第3章-普化电化学

Chapter3

Electrochemistry

电化学Electrochemistrydealswithchemicalchangesproducedbyanelectriccurrentandwiththeproductionofelectricitybychemicalchanges.Theapplicationsinvolveinbattery,electrolysis,electrorefiningofmetals.电化学是研究化学能与电能转换的学科，其主要应用有电池、电解、金属的电化学加工等。1TermsUsedinElectrochemistry(1)

电化学术语(1)Oxidize氧化;reduce还原;electriccurrent电流;oxidizingagent氧化剂;reducingagent还原剂;oxidationnumber氧化数;zincplate锌板;copperstrip铜板

redox

氧化还原;dowork作功;galvaniccell原电池;electrode电极;cathode阳极;anode阴极;saltbridge盐桥;2TermsUsedinElectrochemistry(2)

电化学专业词汇(2)potassiumchloride氯化钾;amperemeter

安培计;pointer指针;zinccompartment锌半室;positivecharge正电荷;redoxcouple电对;cellpotential原电池电动势;volt伏特;voltmeter伏特计;half-cell半电池;inertelectrode惰性电极;platinum铂;calomelelectrode甘汞电极;referenceelectrode参比电极;3RedoxReactions

氧化还原反应Redox,theshortnameforoxidation-reduction,isoneof4typesofchemicalreactions:氧化还原反应的缩写为Redox,是四种化学反应的一种。①combination化合反应

②decomposition分解反应③metathesis复分解反应

④redox

氧化还原反应.E.g.Zn(s)+H2SO4(aq)==ZnSO4(aq)+H2(g)Here,Zincmetalisreducingagent;sulfuricacid(H2SO4)isoxidizingagent金属锌作还原剂，硫酸作氧化剂.4OxidizingandReducingAgents

氧化剂和还原剂1.OxidizingAgent:氧化剂Thespecieswhichcausesoxidationiscalledthe

oxidizingagent.

Thesubstancewhichisoxidizedloseselectronstotheother.

Theoxidizingagentisalwaysreduced.氧化剂发生还原反应。2.ReducingAgent:还原剂Thespecieswhichcausesreductioniscalledthe

reducingagent.Thesubstancewhichisreducesgainselectronsfromtheother.

TheReducingagentisalwaysoxidized.还原剂发生氧化反应。5CommonReducingandOxidizingAgents常见氧化剂和还原剂Thereducedspeciesmaybeusedasareducingagent,e.g.Zincmetal,stannouschloride(SnCl2),iodine(I2),sodiumthiosulfate(Na2S2O3).Theoxidizedspeciesasanoxidizingagent,e.g.copper(Cu2+)ions,potassiumpermanganate(KMnO4),potassiumdichromate(K2Cr2O7),sodiumbismuthate(NaBiO3).6OxidationNumbers氧化数1.Foranatominitselementalform(Na,O2,Cl2…)oxidationnumber=0

元素的氧化数为0。2.

Foramonatomicion:oxidationnumber=ioncharge

单原子离子的电荷等于氧化数。3.

Thesumofoxidationnumbervaluesfortheatomsinacompoundequalszero.Thesumofoxidationnumbervaluesfortheatomsinapolyatomicionequalstheioncharge.

化合物的氧化数为0，多原子离子中所有元素的氧化数之和等于离子电荷。

RulesforAssigninganOxidationNumber氧化数的判定7RulesforSpecificAtomsorPeriodicTableGroups.

周期表中元素的氧化数1.Forfluorine:Ox.no.=1inallcompounds

氟为1。2.Foroxygen:Ox.no.=1inperoxides

过氧化物中为1。Ox.No.=2inallothercompounds(exceptwithF)

氧化物中为2。3.

