第3章-普化电化学解析.ppt

1,Chapter3Electrochemistry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。,2,TermsUsedinElectrochemistry(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;,3,TermsUsedinElectrochemistry(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;,4,RedoxReactions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。combination化合反应decomposition分解反应metathesis复分解反应redox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.,5,OxidizingandReducingAgents氧化剂和还原剂,1.OxidizingAgent:氧化剂Thespecieswhichcausesoxidationiscalledtheoxidizingagent.Thesubstancewhichisoxidizedloseselectronstotheother.Theoxidizingagentisalwaysreduced.氧化剂发生还原反应。2.ReducingAgent:还原剂Thespecieswhichcausesreductioniscalledthereducingagent.Thesubstancewhichisreducesgainselectronsfromtheother.TheReducingagentisalwaysoxidized.还原剂发生氧化反应。,6,CommonReducingandOxidizingAgents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).,7,OxidationNumbers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,8,RulesforSpecificAtomsorPeriodicTableGroups.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。,9,BalancingRedoxReactions氧化还原反应的配平,Forexample,KMnO4+H2S+H2SO4=MnSO4+S+K2SO4+H2O,MnO4+8H+5e=Mn2+4H2O氧化剂,H2S=S+2H+2e还原剂,2,+5,2KMnO4+5H2S+3H2SO4=2MnSO4+5S+K2SO4+8H2O,10,RedoxReactions-IonElectronMethod.离子电子法,Underacidiccondition酸性条件BalancethehalfreactionseparatelyexceptHoxidationalwaysoccursattheanode阴极发生氧化.Asaltbridgeworksasionflowchannel,istobalancetheionchargesineachcompartment盐桥充当离子通道.Potassiumions(K+)movetocoppersulfatesolution;whereaschlorideions(Cl)movetozincsulfatesolution.,17,CellAssembly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。,18,HowaGalvanicCellWorks?原电池工作原理,Anode(-)grayNegativeelectrodegenerateselectronOxidationOccur,Cathode(+)RedPositiveelectrodeacceptselectronReductionOccur,Anode/Anion(-),Cathode/Cation(+),e-,ReducingAgent,e-,e-,OxidizingAgent,PorousDisk,19,TheDiagrammaticRepresentationofaGalvanicCell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,Saltbridge,Cathode正极,20,LineNotationConvention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(+),21,LineNotationofCellExamples电池符号,C,Cr,Ag,()C|I(c1),I2(g,1atm)|MnO4(c2),Mn2+(c3)|C(+),()Cr|Cr3+(c1)|Ag+(c2)|Ag(+),C,22,ElectrodePotentials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。,23,SomeCommonlyUsedElectrodes常用电极,TypesofElectrodesCouplesElectrodesMe-Men+electrodeZn2+/ZnZnZn2+A-An-electrodeCl2/ClClCl2PtRedoxelectrodeFe3+/Fe2+Fe3+,Fe2+PtMe-slightlysolublesaltAgCl/AgAgAgClCl,Anytwoelectrodescanbuildupacell.原电池由两个电极构成。,24,StandardElectrodePotential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.,25,Ecell=Ecathode-Eanode,Eanode=-0.7618,E(Zn2+/Zn)=-0.7618V,H+=1.00,H2(g),e-,Ptgauze,pH2(g)=1.00atm,e-,H2(g),0.76V,Zn,1MZn2+,1MH+,26,ReferenceElectrode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=0.2415V,PtHgHg2Cl2Cl-,27,PotentialValuesofSomeElectrodes一些电极的电极电位,CouplesElectrodeReactionsElectrodepotentials,K+/KK+e,K2.931,Zn2+/ZnZn2+2e,Zn0.7618,H+/H2H+2e,H20.0000,Cu2+/CuCu2+2e,Cu+0.3419,F2/FF2+2e,2F+2.866,电对电极反应电极电位,28,TableofElectrodePotentials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？,29,MeaningsoftheStandardElectrodePotential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.,30,UseoftheTableofStandardElectrodePotentials(1)电极电位表的使用,thepotentialsarestandardreductionpotentials表中为还原电位.Nomatterhowdifferentlytheelectrodereactioniswritten,thestandardelectrodepotentialdoesntvary标准电极电位的数值与电极反应的写法无关.,orZn2eZn2+,Zn2+2eZn,E(Zn2+/Zn)=-0.7618V,31,UseoftheTableofStandardElectrodePotentials(2),theelectrodepotentialkeepsthesamewiththechangeinstiochoimetricnumbersoftheelectrodereaction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+4e4OHor1/2O2+H2O+2e2OH,Theelectrodepotentialsarethecharacteristicvalues.