电极过程动力学 (11).pdf

《电极过程动力学 (11).pdf》由会员分享，可在线阅读，更多相关《电极过程动力学 (11).pdf（36页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、1 1ELECTROCHEMISTRYELECTROCHEMISTRYChapter 18Chapter 18SAVE PAPER AND INK!When you print out the notes on PowerPoint,print Handouts instead of Slides in the print setup.Also,turn off the backgrounds(ToolsOptionsPrintUNcheck Background Printing)!2 2Electron Transfer ReactionsElectron Transfer Reactio

2、nsElectron Transfer ReactionsElectron transfer reactions are Electron transfer reactions are Electron transfer reactions are Electron transfer reactions are oxidation-oxidation-oxidation-oxidation-reductionreductionreductionreduction or or or or redoxredoxredoxredox reactions.reactions.reactions.rea

3、ctions.Results in the generation of an electric current Results in the generation of an electric current Results in the generation of an electric current Results in the generation of an electric current(electricity)or be caused by imposing an electric(electricity)or be caused by imposing an electric

4、(electricity)or be caused by imposing an electric(electricity)or be caused by imposing an electric current.current.current.current.Therefore,this field of chemistry is often Therefore,this field of chemistry is often Therefore,this field of chemistry is often Therefore,this field of chemistry is oft

5、en called called called called ELECTROCHEMISTRY.ELECTROCHEMISTRY.3 3Terminology for Redox ReactionsTerminology for Terminology for RedoxRedox ReactionsReactionsOXIDATIONOXIDATIONloss of loss of electron(selectron(s)by a species;increase)by a species;increase in oxidation number;increase in oxygen.in

6、 oxidation number;increase in oxygen.REDUCTIONREDUCTIONgain of gain of electron(selectron(s);decrease in oxidation);decrease in oxidation number;decrease in oxygen;increase in hydrogen.number;decrease in oxygen;increase in hydrogen.OXIDIZING AGENTOXIDIZING AGENTelectron acceptor;species is electron

7、acceptor;species is reduced.reduced.REDUCING AGENTREDUCING AGENTelectron donor;species is oxidized.electron donor;species is oxidized.OXIDATIONOXIDATIONOXIDATIONOXIDATIONloss of loss of loss of loss of electron(selectron(selectron(selectron(s)by a species;increase)by a species;increase)by a species;

8、increase)by a species;increase in oxidation number;increase in oxygen.in oxidation number;increase in oxygen.in oxidation number;increase in oxygen.in oxidation number;increase in oxygen.REDUCTIONREDUCTIONREDUCTIONREDUCTIONgain of gain of gain of gain of electron(selectron(selectron(selectron(s);dec

9、rease in oxidation);decrease in oxidation);decrease in oxidation);decrease in oxidation number;decrease in oxygen;increase in hydrogen.number;decrease in oxygen;increase in hydrogen.number;decrease in oxygen;increase in hydrogen.number;decrease in oxygen;increase in hydrogen.OXIDIZING AGENTOXIDIZING

10、 AGENTOXIDIZING AGENTOXIDIZING AGENTelectron acceptor;species is electron acceptor;species is electron acceptor;species is electron acceptor;species is reduced.reduced.reduced.reduced.REDUCING AGENTREDUCING AGENTREDUCING AGENTREDUCING AGENTelectron donor;species is oxidized.electron donor;species is

11、 oxidized.electron donor;species is oxidized.electron donor;species is oxidized.4 4You cant have one without the other!Reduction(gaining electrons)canReduction(gaining electrons)cant happen without an oxidation t happen without an oxidation to provide the electrons.to provide the electrons.You canYo

12、u cant have 2 oxidations or 2 reductions in the same t have 2 oxidations or 2 reductions in the same equation.Reduction has to occur at the cost of oxidationequation.Reduction has to occur at the cost of oxidationLEOLEO the lion says the lion says GERGER!GER!GER!5 5Another way to rememberOIL RIGOIL

13、RIG6 6OXIDATION-REDUCTION REACTIONSOXIDATION-REDUCTION REACTIONSDirect Direct Direct Direct RedoxRedoxRedoxRedox Reaction Reaction Reaction ReactionOxidizing and reducing Oxidizing and reducing Oxidizing and reducing Oxidizing and reducing agents in direct contact.agents in direct contact.agents in

14、direct contact.agents in direct contact.Cu(sCu(sCu(sCu(s)+2)+2)+2)+2 AgAgAgAg+(aq(aq)-)-CuCu2+2+(aq)+2(aq)+2 Ag(sAg(s)7 7OXIDATION-REDUCTION REACTIONSOXIDATION-REDUCTION REACTIONSIndirect Indirect Indirect Indirect RedoxRedoxRedoxRedox Reaction Reaction Reaction ReactionA battery functions by transf