ForGroup7A

卤素: Ox.no.=1incombinationwithmetals,nonmetals(exceptO)

金属、非金属元素卤化物中为1,

andotherhalogenslowerinthegroup.4.ForGroup1A

碱金属: Ox.no.=+1inallcompounds

为+1。5.ForGroup2A

碱土金属:Ox.no.=+2inallcompounds为+2。6.Forhydrogen: Ox.no.=+1incombinationwithnonmetals

与非金属化合为+1，Ox.no.=1incombinationswithmetalsandboron

与金属和硼化合为1。8BalancingRedoxReactions

氧化还原反应的配平Forexample,KMnO4+H2S+H2SO4==MnSO4+S+K2SO4+H2OMnO4+8H++5e=Mn2++4H2O氧化剂H2S=S+2H++2e还原剂2+52KMnO4+5H2S+3H2SO4=2MnSO4+5S+K2SO4+8H2O9RedoxReactions-IonElectronMethod.离子电子法Underacidiccondition

酸性条件

BalancethehalfreactionseparatelyexceptH&O’s.

BalanceOxygenbyH2O

用水分子平衡氧原子。

BalanceHydrogenbyH+用H+平衡氢原子。

Balancechargebye

加上电子平衡电荷。

Addthehalf-reactiontogether.Underbasiccondition

碱性条件

thesameprocedureastheacidic,except

处理如酸性条件，注意：EliminateH+byadding:H++OH

H2O10Examples例题BrO4

+CrO2

BrO3

+

CrO42

(basic)配平氧化剂、还原剂：BrO4

+e

BrO3CrO2

CrO42

+e配平氧原子：BrO4

+2H++2e

BrO3

+H2O

2H2O

2H++2OH

BrO4

+2H2O+2e

BrO3

+2OH

碱性条件：CrO2

+2H2OCrO42

+4H++3e4H++4OH

4H2O

CrO2

+4OH

CrO42

+2H2O+3e总反应：3BrO4

+2CrO2

+2OH+2H2O3BrO3

+2CrO42

32+11TheActivitySeriesofMetals金属活泼程度Metalscanbeplacedinorderoftheirtendenciesforlosingelectrons.Thisiscalledtheactivityseries.Activityseriesofmetalinaqueoussolution

水溶液中金属的活泼性PotassiumCalciumSodiumMagnesiumAluminumZincIronTinNickelLeadHydrogenCopperSilverMercuryWhatdetermineswhetherthereactionsoccur?12GalvanicCells(1)原电池

Theredoxreactionisreallyanelectrontransferreaction电子得失.Noelectriccurrentresultswhentheoxidizingagentandreducingagentaremixed.

Zn(s)+Cu2+(aq)Zn2+(aq)+Cu(s)Zincplate,coppersulfatezincsulfate,copperstripZincatomCopperatomNousefulworkisobtained13GalvanicCell(2)原电池Agalvaniccellisanenergy-producingapparatus,inwhichtheoxidationandreductionreactionoccurintwoseparatecompartments,oftencalledhalf-cells.

原电池是将氧化还原反应的化学能转化为电能的装置。ZnCuInitially,thereisaflowofelectron,aftersometime,theprocessstopsbecauseofthechargebuildup.起先电子流动，但因电荷积聚，一会儿便停止14Completingthecircuit

盐桥连接Asaltbridge,anup-side-downUtube,containingsaturatedpotassiumchloride(KCl)solutionandgalatin(凝胶).Anamperemeter(安培计)isconnectedintothecircuit.Therefore,thechemicalenergycanbeconvertedintotheelectricenergy.ZnCu盐桥的作用:

Inorderforelectronstomovethroughanexternalwire,chargemustnotbuildupatanycell.Thisisdonebythesaltbridgeinwhichionsmigratetodifferentcompartmentsneutralizeanychargebuildup.15Thecathode正极:electrodeatwhichelectronsflowin电子流入,Copper(Cu)stripseversasthecathode.Theanode负极:electrodeatwhichelectronsflowout电子流出,Zinc(Zn)plateservesastheanode.Reductionalwaysoccursatthecathode阳极发生还原;oxidationalwaysoccursattheanode阴极发生氧化.Asaltbridgeworksasionflowchannel,istobalancetheionchargesineachcompartment

盐桥充当离子通道.Potassiumions(K+)movetocoppersulfatesolution;whereaschlorideions(Cl)movetozincsulfatesolution.16CellAssembly电池构造Electrontransfercanoccurifthecircuitisclosed.