电极电位是电极的特征值。someelectrodesmayhavedifferentpotentialsindifferentmedia.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)=0.401V,32,NernstEquation能斯特方程式,Foranelectrodereaction,就如下的电极反应Oxidizedform+neReducedform,At25:,33,Examples,Forexample,O2+2H2O+4e4OHNernstequation:能斯特方程式,MnO4+8H+5eMn2+4H2ONernstequation:,34,UsingNernstEquation,nrepresentsthenumberofelectronsgainedorlost得失电子数n.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.therelativeconcentrationofagasisexpressedinthepartialpressure气体的浓度以分压表示.puresolidsandliquidsarenotincluded固体和纯液体不计.,35,Example,Example:calculatethepotentialforpermanganate/manganeseioncoupleinneutralsolution.,Solution:MnO4+8H+5eMn2+4H2OusingNernstequation:,SupposeMnO4=1molL1,Mn2+=1molL1,36,Temperature,(2)Temperaturehaslittleimpactontheelectrodepotentials.温度对电极电位的影响甚微。,37,RelationshipbetweenrGmandCellPotentialsrGm与电动势之间的关系,电功rGm=WmaxweknowWmax=-QE=-nFE,thecellpotentialscanbeusedtojudgethespontaneityofaredoxreaction.电池电动势可判断氧化还原反应的自发性：E0nonspontaneous;非自发E=0G=0atequilibrium;可逆E0G0spontaneous自发,or,rGm=-nFE,rGm=-nFE,38,Example:c(Pb2+)=0.1mol.L-1,c(Sn2+)=1.0mol.L-1.PredictthedirectionforthereactionofPb2+Sn=Sn2+Pb,=-0.1262-0.0296=-0.1558V,Whereas,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+Snactsascathode正极,Pb2+Pbactsasanode负极.Thereversereactionisspontaneous逆反应自发.,Solution:usingNernstequation,利用能斯特方程式,39,UsingElectrodePotentials,(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)ZnZn2+（1.0mol.L-1)(2)ZnZn2+(0.001mol.L-1)Pointoutthecathodeandanodeandcalculatethecellpotential.指出正、负极，并计算电动势。,40,Solution:forelectrode(2),usingNernstequation,E2(Zn2+/Zn)=,Forelectrode(1),E1(Zn2+/Zn)=E=0.7618V,Comparingthetwopotentials,E2E1,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.,=-0.8506V,41,(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:Br2Fe3+I2reducingabilitiesincrease还原能力增强:I-Fe2+Br-,42,PredicatetheSpontaneityofaRedox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+2eH2,43,FortheAg+/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:,44,AgI(s)Ag+(aq)+I(aq)intheAg+/Agcouple,therehasAgI=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。,45,Ifanoxidizingagent,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Br2Fe3+I2,(4)predicatetheextentofaredoxreaction预测反应进度Atequilibrium平衡时G=0-rGm=RTlnK,at298.15K,-rGm=RTlnK,-rGm=nFE电池,46,Calculation,So,thecellpotential,E电池=-0.771V,Example:Calculatetheequilibriumconstantforthereaction.计算如下反应的平衡常数。,2H+2Fe2+,H2+2Fe3+,Solution:E(H+/H2)=0VE(Fe3+/Fe2+)=0.771V,K=8.9110-27,47,ElectrodePotentialorLatimeDiagram元素电位图,Cu2+Cu+Cu,0.167,0.522,ElectrodepotentialDiagramofanelement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,48,Copper(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+eCurGm1o=n1FE1o(1)Cu2+eCu+rGm2o=n2FE2o(2)(1)+(2):Cu2+2eCurGm3o=n3FE3otherefore,n1FE1o+n2FE2o=n3FE3o,49,50,Lead-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。,51,APictureofCarBattery汽车电池,52,AnAlkalineBattery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,53,54,FuelCells燃料电池,Directproductionofelectricityfromfuelsoccursinafuelcell.燃料直接转换成电能。H2-O2fuelcellwastheprimarysourceofelectricityonApollomoonflights.阿波罗登月采用氢氧燃料电池。Cathode正极:reducti