蘇教版選修六專題5 電化學(xué)問題研究課題1 原電池（區(qū)一等獎）

DeterminationofPrimaryCell'sElectromotiveForce原電池電動勢的測定ContentsPurposesandDemandsPrinciple

ApparatusandReagent

Procedure

DataRecordsandProcessing

QuestionAttentions

DemonstratedPurposesandDemandsDetermineelectromotiveforceofCu-ZncellandpotentialforbothelectrodeswithCuandZn.Understandwhat’sthemeaningofreversiblecell,reversibleelectrodesandsaltbridge.Learntoprepareelectrodesandsomeotherdealingmethods.Commandtheprincipleandtechniqueofthemethodwhichusingpotentialtodetermineelectromotiveforce.

potentiometerPrincipleAcelliscomposedofpositiveandnegativeelectrodes.Inthedischargeprocess,deoxidizereactiontakesplaceinpositiveelectrodeandoxidationreactionoccursinanotherelectrode.

Besideconsideredaselectricitysource,acellisalsousedtodoresearchesaboutitspropertiesofthermodynamics.Thermochemistrysaid,providedcertaintemperature,pressureandreversiblestate,thereisanequationfromthereactionthatisexpressed:△G=-nFE(1)Where△Gisthefreeenergychange,FistheFaradayconstant(F=96500C),nisthemolarofelectronswhichtransferthroughtheelectrochemistryreaction,andEistheelectromotiveforce.

Ewhichhasbeendeterminedwillbetakenintoaccountthefreeenergychange△Gandanyotherthermochemistryfunctionsintheexperiment.Itisimportantthattheconditionshouldbecontrolledinconstanttemperature,pressureandreversible.

Firstly,thereactionincellmustbereversible.Itistosaythatboththeprocessofcharginganddischargeoccurinanequilibriumstateandthatitallowshardlyanycurrentrunsthroughthecell.Makeupof

batteries：Zn│ZnS04(C1)‖CuS04(C2)│CuReactioninthenegativeelectrodeZn→Zn2+（аZn2+）+2еReactioninthepositiveelectrodeCu2+(аCu2+)+2е→CuThegeneralreactionisZn+Cu2+(аCu2+)→Zn2+(аZn2+)+CuFreeenergychange△G=△G°+RTlnаZn2+*аCu/аCu2+*аZn

(2)Activityofpuresolidis1.аZn=аCu=1(3)CombiningEq.(2)withEq.(3),weobtain△G=△G°+RTlnаZn2+/аCu2+

(4)MakinguseofEq.(1),wefindtherelationshipbetweenelectromotiveforceandactivityE=E°-RT/2F×lnаZn2+/аCu2+(5)WhereE°istheelectromotiveforcewhenbothactivityofcuprumandzincionareequalto1.Sincethegeneralreactioncontainstworeactionsintwoelectrodes,theelectromotiveforcecanbeexpressedasthepotentialdifferencebythetwoelectrodes.Givenpotentialofpositiveelectrodeisφ+andpotentialofnegativeelectrodeisφ-,thusitcanbestatedasthefollowingequation:E=φ+-φ-(6)ToCu-Znbattery,weobtaintwoequationsforφ+

andφ-φ+=φ°Cu2+-RT/2F×lnаCu/аCu2+(7)φ-=φ°Zn2+-RT/2F×lnаZn/аZn2+(8)At25℃,thetwoelectrodepotentialdeterminedinconditionwhichpresentstwoequilibriumprocessesCu-Cu2+andZn-Zn2+whenactivitybothofCu2+andZn2+areequalto1arecalledstandardpotential.ApparatusandReagentPotentiometer1Standardcell1Calomelelectrode1Zincelectrode1Beaker(50ml)1ZnSO4(0.1000M)CuSO4(0.01000M)CuSO4(0.1000M)KCl(saturated)Sheetcopper(usedforplating)Milliammeter(0－25－50)Procedure1.Preparationofelectrodes1)

ZincelectrodeItshouldbeburnishedwithsandpaperandsoakedindilutedvitriolinordertoremoveoxideonsurface.Thenitwaswashedcleanforstandby.(2)

CuprumelectrodeSoakthesheetcopperinsteadofdilutednitricacidandburnishit.Washandcleanitforstandby.

(3)

Assemblethebattery.Makeasaltbridge.Dissolve3gofagar-agarin20-30mlhotKClsaturated-solutionandimmediatelytransferittoUtubethenallowitstandforseveralmin.AssembletwoelectrodesandthesaltbridgetoformaCu-Znbattery.

2.DeterminationofelectromotiveforceConnecttheelectrocircuitwithinstrumentsofpotentiometerandgalvanometer.(2)AccordingtothefollowingtemperatureproofreadformulaEt/v=1.0186-4.06×10-5(t/℃-20)-9.5×10-7×(t/℃-20)2determineelectromotiveforceofstandardbatteryatroomtemperature.(3)Demarcateelectriccurrentofpotentiometer,baseonthevaluecalculatedabove.(4)Determineelectromotiveforceintermsofbatterybelow

Zn│ZnS04(0.1000M)‖CuS04(0.1000M)│Cu（A）

Cu│CuS04(0.1000M)‖CuS04(0.1000M)│Cu（B）

Zn│ZnS04(0.1000M)‖KCl(sat)│Hg2Cl2│Hg（C）

Hg│Hg2Cl2│KCl(sat)‖CuS04(0.1000M)│Cu（D）

Sketch-mapsofEquipmentsDataRecordsandProcessing1.CalculatepotentialofcalomelelectrodebythetemperatureproofreadformulawritteninattachingtableV-16atroomtemperature.

2.FigureoutacademicvalueofelectromotiveforceofcalomelbatterybyEq.(6),(7)and(8).3.Fillinthetablewithtruevaluesofelectromotiveforcethataredeterminedfromexperiment.battery（A）（B）Etrue

Eacademic

True

Academic

φZn2+

φCu2+

Questions1.Didvoltagecanbeusedtodetermineelectromotiveforce?2.Whydidagalvanometerpointstoonedirectionwhendetermineelectromotiveforce?AttentionsClickthebig-smallkeyrapidly,andstaynomoretime.Don’tmovethegalvanometerinthedeterminationprocess.Regulatesensibilityinthelowestswitchforstartthatturnfrom0.01Xto1XDeterminationofelectromotiveforce1.Connecttheelectrocircuit.AdjustthegalvanometerandU-25typepotentiometertozerovalue.Goonafter