电路闭合，电子流动。Partsofacell:

Twoconductors

两个电极Electrolytesolution

电解质溶液SaltBridge/Porousmembrane

盐桥或多孔膜Threeprocessesmusthappenifeistoflow.etransportthroughexternalcircuit

电子流经外电路Inthecell,ionsamustmigrate

离子在电池内迁移Circuitmustbeclosed(nochargebuildup)

电路闭合，无电荷积聚。17HowaGalvanicCellWorks?

原电池工作原理Anode(-)grayNegativeelectrodegenerateselectronOxidationOccurCathode(+)RedPositiveelectrodeacceptselectronReductionOccurAnode/Anion(-)Cathode/Cation(+)e-ReducingAgente-e-OxidizingAgentPorousDisk18TheDiagrammaticRepresentationofaGalvanicCell原电池符号Theoxidizedspeciesandthereducedspeciesineachcompartment(half-cell)consistofaredoxcouple

氧化还原电对,writtenase.g.Zn2+/Zncouple.Agalvaniccellisdepictedas()Zn|ZnSO4(c1)||CuSO4(c2)|Cu(+)Anode负极Phaseboundary相界面Concentration浓度SaltbridgeCathode正极19LineNotationConvention电池符号Sometimes,aninertelectrodeisnecessary.Itshouldbeaconductor导体

thatdeliverselectricityanddoesnotenterintothecellreaction.E.g.platinumandgraphite.

惰性电极导电，但不参与电池反应，如铂、石墨。

Inhydrogen-zincgalvaniccell,platinumisusedasthecathode.ThelinenotationfortheH2-Zngalvaniccellisasfollows:氢-锌原电池中，铂作正极，电池符号:()Zn|Zn2+(c1)||H+(c2)|H2(g,1atm)|Pt(+)20LineNotationofCellExamples电池符号CCrAg()C|I(c1),I2(g,1atm)||MnO4(c2),Mn2+(c3)|C(+)()Cr|Cr3+(c1)||Ag+(c2)|Ag(+)C21ElectrodePotentials电极电位Electronsgeneratedinacellarethoughttobe“driven”towardthecathodebytheelectromotiveforce原电池中的电子由电动势驱使，流向正极.Cellpotential电池电动势:apotentialdifferenceexistingattwoelectrodes.正、负极间的电势差。AvoltmetersaysthecellpotentialofZn-Cugalvaniccellisabout1volt.锌-铜原电池的电动势为1伏.Cathodehasahigherpotential正极电位高;anodehasalowerpotential负极电位低,eachhasapotential,calledelectrodepotential.

正、负极都有电位，叫作电极电位。22SomeCommonlyUsedElectrodes

常用电极

TypesofElectrodes

Couples

Electrodes

Me-Men+electrode

Zn2+/Zn Zn∣Zn2+

A-An-electrodeCl2/Cl

Cl∣Cl2∣PtRedoxelectrode

Fe3+/Fe2+Fe3+,Fe2+∣PtMe-slightlysolublesalt

AgCl/AgAg∣AgCl∣Cl

Anytwoelectrodescanbuildupacell.原电池由两个电极构成。23StandardElectrodePotential标准电极电位Thereisnosatisfactorymethodtodeterminetheactualpotentialofsingleelectrode.Butcellpotentialcanbedetermined.Soastandardelectrodeorreferenceelectrodemustbeassigned.

