无机化学英文版Chapter20

Copyright1999,PRENTICEHALL,Chapter20,1,Electrochemistry,Chapter20,Copyright1999,PRENTICEHALL,Chapter20,2,Oxidation-ReductionReactions,ZnaddedtoHClyields产量thespontaneous自发的reactionZn(s)+2H+(aq)Zn2+(aq)+H2(g).Theoxidation氧化numberofZnhasincreasedfrom0to2+.TheoxidationnumberofHhasreducedfrom1+to0.Therefore,ZnisoxidizedtoZn2+whileH+isreducedtoH2.H+causesZntobeoxidizedandistheoxidizingagent.ZncausesH+tobereducedandisthereducingagent.Notethatthereducingagentisoxidizedandtheoxidizingagentisreduced.,Copyright1999,PRENTICEHALL,Chapter20,3,BalancingOxidation-ReductionEquations,Lawofconservationofmass:theamountofeachelementpresentatthebeginningofthereactionmustbepresentattheend.Conservationofcharge:electronsarenotlostinachemicalreaction.Incomplicatedredox氧化还原作用reactions,weneedtolookatthetransferofelectronscarefully.Half-ReactionsHalf-reactionsareaconvenientwayofseparatingoxidationandreductionreactions.,Copyright1999,PRENTICEHALL,Chapter20,4,BalancingOxidation-ReductionEquations,Half-ReactionsThehalf-reactionsforSn2+(aq)+2Fe3+(aq)Sn4+(aq)+2Fe3+(aq)areSn2+(aq)Sn4+(aq)+2e-2Fe3+(aq)+2e-2Fe2+(aq)Oxidation:electronsareproducts.Reduction:electronsarereagents.,Copyright1999,PRENTICEHALL,Chapter20,5,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-ReactionsConsiderthetitrationofanacidicsolutionofNa2C2O4(sodiumoxalate,colorless)withKMnO4(deeppurple).MnO4-isreducedtoMn2+(palepink)whiletheC2O42-isoxidizedtoCO2.Theequivalencepointisgivenbythepresenceofapalepinkcolor.IfmoreKMnO4isadded,thesolutionturnspurpleduetotheexcessKMnO4.,Copyright1999,PRENTICEHALL,Chapter20,6,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-Reactions,Copyright1999,PRENTICEHALL,Chapter20,7,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-ReactionsWhatisthebalancedchemicalequation?1.Writedownthetwohalfreactions.2.Balanceeachhalfreaction:a.FirstwithelementsotherthanHandO.b.ThenbalanceObyaddingwater.c.ThenbalanceHbyaddingH+.d.Finishbybalancingchargebyaddingelectrons.,Copyright1999,PRENTICEHALL,Chapter20,8,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-Reactions3.Multiplyeachhalfreactiontomakethenumberofelectronsequal.4.Addthereactionsandsimplify.5.Check!ForKMnO4+Na2C2O4:,Copyright1999,PRENTICEHALL,Chapter20,9,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-Reactions1.ThetwoincompletehalfreactionsareMnO4-(aq)Mn2+(aq)C2O42-(aq)2CO2(g)2.AddingwaterandH+yields8H+MnO4-(aq)Mn2+(aq)+4H2OThereisachargeof7+ontheleftand2+ontheright.Therefore,5electronsneedtobeaddedtotheleft:5e-+8H+MnO4-(aq)Mn2+(aq)+4H2O,Copyright1999,PRENTICEHALL,Chapter20,10,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-ReactionsIntheoxalatereaction,thereisa2-chargeontheleftanda0chargeontheright,soweneedtoaddtwoelectrons:C2O42-(aq)2CO2(g)+2e-3.Tobalancethe5electronsforpermanganateand2electronsforoxalate,weneed10electronsforboth.Multiplyinggives:,Copyright1999,PRENTICEHALL,Chapter20,11,BalancingOxidation-ReductionEquations,BalancingEquationsbytheMethodofHalf-Reactions10e-+16H+2MnO4-(aq)2Mn2+(aq)+8H2O5C2O42-(aq)10CO2(g)+10e-4.Addinggives:16H+(aq)+2