Unit 13: Unit 13: ThermochemistryThermochemistry

Chapter 17Chapter 17

By: Jennie BordersBy: Jennie Borders

Section 17.1 – The Flow of Section 17.1 – The Flow of EnergyEnergy

EnergyEnergy is the capacity to do is the capacity to do workwork or or supply supply heatheat..

Energy has no Energy has no mass or volumemass or volume.. Chemical potential energyChemical potential energy is energy is energy

stored in stored in chemicalschemicals.. The kinds of The kinds of atomsatoms and the and the arrangement arrangement

of the atoms in a substance determine of the atoms in a substance determine the amount of the amount of energyenergy stored in the stored in the substance.substance.

HeatHeat

HeatHeat is a form of is a form of energyenergy that always that always flowsflows from a from a warmerwarmer object to a object to a coolercooler object.object.

Heat is represented Heat is represented by by qq..

ThermochemistryThermochemistry

ThermochemistryThermochemistry is the is the study of the study of the heatheat changes changes that occur during that occur during chemical reactionschemical reactions and and physical changesphysical changes of state. of state.

The The law of conservation law of conservation of energyof energy states that in states that in any chemical or physical any chemical or physical process, process, energy is neither energy is neither created nor destroyedcreated nor destroyed..

The Great The Great DebateDebate

1. Exothermic reactions lose heat.1. Exothermic reactions lose heat.

2. Endothermic reactions absorb heat.2. Endothermic reactions absorb heat.

Exothermic and Exothermic and EndothermicEndothermic

ThermochemistryThermochemistry is concerned with the is concerned with the flow of heatflow of heat between a between a chemical system chemical system (reaction) and its surroundings(reaction) and its surroundings..

A A systemsystem is the specific part of the is the specific part of the universe on which you focus your universe on which you focus your attention.attention.

The The surroundingssurroundings include everything include everything outside the system.outside the system.

The The systemsystem and the and the surroundingssurroundings constitute the constitute the universeuniverse..

Exothermic and Exothermic and EndothermicEndothermic

In thermochemical calculations In thermochemical calculations the direction of the the direction of the heat flowheat flow is is given from the point of view of given from the point of view of the the systemsystem..

A process that A process that absorbsabsorbs heat heat from the surroundings is called from the surroundings is called an an endothermic processendothermic process..

A process that A process that losesloses heat to the heat to the surroundings is called an surroundings is called an exothermic processexothermic process..

Units of HeatUnits of Heat A A caloriecalorie is the quantity of is the quantity of

heat that raises the heat that raises the temperature of temperature of 1 gram1 gram of of pure water pure water 11ooCC..

A A CalorieCalorie, or dietary Calorie, , or dietary Calorie, is equal to is equal to 1000 calories1000 calories..

A A JouleJoule is the SI unit of is the SI unit of heatheat and and energyenergy..

1 Calorie = 1000 cal = 1 kcal 1 Calorie = 1000 cal = 1 kcal = 4184 J= 4184 J

1 cal = 4.184 J1 cal = 4.184 J

Energy ConversionsEnergy ConversionsMake the following conversions.Make the following conversions.

444 calories to Joules444 calories to Joules

850 Joules to calories850 Joules to calories

444 cal x 444 cal x 4.184 J4.184 J = = 1857.7 J 1857.7 J 1 cal1 cal

850 J x 850 J x 1 cal 1 cal = 203.2 cal= 203.2 cal 4.184 J4.184 J

Heat CapacityHeat Capacity

The The heat capacityheat capacity of an object is the amount of an object is the amount of of heatheat it takes to change an object’s it takes to change an object’s temperature by exactly temperature by exactly 11ooCC..

The greater the The greater the massmass of an object, the of an object, the greater the greater the heat capacityheat capacity..

The The heat capacityheat capacity of an object also depends of an object also depends on its on its chemical compositionchemical composition..

Specific HeatSpecific Heat The The specific heat capacityspecific heat capacity

of a substance is the of a substance is the amount of amount of heatheat it takes to it takes to raise the temperature of raise the temperature of 1 1 gramgram of the substance of the substance 11ooCC..

Specific heat is represented Specific heat is represented by by CC..

The The unitsunits of specific heat of specific heat are are J/gJ/gooCC..

WaterWater has a has a higherhigher specific specific heat than most substances.heat than most substances.

