电化学与热力学Electrochemistry and Thermodynamics.docx

电化学与热力学Electrochemistry and Thermodynamics.docx

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电化学与热力学ElectrochemistryandThermodynamics

E10

ElectrochemistryandThermodynamics

E10ElectrochemistryandThermodynamics

INTRODUCTION

Inthisexperimentyouwillstudytheelectrochemistryofvoltaic（galvanic）cellsandintheprocessdeterminethevaluesofthethermodynamicfunctions∆G,∆H,and∆S.Youwillconstructelectrochemicalcellsbycombiningdifferentmetallicsystemsandtheirsolutions.Measuringthepotentialofthepreparedcellsatvarioustemperatureswillrenderthevaluesofthethermodynamicfunctions∆G,∆H,and∆Scorrespondingtotheelectrochemicalsystemstudied.

THEORETICALBACKGROUND

Electrochemistry:

Electrochemistryistheareaofchemistrythatdealswiththerelationbetweenchemicalchangesandelectricalenergy.Chemicalreactionscanbeusedtoproduceelectricalenergyinvoltaic（galvanic）cells.Electricalenergy,ontheotherhandcanbeusedtobringaboutchemicalchangesinwhataretermedelectrolyticcells.Inthisexperimentyouwillinvestigatesomeofthepropertiesofvoltaiccells.

Oxidation-reductionreactionsarethosethatinvolvethetransferofelectronsfromonesubstancetoanother.Aredoxreactionisthesimultaneousoccurrenceofthetwocomponentsorhalfreactions.Oxidationoccurswhenachemicalspecieslosesorgivesupelectronstoanotherchemicalspecies.Reductionoccurswhenachemicalspeciesreceivesorgainelectrons.Theoxidationprocessprovidestheelectronsnecessaryforreductiontooccur.Therefore,theoxidizedspeciesisthereducingagent.,andthereducedspeciesistheoxidizingagent.

Forexample,ifapieceofzincmetalisimmersedintoasolutioncontainingcopper（II）ions,zincwillbeoxidizedbythecopper（II）ions.Zincloseselectronsanditisoxidizedwhilecopper（II）ionsgainelectronsandarereduced.Wecanconvenientlyexpressthesetwoprocessesbythefollowingtwohalf-reactions,whichaddtogivetheoverallredoxreaction.

Thefunctionofavoltaiccellisbaseduponreactionssimilartotheoneillustratedinequation

（1）.ThesystemZn/CuiscalledtheDaniellcellinhonorofJohnFrederickDaniell（1790-1845）whodevelopedthecellforthefirsttime.Asaltbridgemustbeusedtoavoidpolarizationoftheelectrodesbyfacilitatingthecirculationofionsfrombothcellcompartments.（seefigure1）.

Figure1ElectrochemicalCell

Thecellvoltage,ortheelectromotiveforce（abbreviatedemf）,isindicatedonthevoltmeterinvolts.Thecellemfisalsocalledthecellpotential.Themagnitudeoftheemfisaquantitativemeasureofthedrivingforceorthermodynamictendencyforthereactiontooccur.Ingeneral,theemfofavoltaiccelldependsuponthesubstancesthatmakeupthecellaswellasontheirconcentrations.Hence,itiscommonpracticetocomparestandardcellpotentials,symbolizedbyEocell.Thesepotentialscorrespondtocellvoltagesunderstandardstateconditions-gasesat1atmpressure,solutionsat1Mconcentration,andtemperatureat25oC.

Justastheoverallcellreactionmayberegardedasthesumoftwohalf-reactionstheoverallcellemfcanbethoughtofasthesumoftwohalf-cellpotentials,thatis,thesumofthevoltageoftheoxidationhalf-reaction（Eo（ox））andthevoltageofthereductionhalf-reaction（Eo（red））

（2）

Becauseitisimpossibletomeasuredirectlythepotentialofanisolatedhalf-cell,thestandardhydrogenhalf-reactionhasbeenselectedasareferenceandhasbeenassignedastandardreductionpotentialofexactly0.000V;

（3）

Let’slookagainattheDaniellcelldescribedabove.Thestandardhalf-cellpotentialsare:

Therefore,

（4）

Thecompletenotationschemeforagalvaniccelliswrittenwiththeanodeontheleftandadoubleverticallinedenotingthepresenceofaporousplateorsaltbridge.FortheDaniellcell:

Zn|Zn2+||Cu2+|Cu.

Eachelectrodeisconnectedtothevoltmeterbyalligatorclipsandmetalwiring.Thevoltmetermeasuresthevoltagegeneratedbytheredoxreaction.Thevoltagereadingwillbepositivewhentheelectrodesareconnectedproperlyforspontaneousreaction.Aspontaneousredoxreactionoccurswhenthespecieswithhigherreductionpotentialisconnectedasthecathode.Otherwise,thevoltagereadingwillbenegative.Themeterreadingwillbepositivewhenthecathodeisconnectedtothe（+）outletandtheanodeisconnectedtothe（-）outlet.Physically,anegativevoltagereadingmeansthatyouhaveconnectedthewrongelectrodeascathode.Thisisequivalenttoreversingequation

（1）.Whenequation

（1）isreversed,themeasuredcellpotentialdifferencebecomes

Eocell=-1.10V.Theabsolutevalueof∆Eocellisthesameinbothcases,butthesignisdifferent.Thesignof∆Eocellispositiveforaspontaneousreactionandnegativefornon-spontaneousreaction.