单个电极的电为不可测，但电池电动势可测。所以，必须指定一参考电极.Thepotentialofstandardhydrogenelectrodeisassignedaszeroatstandardconditions

标准状态下的氢电极的电极电位被指定为0伏.Forexample,acellcomposedofzincelectrodeandhydrogenelectrode,hasacellpotentialof0.786Vatstandardconditions.Zincelectrodebehavesasanode.24Ecell=E

cathode

-

E

anode

E

anode=-0.7618E(Zn2+/Zn)=-0.7618V[H+]=1.00H2(g)e-Ptgauzep{H2(g)}=1.00atme-H2(g)0.76VZn1MZn2+1MH+25ReferenceElectrode参比电极Itisinconvenienttousestandardhydrogenelectrodebecauseofuseofpurehydrogengasandthespecialtreatmentoftheelectrode.标准氢电极因使用纯氢、并需作特别处理，故使用不便。Usuallyweusecalomelelectrode,whichconsistingof

常用的参比电极是甘汞电极，其电池符号和结构如下：WhenusingsaturatedKClsolution使用饱和氯化钾溶液时，其电位:E甘汞电极o=0.2415VPt∣Hg∣Hg2Cl2∣Cl-26PotentialValuesofSomeElectrodes

一些电极的电极电位CouplesElectrodeReactionsElectrodepotentials

K+/K K++eK 2.931 Zn2+/ZnZn2++2e

Zn

0.7618H+/H

2H++2eH20.0000Cu2+/CuCu2++2eCu

+0.3419F2/F

F2+2e2F

+2.866电对电极反应电极电位27TableofElectrodePotentials

电极电位表Inthetable,fromthetoptothebottom,thealgebraicvalueofaselectrodepotentialsincreases;thereducedspeciesofanelectrodeisalessstrongerreducingagent;andtheoxidizedspeciesofanelectrodeisamuchstrongeroxidizingagent.

表中，上方电对的还原型物质为还原剂，下方电对的氧化型物质为氧化剂。自上而下，电极电位增加。还原型物质的还原性降低，而氧化型物质的氧化性增强。

电极电位最低的是Li+/Li，但金属锂不是最活泼的，为什么？28MeaningsoftheStandardElectrodePotential

标准电极电位的意义Ifanelectrodehasalargenegativepotential,thereducedspeciesoftheelectrodeismosteasilyoxidized电极电位愈小，其还原型物质愈易被氧化.

Ifanelectrodehasalargepositivepotential,theoxidizedspeciesisastrongoxidizingagent电极电位愈正，其氧化型物质是强氧化剂.29UseoftheTableofStandardElectrodePotentials(1)

电极电位表的使用①thepotentialsarestandardreductionpotentials

表中为还原电位.②Nomatterhowdifferentlytheelectrodereactioniswritten,thestandardelectrodepotentialdoesn’tvary标准电极电位的数值与电极反应的写法无关.orZn2e

Zn2+Zn2++2e

ZnE(Zn2+/Zn)=-0.7618V30UseoftheTableofStandardElectrodePotentials(2)③theelectrodepotentialkeepsthesamewiththechangeinstiochoimetricnumbersoftheelectrodereaction

电极电位的数值并不随电极反应的计量系数的变化而变化.

O2+2H2O+4e4OH

or1/2O2+H2O+2e2OH④Theelectrodepotentialsarethecharacteristicvalues.电极电位是电极的特征值。⑤someelectrodesmayhavedifferentpotentialsindifferentmedia.有些电极在不同介质中的电极电位的数值有所不同.E(O2/OH)=0.401V31NernstEquation

能斯特方程式Foranelectrodereaction,就如下的电极反应

Oxidizedform+

ne

ReducedformAt25℃:32ExamplesForexample,O2+2H2O+4e4OH

Nernstequation:能斯特方程式

MnO4+8H++5e

Mn2++4H2O

Nernstequation:33UsingNernstEquation①nrepresentsthenumberofelectronsgainedorlost得失电子数n.