MnO4-(aq)+5C2O42-(aq)2Mn2+(aq)+8H2O(l)+10CO2(g)5.Whichisbalanced!,Copyright1999,PRENTICEHALL,Chapter20,12,BalancingOxidation-ReductionEquations,BalancingEquationsforReactionsOccurringinBasicSolutionWeuseOH-andH2OratherthanH+andH2O.Thesamemethodasaboveisused,butOH-isaddedto“neutralize”theH+used.,Copyright1999,PRENTICEHALL,Chapter20,13,Voltaic(Galvanic)Cells,Theenergyreleasedinaspontaneousredoxreactionisusedtoperformelectricalwork.Voltaicorgalvaniccellsaredevicesinwhichelectrontransferoccursviaanexternalcircuit.Voltaiccellsarespontaneous.IfastripofZnisplacedinasolutionofCuSO4,CuisdepositedontheZnandtheZndissolvesbyformingZn2+.ZnisspontaneouslyoxidizedtoZn2+byCu2+.TheCu2+isspontaneouslyreducedtoCu0byZn.Theentireprocessisspontaneous.,Copyright1999,PRENTICEHALL,Chapter20,14,VoltaicCells,Copyright1999,PRENTICEHALL,Chapter20,15,VoltaicCells,VoltaiccellsconsistofAnode:Zn(s)Zn2+(aq)+2e2Cathode:Cu2+(aq)+2e-Cu(s)Saltbridge(usedtocompletetheelectricalcircuit):cationsmovefromanodetocathode,anionsmovefromcathodetoanode.Thetwosolidmetalsaretheelectrodes(cathodeandanode).Asoxidationoccursinanode,ZnisconvertedtoZn2+and2e-.Theelectronsflowtowardsthecathodewheretheyareusedinthereductionreaction.,Copyright1999,PRENTICEHALL,Chapter20,16,VoltaicCells,WeexpecttheZnelectrodetolosemassandtheCuelectrodetogainmass.“Rules”ofvoltaiccells:1.Attheanodeelectronsareproducts.(Oxidation)2.Atthecathodeelectronsarereagents.(Reduction)3.Electronscannotswim.Electronsflowfromtheanodetothecathode.Therefore,theanodeisnegativeandthecathodeispositive.Electronscannotflowthroughthesolution,theyhavetobetransportedthroughanexternalwire.(Rule3.),Copyright1999,PRENTICEHALL,Chapter20,17,VoltaicCells,Copyright1999,PRENTICEHALL,Chapter20,18,VoltaicCells,Anionsandcationsmovethroughaporousbarrierorsaltbridge.Cationsmoveintothecathodiccompartmenttoneutralizetheexcessnegativelychargedions(Cathode:Cu2+2e-Cu,sothecounterionofCuisinexcess).AnionsmoveintotheanodiccompartmenttoneutralizetheexcessZn2+ionsformedbyoxidation.,Copyright1999,PRENTICEHALL,Chapter20,19,VoltaicCells,AMolecularViewofElectrodeProcessesConsiderthespontaneousredoxreactionbetweenZn(s)andCu2+(aq).Duringthereaction,Zn(s)isoxidizedtoZn2+(aq)andCu2+(aq)isreducedtoCu(s).Ontheatomiclevel,aCu2+(aq)ioncomesintocontactwithaZn(s)atomonthesurfaceoftheelectrode.TwoelectronsaredirectlytransferredfromtheZn(s)(formingZn2+(aq)totheCu2+(aq)(formingCu(s).,Copyright1999,PRENTICEHALL,Chapter20,20,CellEMF,Theflowofelectronsfromanodetocathodeisspontaneous.Electronsflowfromanodetocathodebecausethecathodehasalowerelectricalpotentialenergythantheanode.Potentialdifference:differenceinelectricalpotential.Measuredinvolts.Onevoltisthepotentialdifferencerequiredtoimpartonejouleofenergytoachargeofonecoulomb:,Copyright1999,PRENTICEHALL,Chapter20,21,CellEMF,Electromotiveforce(emf)istheforcerequiredtopushelectronsthroughtheexternalcircuit.Cellpotential:Ecellistheemfofacell.For1Msolutionsat25C(standardconditions),thestandardemf(standardcellpotential)iscalledEcell.StandardReductionPotentialsConvenienttabulationofelectrochemicaldata.Standardreductionpotentials,Eredaremeasuredrelativetothestandardhydrogenelectrode(SHE).,Copyright1999,PRENTICEHALL,Chapter20,22,CellEMF,StandardReductionPotentials,Copyright1999,PRENTICEHALL,Chapter20,23,CellEMF,StandardReductionPotentialsTheSHEisthecathode.ItconsistsofaPtelectrodeinatubeplacedin1MH+solution.H2isbubbledthroughthetube.FortheSHE,weassign2H+(aq,1M)+2e-H2(g,1atm)Eredofzero.Theemfofacellcanbecalculatedfromstandardreductionpotentials:Ecell=Ered(cathode)-Ered(anode),Copyright1999,PRENTICEHALL,Chapter20,24,CellEMF,StandardReductionPotentials,Copyright1999,PRENTICEHALL,Chapter20,25,CellEMF,StandardReductionPotentialsConsiderZn(s)Zn2+(aq)+2e-.WemeasureEcellrelativetotheSHE(cathode):Ecell=Ered(cathode)-Ered(anode)0.76V=0V-Ered(anode).Therefore,Ered(anode)=-0.76V.Standardreductionpotentialsmustbewrittenasreductionreactions:Zn2+(aq)+2e-Zn(s),Ered=-0.76V.SinceEred=-0.76VweconcludethatthereductionofZn2+inthepresenceoftheSHEisnotspontaneous.,Copyright1999,PRENTICEHALL,Chapter20,26,CellEMF,StandardReductionPotentialsTheoxidationofZnwiththeSHEisspontaneous.ChangingthestoichiometriccoefficientdoesnotaffectEred.Therefore,2Zn2+(aq)+4e-2Zn(s),Ered=-0.76V.ReactionswithEred0arespontaneousreductionsrelativetotheSHE.ReactionswithEred0thenE0.,Copyright1999,PRENTICEHALL,Chapter20,33,EffectofConcentrationonCellEMF,AvoltaiccellisfunctionaluntilE=0atwhichpointequilibriumhasbeenreached.ThepointatwhichE=0isdeterminedbytheconcentrationsofthespeciesinvolvedintheredoxreaction.TheNernstEquationTheNernstequationrelatesemftoconcentrationusingandnotingthat,Copyright1999,PRENTICEHALL,Chapter20,34,EffectofConcentrationonCellEMF,TheNernstEquationThisrearrangestogivetheNernstequation:TheNernstequationcanbesimplifiedbycollectingalltheconstantstogetherusingatemperatureof298K:(Notethatchangefromnaturallogarithmtobase-10log.)Rememberthatnisnumberofmolesofelectrons.,Copyright1999,PRENTICEHALL,Chapter20,35,EffectofConcentrationonCellEMF,ConcentrationCellsWecanusetheNernstequationtogenerateacellthathasanemfbasedsolelyondifferenceinconcentration.Onecompartmentwillconsistofaconcentratedsolution,whiletheotherhasadilutesolution.Example:1.00MNi2+(aq)and1.0010-3MNi2+(aq).ThecelltendstoequalizetheconcentrationsofNi2+(aq)ineachcompartment.TheconcentratedsolutionhastoreducetheamountofNi2+(aq)(toNi(s),somustbethecathode.,Copyright1999,PRENTICEHALL,Chapter20,36,EffectofConcentrationonCellEMF,ConcentrationCellsSincethetwohalf-reactionsarethesame,Ewillbezero.,Copyright1999,PRENTICEHALL,Chapter20,37,EffectofConcentrationonCellEMF,CellEMFandChemicalEquilibriumAsystemisatequilibriumwhenG=0.FromtheNernstequation,atequilibrium:,Copyright1999,PRENTICEHALL,Chapter20,38,Batteries,Lead-AcidBatteryA12Vcarbatteryconsistsof6cathode/anodepairseachproducing2V.Cathode:PbO2onametalgridinsulfuricacid:PbO2(s)+SO42-(aq)+4H+(aq)+2e-PbSO4(s)+2H2O(l)Anode:Pb:Pb(s)+SO42-(aq)PbSO4(s)+2e-,Copyright1999,PRENTICEHALL,Chapter20,39,Batteries,Lead-AcidBatteryTheoverallelectrochemicalreactionisPbO2(s)+Pb(s)+2SO42-(aq)+4H+(aq)2PbSO4(s)+2H2O(l)forwhichEcell=Ered(cathode)-Ered