TheNernstEquation:

Formeasurementstakenunderstandardconditions（1atm,1Msolutions）,∆Eocellmeasurestheelectricpotentialdifferencebetweenthehalf-cells.Formeasurementstakenundernon-standardconditions（theusuallaboratorysituation）,theNernstequationisusedtocalculate∆Ecell.TheNernstequationgivesustherelationshipbetweentheoverallcellpotentialdifferenceforaredoxreaction∆Ecellandtheconcentrationsofthemetal-ionsolutions.TheNernstequationis;

（5）

whereFisFaradayconstant.Rtheuniversalgasconstant,nthenumberofelectronstransferred,andQisthereactionquotient.Whennatural（ln）isconvertedtobaseten（log）and（RT/nF）areevaluatedusingR=8.315JK-1mol-1,T=295.15K,andF=96,485Cmol-1theequationbecomes;

（6）

Inthisform,galvaniccellsareusedtodeterminetheconcentrationofthemetalionspresentunderconditionsotherthanstandardconditions.IfweapplytheNernstequationtoequation

（1）wewillget

（7）

Thermodynamics:

Morethanacenturyago,scientistsobservedtheeffectoftemperatureonnaturalprocessesandformulatedthelawsofthermodynamics.Thermodynamicsisthestudyoftheflowofenergy,especiallyintheformofheat,anditsconversionfromoneformtoanother（forexample,fromchemicalenergytoheatenergy）.Thelawsofthermodynamicsareasetofmathematicalfunctions,whichgovernallformsofenergyandtheirinter-conversion.

Thezerothlawofthermodynamicsisconcernedwithtemperatureequilibrium;itstatesthatheatalwaysflowsfromahottoacoldbody.

Thefirstlawofthermodynamicsdescribestheconservationofenergy.Itstatesthatenergycannotbecreatedordestroyed;itcanonlybeconvertedfromoneformtoanother.

Thesecondlawofthermodynamicsisconcernedwiththereversibilityanddirectionofnaturalevents;itstatesthattheentropyoftheuniversetendstoincrease.Therefore,allspontaneouseventsareaccompaniedbyanincreaseintheentropyoftheuniverse.Entropy（S）isameasureofdisorder.Thehighertheentropyofasystem,thelowertheorderofthatsystem.Forexample,theentropyofheliumsealedinsideaballoonislowerthanitsentropywhenisreleasedtoopenspace.Whenintheballoon,theindividualheliumatomsareconfinedtothespacewithintheballoon.Whenthegasisreleasedtheatomsrushoutintoalessorderedstate.

TheThirdlawdefinesthezeroofentropyforasystem.Itstatesthatpuresubstancesatabsolutezero（0Kelvin）,thetemperatureatwhichallatomicmotionceases,haveentropyofzero.

Whentheselawsareappliedtochemicalreactions,theyrelatetheequilibriumcondition（whetherthereactionisproceedingintheforwardorreversedirection）andthetemperatureofthereactiontothethermodynamicfunctions,namelychangeinenthalpy（∆H）,changeinentropy（∆S）,andchangeinfreeenergy（∆G）.Bystudyingtheeffectoftemperatureontheequilibriumconditionforachemicalreaction,wecancalculatethesethermodynamicfunctions.

Byexamining∆H,∆Sand∆Ginachemicalsystem,awealthofinformationmaybeextracted.Inthermodynamicsasystemistheparticularpartoftheuniversebeingstudied.

Enthalpydescribeschangesofheatinasystem.Ifthesystemabsorbsheatduringagivenprocess,thisprocessissaidtobeendothermic,andthechangeinenthalpy（∆H）isgreaterthanzero（H>0）.Onthecontrary,ifthesystemreleasesheat,theprocessiscalledexothermic,and（∆H<0）.

Asmentionedabove,entropyisthemeasureofdisorderorrandomnessinasystem.Thegreatertheorderinasystem,thelowertheentropy.Thelowertheorderinasystem,thehighertheentropy.

TheGibbsFreeEnergy（G）isthefunctionderivedfromHandS.Thechangeinfreeenergy（∆G）isacompositefunctionthatgivesameasureofthespontaneityofareaction.If∆G<0,areactionwilloccurspontaneously.If∆G>0,thereactionisnotspontaneous.If∆G=0thesystemisatequilibrium.Themathematicalformof∆Gisthefollowing:

（8）

WhereTistheabsolutetemperature（inKelvin）.Theeffectofthesignof∆Hand∆Sandtheeffectoftemperatureonspontaneityofareactioncanbesummarizedasfollows:

∆Hsystem

∆Ssystem

Process

-

+

Spontaneousatalltemperatures

+

-

Non-spontaneousatalltemperatures

+

+

Spontaneousathightemperatures

-

-

Spontaneousatlowtemperatures

ThermodynamicsandEquilibrium:

ForageneralreactionorprocessofAandBgivingproductsCandD:

thereactionquotient,Q,is:

（9）

The∆GofaprocessatanyconcentrationofA,B,C,andD,isgivenby:

（10）

Where∆Goisthefreeenergyofthereactionunderthedefinedstandardconditionsof1atmand1Mconcentrationsofreactants.∆Gistheobservedchangeinfreeenergyatconditionsotherthanstandard.AtequilibriumQ=Keq.Thereactionfavorsneitherproductsnorreactantsatequilibrium,a