15、erring electrons through an A battery functions by transferring electrons through an A battery functions by transferring electrons through an A battery functions by transferring electrons through an external wire from the reducing agent to the external wire from the reducing agent to the external wi

16、re from the reducing agent to the external wire from the reducing agent to the oxidizing agent.oxidizing agent.oxidizing agent.oxidizing agent.8 8Why Study Electrochemistry?Why Study Electrochemistry?Why Study Electrochemistry?BatteriesBatteries CorrosionCorrosion Industrial Industrial production of

17、 production of chemicalschemicals such as such as ClCl2 2,NaOHNaOH,F,F2 2 and and AlAl Biological Biological redoxredox reactionsreactionsThe The The The hemehemehemeheme group group group group9 9Electrochemical CellsElectrochemical CellsElectrochemical CellsAn apparatus that allows a An apparatus

18、that allows a An apparatus that allows a An apparatus that allows a redoxredoxredoxredox reaction to occur by reaction to occur by reaction to occur by reaction to occur by transferring electrons through transferring electrons through transferring electrons through transferring electrons through an

19、external connector.an external connector.an external connector.an external connector.Product favored reaction -Product favored reaction -Product favored reaction -Product favored reaction -voltaic or galvanic cellvoltaic or galvanic cellvoltaic or galvanic cellvoltaic or galvanic cell-electric curre

20、nt-electric current-electric current-electric currentReactant favored reaction-Reactant favored reaction-Reactant favored reaction-Reactant favored reaction-electrolytic cellelectrolytic cellelectrolytic cellelectrolytic cell-electric current used to cause electric current used to cause electric cur

21、rent used to cause electric current used to cause chemical change.chemical change.chemical change.chemical change.Batteries are voltaic Batteries are voltaic Batteries are voltaic Batteries are voltaic cellscellscellscells1010AnodeAnodeAnodeAnodeCathodeCathodeCathodeCathodeBasic Concepts Basic Conce

22、pts of Electrochemical Cellsof Electrochemical CellsZnZn2+ionsCuCu2+ionswiresaltbridgee e l le e c c t tr ro o n n s s1111CHEMICAL CHANGE-ELECTRIC CURRENTCHEMICAL CHANGE-CHEMICAL CHANGE-ELECTRIC CURRENTELECTRIC CURRENTZn metalCu2+ionsWith time,Cu plates out onto Zn metal strip,and Zn strip“disappear

23、s.”With time,Cu plates out onto Zn metal strip,and Zn strip“disappears.”Zn is oxidized Zn is oxidized and is the reducing agent and is the reducing agent Zn(sZn(s)-Zn)-Zn2+2+(aq)+2e-(aq)+2e-CuCu2+2+is reduced is reduced and is the oxidizing agentand is the oxidizing agentCuCu2+2+(aq)+2e-(aq)+2e-Cu(s

24、Cu(s)1212 To obtain a useful To obtain a useful current,we separate the current,we separate the oxidizing and reducing oxidizing and reducing agents so that electron agents so that electron transfer occurs thru an transfer occurs thru an external wire.external wire.ZnZn2+ionsCuCu2+ionswiresaltbridge

25、e el l e ec ct tr ro on ns sCHEMICAL CHANGE-ELECTRIC CURRENTCHEMICAL CHANGE-CHEMICAL CHANGE-ELECTRIC CURRENTELECTRIC CURRENTThis is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell.cell.A group of such cells is called a A group of such cells is called a batteryb

26、attery.http:/ el l e ec ct tr ro on ns sElectrons travel thru external wire.Salt bridge allows anions and cations to move between electrode compartments.Electrons travel thru external wire.Electrons travel thru external wire.Salt bridge Salt bridge allows anions and allows anions and cationscations

27、to move to move between electrode compartments.between electrode compartments.Zn-ZnZn-Zn2+2+2e-+2e-CuCu2+2+2e-Cu+2e-Cu-Anions-OxidationAnodeNegativeOxidationOxidationAnodeAnodeNegativeNegativeReductionCathodePositiveRedReductionuctionCatCathodehodePositivePositiveRED CATRED CAT1414Terms Used for Vol

28、taic CellsTerms Used for Voltaic Cells1515CELL POTENTIAL,ECELL POTENTIAL,E For Zn/Cu cell,For Zn/Cu cell,potentialpotential is is+1.10 V+1.10 V at 25 at 25 C C and when Znand when Zn2+2+and Cu and Cu2+2+=1.0 M.=1.0 M.This is the This is the STANDARD CELL STANDARD CELL POTENTIAL,POTENTIAL,E Eo o a qu

