Ch.6:Thermochemistry

Energy

TheNatureofEnergy•  Energy(E)isthecapacitytodoworkortoproduceheat– Workisdefinedasforceactingoveradistance

•  Inthischapter,wewillspecificallyfocusontheheattransferthataccompanieschemicalprocesses

•  Don’tforget:potentialvs.kineticenergy

TheNatureofEnergy•  LawofConservationofEnergy

– EnergyCANNOTbecreatedordestroyed– EnergyCANbeconvertedfromoneformtoanother

•  FirstLawofThermodynamics– Appliesthisprincipletosystemswherewe’relookingatheattransfer

TheProcessofHeat

TheProcessofHeat•  Heat(q)isatransferofenergybetweentwoobjectsduetoatemperaturedifference.–  Etransfersfromwarmertocoolerobject

•  Recall:Temperature(T)isproportionaltotheaverageKEofthemolecules

TheProcessofHeat•  Whentwoobjectsatdifferenttemperaturesareincontact,theywilleventuallyreachthesameintermediatetemperature.– Ex/Aglassofcoldwaterandacupofhotteaareleftinaroom

Types of Heat Transfer •  Conduction•  Convention•  Radiation

EnergyandWork

Energy •  Whatisenergy?•  Energy=capacityfordoingworkorsupplying

heat–  weightless,odorless,tasteless

•  Gasolinecontainsasignificantamountofchemicalpotentialenergy

EnergyandWorkΔE=q+w

– E=energy– q=heat– w=work

•  Heatis(+)ifitisabsorbed,(-)ifitisreleased•  Workis(+)ifdoneonthesystem(compression),(-)ifitisdonebythesystem(expansion)

EnergyandWork•  ΔE=q+w•  IfΔE>0,thesystemisgainingenergy(workisdoneonthesystemorheatistransferredtothesystem)

•  IfΔE<0,thesystemisreleasingenergy(workisdonebythesystemorheatistransferredtothesurroundings)

•  Let’stryusingthisequation!•  Ex/CalculateΔEforasystemundergoinganendothermicprocessinwhich15.6kJofheatflowsandwhere1.4kJofworkisdoneonthesystem.

EnergyandWork•  Recall:Workistheenergyrequiredtomovesomethingagainstaforce.Pressureisdefinedasforceperunitofarea

•  w=-PΔV– w=work– P=pressure– V=volume

•  Ex/Calculatetheworkassociatedwiththeexpansionofagasfrom46Lto64Lataconstantexternalpressureof15atm

TermstoKnow

Terms to Know •  Thermochemistry=studyofenergyand

heatassociatedwithchemicalrxns

TermstoKnow•  System

– Theareaoftheuniversethatwearefocusingon

– Forexample,theexperiment

•  Surroundings– Everythingoutsideofthesystem

TermstoKnow•  StateFunction

–  Propertywhosevaluedoesnotdependonthepathtakentoreachthatspecificvalue

–  Ex/Changeinenergy

TermstoKnow•  StandardConditions

– NottobeconfusedwithSTP– Standardconditionsare1.0M,1atm,and25oC

– IfyouseeGo,Ho,orSo,theseareatstandardconditions

– Thismeansyoualreadyknowpressure,temperature,andconcentration

TermstoKnow•  Exothermic

– Netreleaseofenergy(heatexchange)bythesystem–  Energyisaproduct–  Ex/Burningmethanegasinthelabburnerproducesheat–  Ex/Lightsticksgiveofflight–  -ΔH

TermstoKnow•  Endothermic

– Netabsorptionofenergy(heatexchange)bythesystem–  Energyisareactant–  Ex/Bakingsodaandvinegarmixedturnscold–  +ΔH

Exothermic and Endothermic •  Everyreactionhasanenergychange

associatedwithit•  Exothermicreactionsreleaseenergy,

usuallyintheformofheat.•  Endothermicreactionsabsorbenergy•  Energyisstoredinbondsbetween

atoms

TermstoKnow•  Entropy(S)

