, so they are obtained using directly, so they are obtained using HessHess’’ Law. They will vary greatly Law. They will vary greatly with ionic charge, and, to a lesser with ionic charge, and, to a lesser degree, with ionic size. degree, with ionic size.

Starting with the elements is their Starting with the elements is their standard states, we know the standard states, we know the enthalpies of sublimation, ionization, enthalpies of sublimation, ionization, bond dissociation, electron affinity, bond dissociation, electron affinity, and the enthalpy of formation for the and the enthalpy of formation for the ionic solid. ionic solid.

1/2 bond energy of Cl2

Ionization energy of K

∆Hf of KCl

∆Hsub of K}

Electron Affinity of Cl

–Lattice Energy of KCl

Ionic charge has a huge effect on lattice energy.

Ionic vs. Covalent Ionic vs. Covalent CompoundsCompounds

Ionic Compounds Ionic Compounds Crystalline solids Crystalline solids (made of ions)(made of ions)

High melting and High melting and boiling pointsboiling points

Conduct electricity Conduct electricity when meltedwhen melted

Many soluble in Many soluble in water but not in water but not in nonpolar liquidnonpolar liquid

Covalent CompoundsCovalent Compounds Gases, liquids, or Gases, liquids, or solids (made of solids (made of molecules)molecules)

Low melting and Low melting and boiling pointsboiling points

Poor electrical Poor electrical conductors in all conductors in all phasesphases

Many soluble in Many soluble in nonpolar liquids but nonpolar liquids but not in waternot in water

Properties of NaCl and Properties of NaCl and CClCCl44

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Thermochemistry The study of energy and its transformations