HeatHeat

Heat = mass x specific heat x change in Heat = mass x specific heat x change in temptemp

q = mq = m..CC..TT

Mass is in gramsMass is in gramsSpecific heat is in J/gSpecific heat is in J/gooCCChange in temp is in Change in temp is in ooCC

Section 17.1 ReviewSection 17.1 Review1.1. In what direction does heat flow between In what direction does heat flow between

two objects?two objects?

2.2. How do endothermic processes differ How do endothermic processes differ from exothermic processes?from exothermic processes?

3.3. On what factors does the heat capacity On what factors does the heat capacity of an object depend?of an object depend?

4.4. Using calories, calculate how much heat Using calories, calculate how much heat 32.0g of water absorbs when it is heated 32.0g of water absorbs when it is heated from 25from 25ooC to 80C to 80ooC. How many joules is C. How many joules is this?this?q = 32g (4.184 J/goC) (55oC) = 7363.8J

7363.8J x 1 cal = 1759.9 cal 4.184 J

Section 17.1 ReviewSection 17.1 Review

4. How many kilojoules of heat are 4. How many kilojoules of heat are absorbed when 1000g of water is absorbed when 1000g of water is heated from 18heated from 18ooC to 85C to 85ooC?C?

q = 1000g (4.184J/goC) (67oC) = 280328J

280328J x 1kJ = 280.328kJ 1000 J

Section 17.2 – Measuring and Section 17.2 – Measuring and Expressing Enthalpy ChangesExpressing Enthalpy Changes

CalorimetryCalorimetry is the accurate and is the accurate and precise measurement of heat change precise measurement of heat change for for chemical and physical processeschemical and physical processes..

CalorimetersCalorimeters are devices used to are devices used to measure the amount of measure the amount of heat absorbed heat absorbed or releasedor released during chemical and during chemical and physical processes.physical processes.

EnthalpyEnthalpy is the is the heat contentheat content of a of a system at system at constant pressureconstant pressure..

Enthalpy is represented by Enthalpy is represented by HH..

CalorimeterCalorimeterq = q = H = m H = m . . C C . . TT

Heat Change Sign Heat Change Sign ConventionConvention

Direction of Direction of Heat FlowHeat Flow SignSign Reaction TypeReaction Type

Heat Flows Heat Flows Out of the Out of the

SystemSystem--HH ExothermicExothermic

Heat Flows Heat Flows Into the Into the SystemSystem

++HH EndothermicEndothermic

Thermochemical EquationsThermochemical Equations

An An equationequation that included the that included the heat heat changechange is a is a thermochemical equationthermochemical equation..

A A heat of reactionheat of reaction is the heat change for is the heat change for the equation the equation exactly as writtenexactly as written..

Ex:Ex:

CaOCaO(s)(s) + H + H22OO(l)(l) Ca(OH) Ca(OH)2(s)2(s) H = -65.2 KJH = -65.2 KJ

2NaHCO2NaHCO3(s)3(s) Na Na22COCO3(s)3(s) +H +H22OO(g)(g) +CO +CO2(g) 2(g) H = +129 KJH = +129 KJ

Section 17.2 ReviewSection 17.2 Review

1.1. When 2 mol of solid magnesium When 2 mol of solid magnesium combines with 1 mol of oxygen gas, combines with 1 mol of oxygen gas, 2 mol of solid magnesium oxide is 2 mol of solid magnesium oxide is formed and 1204kJ of heat is formed and 1204kJ of heat is release. Write the thermochemical release. Write the thermochemical equation for this combustion equation for this combustion reaction.reaction.

2Mg(s) + O2(g) 2MgO H = -1204kJ

Section 17.2 ReviewSection 17.2 Review

2. How much heat is released when 2. How much heat is released when 12.5g of ethanol burns?12.5g of ethanol burns?CC22HH55OH + 3OOH + 3O22 2CO 2CO22 + 3H + 3H22O O H = -H = -

1368kJ1368kJ

12.5g x 1mol x -1368kJ = -371.74kJ 46g 1 mol

Section 17.3 – Heat in Changes Section 17.3 – Heat in Changes of Stateof State

The The heat of combustionheat of combustion is the heat of reaction is the heat of reaction for the for the complete burning complete burning of one mole of a of one mole of a substancesubstance..