29、antitative measure of the tendency of a quantitative measure of the tendency of reactants to proceed to products when all reactants to proceed to products when all are in their standard states at 25 are in their standard states at 25 C.C.ZnZn2+ionsCuCu2+ionswiresaltbridgee el l e ec ct tr ro on ns s

30、1616Calculating Cell VoltageCalculating Cell Voltage Balanced half-reactions can be added Balanced half-reactions can be added together to get overall,balanced together to get overall,balanced equation.equation.Zn(s)-Zn2+(aq)+2e-Cu2+(aq)+2e-Cu(s)-Cu2+(aq)+Zn(s)-Zn2+(aq)+Cu(s)Zn(sZn(s)-Zn)-Zn2+2+(aq)

31、+2e-(aq)+2e-CuCu2+2+(aq)+2e-(aq)+2e-Cu(sCu(s)-CuCu2+2+(aq)+(aq)+Zn(sZn(s)-Zn)-Zn2+2+(aq)+(aq)+Cu(sCu(s)If we know If we know E Eo o for each half-reaction,we for each half-reaction,we could get could get E Eo o for net reaction.for net reaction.1717TABLE OF STANDARD REDUCTION POTENTIALSTABLE OF STAN

32、DARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS 2 2 E Eo o(V)(V)CuCu2+2+2e-Cu +2e-Cu+0.34+0.342 H2 H+2e-H +2e-H 0.000.00ZnZn2+2+2e-Zn +2e-Zn-0.76-0.76oxidizingoxidizingability of ionability of ionreducing abilityreducing abilityof elementof elementTo determine an oxidation To determi

33、ne an oxidation from a reduction table,just from a reduction table,just take the opposite sign of the take the opposite sign of the reduction!reduction!1818Zn/Cu Electrochemical CellZn/Cu Electrochemical CellZn(sZn(s)-Zn)-Zn2+2+(aq)+2e-(aq)+2e-E Eo o=+0.76 V=+0.76 VCuCu2+2+(aq)+2e-(aq)+2e-Cu(sCu(s)E

34、 Eo o=+0.34 V=+0.34 V-CuCu2+2+(aq)+(aq)+Zn(sZn(s)-Zn)-Zn2+2+(aq)+(aq)+Cu(sCu(s)E Eo o =+1.10 V =+1.10 VCathode,Cathode,positive,positive,sink for sink for electronselectronsAnode,Anode,negative,negative,source of source of electronselectronsZnZn2+ionsCuCu2+ionswiresaltbridgee el l e ec ct tr ro on n

35、s s+19192020VoltsCdSalt BridgeCd2+FeFe2+CdCd -Cd -Cd2+2+2e-+2e-ororCdCd2+2+2e-+2e-CdCdFe -FeFe -Fe2+2+2e-+2e-ororFeFe2+2+2e-Fe+2e-FeE Eo o for a Voltaic Cell for a Voltaic CellAll ingredients are present.Which way does All ingredients are present.Which way does reaction proceed?reaction proceed?2121

36、From the table,you see From the table,you see Fe is a better reducing Fe is a better reducing agent than agent than CdCd CdCd2+2+is a better is a better oxidizing agent than oxidizing agent than FeFe2+2+VoltsCdSalt BridgeCd2+FeFe2+E Eo o for a Voltaic Cell for a Voltaic Cell2222More About More About

37、 Calculating Cell VoltageCalculating Cell VoltageAssume IAssume I-ion can reduce water.ion can reduce water.2 H2 H2 2O +2e-O +2e-H H2 2 +2 OH +2 OH-CathodeCathode2 I2 I-I I2 2 +2e-+2e-AnodeAnode-2 I2 I-+2 H +2 H2 2O O -I I2 2 +2 OH+2 OH-+H +H2 22 H2 H2 2O +2e-O +2e-H H2 2 +2 OH +2 OH-CathodeCathode2

38、 I2 I-I I2 2 +2e-+2e-AnodeAnode-2 I2 I-+2 H +2 H2 2O O -I I2 2 +2 OH+2 OH-+H +H2 2Assuming reaction occurs as written,Assuming reaction occurs as written,E E =E E catcat+E E anan=(-0.828 V)-(-+0.535 V)=(-0.828 V)-(-+0.535 V)=-1.363 V-1.363 VMinus EMinus E means means rxnrxn.occurs in opposite direct