②

thestiochoimetricnumberinahalf-reactionisthenumberofpowerinthec(Ox)orc(Red)半反应的计量系数为能斯特方程式中的浓度的方次数.③therelativeconcentrationofagasisexpressedinthepartialpressure

气体的浓度以分压表示.④puresolidsandliquidsarenotincluded

固体和纯液体不计.34ExampleExample:calculatethepotentialforpermanganate/manganeseioncoupleinneutralsolution.Solution:MnO4+8H++5e

Mn2++4H2O

usingNernstequation:Suppose[MnO4]=1molL1,[Mn2+]=1molL1

35Temperature(2)Temperaturehaslittleimpactontheelectrodepotentials.温度对电极电位的影响甚微。36RelationshipbetweenΔrGmandCellPotentials

ΔrGm与电动势之间的关系电功ΔrGm=Wmax′weknowWmax′=-QE

=-

nFE

thecellpotentialscanbeusedtojudgethespontaneityofaredoxreaction.电池电动势可判断氧化还原反应的自发性：E<0ΔG>0

nonspontaneous;非自发E=0ΔG=0atequilibrium;可逆E>0ΔG<0

spontaneous

自发orΔrGm=-

nFEΔrGm=-

nFE37Example:c(Pb2+)=0.1mol.L-1,c(Sn2+)=1.0mol.L-1.PredictthedirectionforthereactionofPb2++Sn==Sn2++Pb=-0.1262-0.0296=-0.1558VWhereas,E(Sn2+/Sn)=E(Sn2+/Sn)=-0.1375V.Comparingthetwopotentials比较正、负极的电极电位,

E(Pb2+/Pb)<E(Sn2+/Sn)Sn2+│Sn

actsascathode正极,Pb2+│Pbactsasanode负极.

Thereversereactionisspontaneous逆反应自发.Solution:usingNernstequation,利用能斯特方程式38UsingElectrodePotentials(1)judgewhethertheelectrodeiscathodeoranode.推测哪个电极为正极、哪个为负极。

Cathode:reductionreactionoccurs正极发生还原。∴hasahighpotential电位高。

Anode:oxidationreactionoccurs

负极发生氧化。∴hasalowpotential电位低。Example:ifacelliscomposedofthefollowingelectrodes,(1)Zn∣Zn2+（1.0mol.L-1)

(2)Zn∣Zn2+(0.001mol.L-1)Pointoutthecathodeandanodeandcalculatethecellpotential.指出正、负极，并计算电动势。39Solution:forelectrode(2),usingNernstequation,E2(Zn2+/Zn)=Forelectrode(1),E1(Zn2+/Zn)=E=0.7618VComparingthetwopotentials,E2<E1,thus,Electrode(1)actsascathode电极1为正极;

electrode(2)actsasanode电极2作负极.

Thecellpotentialofbothelectrodesis电动势为

E=E+-E-=E1-E2=0.089VThiscelliscalledconcentrationcell该电池为浓差电池.=-0.8506V40(2)

CalculatingcellPotentials计算电动势

(3)comparetheredoxabilityofoxidizingagentsandreducingagents比较氧化剂、还原剂的氧化还原能力E=E+E-Example:Couples电对:

I2/I-，Br2/Br-，Fe3+/Fe2+E/V:0.5355,1.066,0.771Comparetheredoxabilityofthethreecouples.Solution:oxidizingabilitiesdecrease氧化能力下降:

Br2—Fe3+—I2reducingabilitiesincrease还原能力增强:

I-—Fe2+—Br-41PredicatetheSpontaneityofaRedox

预测反应的自发性Example:Silverisaninertelement.Itcannotreactwithdilutesulfuricorhydrochloricacidtoevolvethehydrogengas.Whensilverisdippedintothe1.0Mhydroiodicacidsolution,predicatethespontaneityofthereaction,银是惰性元素，它不能与稀硫酸或稀盐酸作用，产生氢气。若银与1.0molL1HI反应，计算说明如下反应的方向.Ag+2HIAgI+H2