– Measureofthedispersalofmatterandenergy•  Naturetendsfromordertodisorder

– Increaseindispersal/disorder+ΔS– Decreaseindispersal/disorder-ΔS

TermstoKnow•  GibbsFreeEnergy(G)

– Criteriafordeterminingthermodynamicfavorabilityandcalculatingthetheoreticalamountofenergytodowork

Enthalpy

Pair-Share-Respond1.  Define“energy”and“heat”.2.  Identifyanddescribethreewaysthatheatcanbetransferred

3.  Whatisthefirstlawofthermodynamics?

4.  Whatisastatefunction,andwhatisanexampleofastatefunction?

5.  Distinguishbetweenexothermicandendothermicprocesses

Enthalpy•  Enthalpy(H)=Flowofenergy(heatexchange)atconstantpressurewhentwosystemsareincontact– Wemeasurethechangeinenthalpy,orthedifferencebetweenthepotentialenergiesoftheproductsandthereactants

Enthalpy•  ΔHisastatefunction.•  ΔH=qatconstantpressure(i.e.atmosphericpressure)

•  Wecancalculateenthalpymanyways– Stoichiometry– Calorimetry– Tablesofstandardvalues– Hess’sLaw– Bondenergies(weknowthisway!)

Enthalpy•  Stoichiometry•  Ex/Uponaddingsolidpotassiumhydroxidepelletstowater,thefollowingreactiontakesplace:

KOH(s)àKOH(aq)+43kJ/mol•  Answerthefollowingquestionsregardingtheadditionof14.0gofKOHtowater:– Doesthebeakergetwarmerorcolder?–  Isthereactionendothermicorexothermic?– Whatistheenthalpychangeforthedissolutionofthe14.0gramsofKOH?

Enthalpy•  Enthalpyofreaction(ΔHrxn)=amountofheatreleased(negativevalues)orabsorbed(positivevalues)byachemicalrxnatconstantpressureknkJ/mol

Enthalpy•  Enthalpyofcombustion(ΔHcomb)=heatabsorbedorreleasedbyburning(usuallywithO2)inkJ/mol;notethatcombustionreactionsyieldoxidesofthatwhichiscombusted

Enthalpy•  Enthalpyofformation(ΔHf)=HeatabsorbedorreleasedwhenONEmoleofcompoundisformedfromelementsintheirstandardstatesinkJ/mol

Enthalpy•  Enthalpyoffusion(ΔHfus)=heatasborbedtomelt(overcomeIMFs)1moleofsolidtoliquidatMPexpressedinkJ/mol

Enthalpy•  Enthalpyofvaporization(ΔHvap)=heatabsorbedtovaporizeorboil(overcomeIMFs)1moleliquidtovaporattheBPinkJ/mol

UnitsforMeasuringHeatFlow

Units for Heat Flow Whataresomeunitswecanusetomeasureheat/energy?•  Calorie•  Kilocalorie•  Joule(SIUnit)

Units for Heat Flow •  Calorie=quantityofheatneededtoraisethetemperatureof1gofpurewaterby1oC–  Usedexceptwhenreferringtofood–  aCalorie,(writtenwithacapitalC),always

referstotheenergyinfood–  1Calorie=1kilocalorie=1000cal.

Units for Heat Flow •  Joule=SIunit

– namedafterJamesPrescottJoule– 4.184J=1cal

HeatCapacity&SpecificHeat

Heat Capacity & Specific Heat • HeatCapacity=amountofheatneededtoincreasethetempofanobjectexactly1oC– Dependsonboththeobject’smassanditschemicalcomposition

Heat Capacity & Specific Heat •  SpecificHeat(C)=amtofheatneededtoraisethetempof1gofasubstanceby1oC– UnitsareusuallyJ/(goC)

• WaterhasaHUGEvalue,whenitiscomparedtootherchemicalsduetohydrogenbonding

Table of Specific Heats Note the tremendous difference in

Specific Heat.

Water’s value is

VERY HIGH.

Heat Capacity & Specific Heat •  Forwater,C=4.18J/(goC)inJoules,andC=1.00cal/(goC)incalories.