HH

The The heat absorbedheat absorbed by one mole of a by one mole of a substance substance melting from a solid to a melting from a solid to a liquidliquid at constant temperature is the at constant temperature is the molar heat of fusionmolar heat of fusion..

The The heat lostheat lost when one mole of a when one mole of a liquid changes to a solidliquid changes to a solid at a at a constant temperature is the constant temperature is the molar molar heat of solidificationheat of solidification..

HHfusfus = - = - HHsolidsolid

HH

The The heat absorbedheat absorbed by one mole of a by one mole of a substance changing from substance changing from a liquid to a a liquid to a vaporvapor is the is the molar heat of molar heat of vaporizationvaporization..

The The heat releasedheat released by one mole of a by one mole of a substance changing from substance changing from a vapor to a a vapor to a liquidliquid is the is the molar heat of molar heat of condensationcondensation..

HHvapvap = - = - HHcondcond

HH

The The heat changeheat change caused by caused by dissolutiondissolution of of one mole of a substance is the one mole of a substance is the molar heat molar heat of solutionof solution..

Ex.Ex.

NaOHNaOH(s)(s) Na Na++(aq)(aq) + OH + OH--

(aq)(aq)

HHsolnsoln = -445.1 KJ = -445.1 KJ

Heating Curve for WaterHeating Curve for Water

Section 17.3 ReviewSection 17.3 Review1.1. How does the molar heat of fusion How does the molar heat of fusion

of a substance compare to its molar of a substance compare to its molar heat of solidification?heat of solidification?

2.2. How does the molar heat of How does the molar heat of vaporization of a substance vaporization of a substance compare to its molar heat of compare to its molar heat of condensation?condensation?

Section 17.3 ReviewSection 17.3 Review

3. Identify each enthalpy change by 3. Identify each enthalpy change by name and classify each change as name and classify each change as exothermic or endothermic.exothermic or endothermic.

a.a. 1 mol C1 mol C33HH8(l)8(l) 1 mol C 1 mol C33HH8(g)8(g)

b.b. 1 mol Hg1 mol Hg(l)(l) 1 mol Hg 1 mol Hg(s)(s)

c.c. 1 mol NH1 mol NH3(g)3(g) 1 mol NH 1 mol NH3(l)3(l)

d.d. 1 mol NaCl1 mol NaCl(s)(s) + 3.88kJ/mol + 3.88kJ/mol 1 mol 1 mol NaClNaCl(aq)(aq)

e.e. 1 mol NaCl1 mol NaCl(s)(s) 1 mol NaCl 1 mol NaCl(l)(l)

Section 17.4 – Calculating Section 17.4 – Calculating Heats of ReactionHeats of Reaction

Hess’ Law of heat summationHess’ Law of heat summation states that states that if you add two or more if you add two or more thermochemical thermochemical equationsequations to give a final equation, then to give a final equation, then you can also add the you can also add the heat changesheat changes to give to give the the final heat changefinal heat change..

HH

The The standard heat of formationstandard heat of formation of a of a compound is the change in enthalpy that compound is the change in enthalpy that accompanies the accompanies the formation of one moleformation of one mole of of the compound from its element with all the compound from its element with all substances in their substances in their standard states at 25standard states at 25ooCC..

The The HHffoo of a of a free elementfree element in its standard in its standard

state is state is zerozero..

HHoo = = HHffoo (products) – (products) – HHff

oo (reactants) (reactants)

Section 17.4 ReviewSection 17.4 Review

1.1. Calculate the enthalpy change in kJ Calculate the enthalpy change in kJ for the following reaction.for the following reaction.

2Al + Fe2Al + Fe22OO33 2Fe + Al 2Fe + Al22OO33

Use the enthalpy changes for the Use the enthalpy changes for the combustion of aluminum and iron:combustion of aluminum and iron:

2Al + 3/2O2Al + 3/2O22 Al Al22OO33 H = -1669.8kJH = -1669.8kJ

2Fe + 3/2O2Fe + 3/2O22 Fe Fe22OO33 H = -824.2kJH = -824.2kJ

Section 17.4 ReviewSection 17.4 Review

2. What is the standard heat of 2. What is the standard heat of reaction for the decomposition of reaction for the decomposition of hydrogen peroxide?hydrogen peroxide?

2H2H22OO2(l)2(l) 2H 2H22OO(l)(l) + O + O2(g)2(g)

THE ENDTHE END