39、ion.occurs in opposite direction(the connection is backwards or you are(the connection is backwards or you are recharging the battery)recharging the battery)2323Charging a BatteryCharging a BatteryWhen you charge a battery,you are When you charge a battery,you are forcing the electrons backwards(fro

40、m forcing the electrons backwards(from the+to the-).To do this,you will the+to the-).To do this,you will need a higher voltage backwards than need a higher voltage backwards than forwards.This is why the ammeter in forwards.This is why the ammeter in your car often goes slightly higher your car ofte

41、n goes slightly higher while your battery is charging,and while your battery is charging,and then returns to normal.then returns to normal.In your car,the battery charger is In your car,the battery charger is called an alternator.If you have a called an alternator.If you have a dead battery,it could

42、 be the dead battery,it could be the battery needs to be replaced OR battery needs to be replaced OR the alternator is not charging the the alternator is not charging the battery properly.battery properly.2424Dry Cell BatteryDry Cell BatteryAnode(-)Anode(-)Zn-ZnZn-Zn2+2+2e-+2e-Cathode(+)Cathode(+)2

43、NH2 NH4 4+2e-+2e-2 NH2 NH3 3 +H +H2 22525Alkaline BatteryAlkaline BatteryNearly same reactions as Nearly same reactions as in common dry cell,but in common dry cell,but under basic conditions.under basic conditions.Anode(-):Anode(-):Zn+2 OHZn+2 OH-ZnOZnO+H+H2 2O+2e-O+2e-Cathode(+):Cathode(+):2 MnO2

44、MnO2 2+H+H2 2O +2e-O +2e-MnMn2 2OO3 3+2 OH+2 OH-2626Mercury BatteryMercury BatteryAnode:Anode:Zn is reducing agent under basic conditionsZn is reducing agent under basic conditionsCathode:Cathode:HgOHgO +H +H2 2O +2e-Hg +2 OHO +2e-Hg +2 OH-2727Lead Storage BatteryLead Storage BatteryAnode(-)Anode(-)

45、E Eo o=+0.36 V=+0.36 VPbPb+HSO+HSO4 4-PbSO -PbSO4 4+H+H+2e-+2e-Cathode(+)Cathode(+)E Eo o =+1.68 V =+1.68 VPbOPbO2 2+HSO+HSO4 4-+3 H+3 H+2e-+2e-PbSO-PbSO4 4+2 H+2 H2 2OO2828Ni-Cad BatteryNi-Cad BatteryAnode(-)Anode(-)CdCd +2 OH +2 OH-Cd(OH)-Cd(OH)2 2 +2e-+2e-Cathode(+)Cathode(+)NiO(OHNiO(OH)+H)+H2 2

46、O+e-Ni(OH)O+e-Ni(OH)2 2+OH+OH-2929HH2 2 as a Fuel as a FuelCars can use electricity generated by HCars can use electricity generated by H2 2/O/O2 2 fuel cells.fuel cells.H H2 2 carried in tanks or generated from carried in tanks or generated from hydrocarbonshydrocarbons3030Balancing Equations Balan

47、cing Equations for for RedoxRedox Reactions ReactionsSome Some redoxredox reactions have equations that must be balanced by reactions have equations that must be balanced by special techniques.special techniques.MnOMnO4 4-+5 Fe +5 Fe2+2+8 H +8 H+-Mn-Mn2+2+5 Fe+5 Fe3+3+4 H +4 H2 2OOMnMn=+7=+7Fe=+2Fe=

48、+2Fe=+3Fe=+3MnMn=+2=+23131Balancing EquationsBalancing EquationsConsider the Consider the reduction of Agreduction of Ag+ions with copper ions with copper metal.metal.Cu +AgCu +Ag+-give-Cu -give-Cu2+2+Ag +Ag3232Balancing EquationsBalancing EquationsStep 1:Step 1:Divide the reaction into half-reactio

49、ns,one Divide the reaction into half-reactions,one for oxidation and the other for reduction.for oxidation and the other for reduction.OxOxCu -CuCu -Cu2+2+RedRed Ag Ag+-Ag -AgStep 2:Step 2:Balance each element for mass.Already Balance each element for mass.Already done in this case.done in this case

50、.Step 3:Step 3:Balance each half-reaction for charge by Balance each half-reaction for charge by adding electrons.adding electrons.OxOx Cu -Cu Cu -Cu2+2+2e-2e-RedRed Ag Ag+e-e-Ag-Ag3333Balancing EquationsBalancing EquationsStep 4:Step 4:Multiply each half-reaction by a factor so Multiply each half-r