•  Thus,forwater:– ittakesalongtimetoheatup,and– ittakesalongtimetocooloff!

• Waterisusedasacoolant!

HeatCapacity(C)•  Molarheatcapacity:EnergyrequiredtoraisethetemperatureofonemoleofasubstancebyonedegreeCelsius– Units-J/°C·molorJ/K·mol

•  Heatcapacitiesofmetalsaredifferentfromthatofwater–  Ittakeslessenergytochangethetemperatureofagramofametalby1°Cthanforagramofwater

Heat Capacity & Specific Heat •  Ausefulformula:

q=mxΔTxC– m=massingrams– ΔT=changeintemperature– C=SpecificHeat

• Unitsareeither:J/(goC)orcal/(goC)

Example•  Thetemperatureofa95.4gpieceofcopperincreasesfrom25.0oCto48.0oCwhenthecopperabsorbs849Jofheat.Whatisthespecificheatofcopper?

Calorimetry

Calorimetry •  A burning match releases heat to its

surroundings in all directions. •  Is this exothermic or endothermic?

Calorimetry •  Can we determine the amount of heat

given off?

Calorimetry•  Calorimetryistheprocessofmeasuringheatbasedonobservingthetemperaturechangewhenabodyabsorbsordischargesenergyasheat•  Based on the fact that the heat released = the heat

absorbed •  Acalorimeterisfilledwithwaterandtheinitialtemperatureisrecorded

•  The mass of the sample and water is taken •  The substance is heated, and the heat should be

transferred to the water •  The final temperature of the water and the mass

burned is recorded •  This data can be used to calculate the heat released

and the energy content

Calorimetry •  Calorimeters =

used to measure the absorption or release of heat – Ex/ Foam Cups

Coffee-CupCalorimeter

•  ContainstwonestedStyrofoamcupswithacoverthroughwhichastirrerandthermometercanbeinserted– Outercupisusedtoprovideextrainsulation

–  Innercupholdsthesolutioninwhichthereactionoccurs

Calorimetry •  Calorimeters are

used to measure the absorption or release of heat – Ex/ Soda Cans

Example •  Recall:

q=mxΔTxC• Wecanalsoexpressthisintermsofspecificheat:

C=q/mxΔT

Example • When100.0mLofawatersolutionisheatedfrom22.5oCto26.0oC,heatisgivenoff.Calculatetheheatgivenoff.

•  (Recall–thespecificheatofwateris4.18J/goC,andthedensityis1.00g/mL)

Example •  Howmuchheatisneededtowarm250gofwaterfrom22oCto98oC?

SampleQuestion•  Whichofthefollowingstatementsistrue?

a.  Changeinenthalpyisastatefunctionb.  Inexothermicreactions,thereactantsarelowerin

potentialenergythantheproductsc.  Achemisttakesthesurroundingspointofviewwhen

determiningthesignforworkorheatd.  Heatofreactionandchangeinenthalpycanalwaysbe

usedinterchangeably

SampleQuestion•  Twodifferentmetalsofequalmasswithdifferentheatcapacitiesaresubjectedtothesameamountofheat– Whichundergoesthesmallestchangeintemperature?

a.  Themetalwiththehigherheatcapacityshowsthesmallestchangeintemperature

b.  Themetalwiththelowerheatcapacityshowsthesmallestchangeintemperature

c.  Becausetheyhaveequalmass,bothmetalsundergothesamechangeintemperature.distilledmixture

SampleQuestion•  A50.0-gsampleofwaterat80°Cisaddedtoa50.0-gsampleofwaterat20°C– Thefinaltemperatureofthewatershouldbe:

a.  between20°Cand50°Cb.  50°Cc.  between50°Cand80°C

SampleQuestion•  A50.0-gsampleofwaterat80°Cisaddedtoa100.0-gsampleofwaterat20°C– Thefinaltemperatureofthewatershouldbe:

a.  between20°Cand50°Cb.  50°Cc.  between50°Cand80°C