(KspAgI=8.31017,EAg+/Ag=0.799V)Solution:inthereaction,anodeis负极为

AgI+eAg+I;cathodeis

正极是

2H++2eH2

42FortheAg+/Agcouple,itsnernstequation,银电极的能斯特方程式FortheAgI/Agcouple,itsnernstequation,碘化银/银电极的能斯特方程式:AgI+eAg+I;RewritethethenernstequationofAg+/Agcouple改写Ag+/Ag的能斯特方程式:Ifthereexisttheequilibriumnow,若溶液中有平衡:43AgI(s)Ag+(aq)+I(aq)intheAg+/Agcouple,therehas[Ag][I]=Ksp(AgI)则银电极的能斯特方程式可写为:

Thevalueof“Ag+/Ag”electrodepotential,代入数据，“银电极”的电极电位:

比较上式和碘化银/银电极的能斯特方程式，可得:反应可进行。有氢气释放。44Ifanoxidizingagent,forinstance,KMnO4,reactswiththem,thereadinessofthereactionis:如KMnO4与它们

作用，

I-—

Fe2+

—Br-Ifanreducingagent,forinstance,Zn,reactswiththem,thereadinessofthereactionis:锌与它们作用

Br2

—

Fe3+

—

I2(4)predicatetheextentofaredoxreaction预测反应进度Atequilibrium平衡时ΔG

=0-ΔrGm

=RTlnKat298.15K-ΔrGm

=RTlnK-ΔrGm

=nFE电池45CalculationSo,thecellpotential,E电池

=-0.771VExample:Calculatetheequilibriumconstantforthereaction.计算如下反应的平衡常数。2H++2Fe2+H2+2Fe3+Solution:E(H+/H2)=0V

E(Fe3+/Fe2+)=0.771VK

=8.91×10-2746ElectrodePotentialorLatimeDiagram

元素电位图Cu2+——Cu+——Cu0.1670.522ElectrodepotentialDiagramofanelement元素电位图:

thediagrammaticrepresentationofpotentialsinrelationshiptoalloxidationstatesofanelement.图形化表示元素各氧化态电极电位间的关系。Because根据铜元素电位图47Copper(I)ioncanbehavebothasanoxidizingagentandasadeducingagent.亚铜离子既作氧化剂，又作还原剂2Cu+Cu2++CuThisreactioniscalldisproportionreaction歧化反应CalculatethepotentialforCu2+/CucoupleCu++eCu△rGm1o=–n1FE1o(1)Cu2++eCu+

△rGm2o=–n2FE2o(2)(1)+(2):Cu2++2eCu△rGm3o=–n3FE3otherefore,–n1FE1o+–n2FE2o=–n3FE3o

4849Lead-AcidBattery铅酸电池Theoverallelectrochemicalreactionis

电池总反应PbO2(s)+Pb(s)+2SO42-(aq)+4H+(aq)2PbSO4(s)+2H2O(l)

forwhich

电动势Ecell=ERHS-ELHS=(+1.685V)-(-0.356V)=+2.041V.Woodorglass-fiberspacersareusedtopreventtheelectrodesformtouching.

电极衬有木头或玻璃纤维以防电极腐蚀。50APictureofCarBattery汽车电池51AnAlkalineBattery碱性电池在普通电池中Anode:Zncap:

锌负极

Zn(s)Zn2+(aq)+2e-Cathode正极:MnO2,NH4Clandcarbonpaste

氯化铵与碳粉的糊状物:2NH4+(aq)+2MnO2(s)+2e-Mn2O3(s)+2NH3(aq)+2H2O(l)Graphiterodinthecenter-inertcathode

堕性石墨电极位与中央.Alkalinebattery,NH4ClisreplacedwithKOH

氢氧化钾替代了氯化铵.Anode负极:Znpowdermixedinagel:

锌粉胶5253FuelCells燃料电池Directp