(anode)=(+1.685V)-(-0.356V)=+2.041V.Woodorglass-fiberspacersareusedtopreventtheelectrodesformtouching.,Copyright1999,PRENTICEHALL,Chapter20,40,Batteries,Lead-AcidBattery,Copyright1999,PRENTICEHALL,Chapter20,41,Batteries,AlkalineBatteryAnode:Zncap:Zn(s)Zn2+(aq)+2e-Cathode:MnO2,NH4ClandCpaste:2NH4+(aq)+2MnO2(s)+2e-Mn2O3(s)+2NH3(aq)+2H2O(l)Thegraphiterodinthecenterisaninertcathode.Foranalkalinebattery,NH4ClisreplacedwithKOH.Anode:Znpowdermixedinagel:Zn(s)Zn2+(aq)+2e-Cathode:reductionofMnO2.,Copyright1999,PRENTICEHALL,Chapter20,42,Batteries,AlkalineBattery,Copyright1999,PRENTICEHALL,Chapter20,43,Batteries,FuelCellsDirectproductionofelectricityfromfuelsoccursinafuelcell.OnApollomoonflights,theH2-O2fuelcellwastheprimarysourceofelectricity.Cathode:reductionofoxygen:2H2O(l)+O2(g)+4e-4OH-(aq)Anode:2H2(g)+4OH-(aq)4H2O(l)+4e-,Copyright1999,PRENTICEHALL,Chapter20,44,Batteries,FuelCells,Copyright1999,PRENTICEHALL,Chapter20,45,Corrosion,CorrosionofIronSinceEred(Fe2+)Ered(O2)ironcanbeoxidizedbyoxygen.Cathode:O2(g)+4H+(aq)+4e-2H2O(l).Anode:Fe(s)Fe2+(aq)+2e-.Dissolvedoxygeninwaterusuallycausestheoxidationofiron.Fe2+initiallyformedcanbefurtheroxidizedtoFe3+whichformsrust,Fe2O3.xH2O(s).OxidationoccursatthesitewiththegreatestconcentrationofO2.,Copyright1999,PRENTICEHALL,Chapter20,46,Corrosion,CorrosionofIron,Copyright1999,PRENTICEHALL,Chapter20,47,Corrosion,PreventingtheCorrosionofIronCorrosioncanbepreventedbycoatingtheironwithpaintoranothermetal.Galvanizedironiscoatedwithathinlayerofzinc.ZincprotectstheironsinceZnistheanodeandFethecathode:Zn2+(aq)+2e-Zn(s),Ered=-0.76VFe2+(aq)+2e-Fe(s),Ered=-0.44VWiththeabovestandardreductionpotentials,ZniseasiertooxidizethanFe.,Copyright1999,PRENTICEHALL,Chapter20,48,Corrosion,PreventingtheCorrosionofIron,Copyright1999,PRENTICEHALL,Chapter20,49,Corrosion,PreventingtheCorrosionofIronToprotectundergroundpipelines,asacrificialanodeisadded.Thewaterpipeisturnedintothecathodeandanactivemetalisusedastheanode.Often,Mgisusedasthesacrificialanode:Mg2+(aq)+2e-Mg(s),Ered=-2.37VFe2+(aq)+2e-Fe(s),Ered=-0.44V,Copyright1999,PRENTICEHALL,Chapter20,50,Corrosion,PreventingtheCorrosionofIron,Copyright1999,PRENTICEHALL,Chapter20,51,Electrolysis,ElectrolysisofAqueousSolutionsNonspontaneousreactionsrequireanexternalcurrentinordertoforcethereactiontoproceed.Electrolysisreactionsarenonspontaneous.Involtaicandelectrolyticcells:reductionoccursatthecathode,andoxidationoccursattheanode.However,inelectrolyticcells,electronsareforcedtoflowfromtheanodetocathode.Inelectrolyticcellstheanodeispositiveandthecathodeisnegative.(Ingalvaniccellstheanodeisnegativeandthecathodeispositive.),Copyright1999,PRENTICEHALL,Chapter20,52,Electrolysis,ElectrolysisofAqueousSolutions,Copyright1999,PRENTICEHALL,Chapter20,53,Electrolysis,ElectrolysisofAqueousSolutionsExample,decompositionofmoltenNaCl.Cathode:2Na+(l)+2e-2Na(l)Anode:2Cl-(l)Cl2(g)+2e-.Industrially,electrolysisisusedtoproducemetalslikeAl.ElectrolysiswithActiveElectrodesActiveelectrodes:electrodesthattakepartinelectrolysis.Example:electrolyticplating.,Copyright1999,PRENTICEHALL