EnergyandThermochemistry Lesson3.1

Introduction

Everythingwedoisconnectedinonewayoranotherwithenergy.Notonlyourmodernsocietybutlifeitselfdependsonenergyforitsexistence.Theissuessurroundingenergy–itssources,production,distribution,andconsumption–pervadeconversationsinscience,politics,andeconomics,andrelatetoenvironmentalconcernsandpublicpolicy.Think:Listonecurrenteventtopicsthatyoumightunderstandbetterifyouhadabetterunderstandingofenergy.WiththeexceptionofenergyfromtheSun,mostoftheenergyusedinourdailylivescomesfromchemicalreactions.Thecombustionofgasoline,theproductionofelectricityfromcoal,theheatingofhomesbynaturalgas,andtheuseofbatteriestopowerelectronicdevicesareallexamplesofhowchemistryisusedtoproduceenergy.Evensolarcellsrelyonchemistrytoproducethesiliconandothermaterialsthatconvertsolarenergydirectlytoelectricity.Inaddition,chemicalreactionsprovidetheenergythatsustainslivingsystems.Plantsusesolarenergytocarryoutphotosynthesis,allowingthemtogrow.Theplantsinturnprovidefoodfromwhichwehumansderivetheenergyneededtomove,maintainbodytemperature,andcarryoutallotherbodilyfunctions.Listthreewaysweuseenergyinourdailylives:Think:Itisevidentthatthetopicofenergyisintimatelyrelatedtochemistry.Whatexactlyisenergy,though,andwhatprinciplesareinvolvedinitsproduction,consumption,andtransformationfromoneformtoanother?Writedownyourthoughtsaboutthisquestionnow.Thermodynamicsdefinition:Energy:Work:Heat:

EnergyandThermochemistry Lesson3.1

Howcanenergybetransferredfromoneobjecttoanother:KineticEnergyPotentialEnergy

ElectrostaticPotentialEnergyandCoulomb’sLaw

EnergyandThermochemistry Lesson3.1

ConservationofEnergyUnitsofEnergySystemandSurroundings

Twowaystotransferenergy:WorkandHeatWork:Heat:

EnergyandThermochemistry Lesson3.1

FirstLawofThermodynamicsInternalEnergyRelatingΔEtoHeatandWorkSignconventionsSampleProblem:GasesA(g)andB(g)areconfinedinacylinderandpistonarrangementlikethatinFigure5.4andreacttoformasolidproductC(s):A(g)+B(g)èC(g).Asthereactionoccurs,thesystemloses1150Jofheattothesurroundings.Thepistonmovesdownwardasthegasesreacttoformasolid.Asthevolumeofthegasdecreasesundertheconstantpressureoftheatmosphere,thesurroundingsdo480Jofworkonthesystem.Whatisthechangeintheinternalenergyofthesystem?

EnergyandThermochemistry Lesson3.1

EndothermicandExothermicProcessesEndothermicExothermic

EnergyandThermochemistry Lesson3.1

StateFunctionsEnthalpydefinition:Pressure-VolumeWork

EnergyandThermochemistry Lesson3.1

SampleProblem:Afuelisburnedinacylinderwithapiston.Theinitialvolumeofthecylinderis0.250Landthefinalvolumeis0.980L.Ifthepistonexpandsagainstaconstantpressureof1.35atm,howmuchwork(inJ)isdone?(1L-atm=101.3J)

EnergyandThermochemistry Lesson3.1

EnthalpyChangeSampleProblemIndicatethesignoftheenthalpychange,ΔH,inthefollowingprocessescarriedoutunderatmosphericpressureandindicatewhethereachprocessisendothermicorexothermic.(a)Anicecubemelts;(b)1gofbutane(C4H10)iscombustedinsufficientoxygentogivecompletecombustiontoCO2andH2O.