Ch18 electrochemistry

1. 11 ELECTROCHEMISTRYELECTROCHEMISTRY Chapter 18Chapter 18 SAVE 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 (Tools>Options>Print>UNch eck "Background Printing")!

2. 22 Electron Transfer ReactionsElectron Transfer Reactions • Electron transfer reactions areElectron transfer reactions are oxidation-reductionoxidation-reduction oror redoxredox reactions.reactions. • Results in the generation of anResults in the generation of an electric current (electricity) or beelectric current (electricity) or be caused by imposing an electriccaused by imposing an electric current.current. • Therefore, this field of chemistry isTherefore, this field of chemistry is

3. 33 Terminology for RedoxTerminology for Redox ReactionsReactions • OXIDATIONOXIDATION—loss of electron(s) by a—loss of electron(s) by a species; increase in oxidation number;species; increase in oxidation number; increase in oxygen.increase in oxygen. • REDUCTIONREDUCTION—gain of electron(s);—gain of electron(s); decrease in oxidation number; decrease indecrease in oxidation number; decrease in oxygen; increase in hydrogen.oxygen; increase in hydrogen. • OXIDIZING AGENTOXIDIZING AGENT—electron acceptor;—electron acceptor; species is reduced.species is reduced. • REDUCING AGENTREDUCING AGENT—electron donor;—electron donor; species is oxidized.species is oxidized. • OXIDATIONOXIDATION—loss of electron(s) by a—loss of electron(s) by a species; increase in oxidation number;species; increase in oxidation number; increase in oxygen.increase in oxygen. • REDUCTIONREDUCTION—gain of electron(s);—gain of electron(s); decrease in oxidation number; decrease indecrease in oxidation number; decrease in oxygen; increase in hydrogen.oxygen; increase in hydrogen. • OXIDIZING AGENTOXIDIZING AGENT—electron acceptor;—electron acceptor; species is reduced.species is reduced. • REDUCING AGENTREDUCING AGENT—electron donor;—electron donor; species is oxidized.species is oxidized.

4. 44 You can’t have one… without the other! • Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. • You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation LEOLEO the lion saysthe lion says GERGER!! GER!GER!

5. 55 Another way to remember •OIL RIG

6. 66 OXIDATION-REDUCTION REACTIONS Direct RedoxDirect Redox ReactionReaction Oxidizing andOxidizing and reducing agents inreducing agents in direct contact.direct contact. Cu(s) + 2 AgCu(s) + 2 Ag++ (aq)(aq) ---> Cu---> Cu2+2+ (aq) + 2(aq) + 2 Ag(s)Ag(s)

7. 77 OXIDATION-REDUCTION REACTIONS Indirect Redox ReactionIndirect Redox Reaction A battery functions by transferringA battery functions by transferring electrons through an external wire fromelectrons through an external wire from the reducing agent to the oxidizingthe reducing agent to the oxidizing agent.agent.

8. 88 Why Study Electrochemistry?Why Study Electrochemistry? • BatteriesBatteries • CorrosionCorrosion • IndustrialIndustrial production ofproduction of chemicalschemicals such assuch as ClCl22, NaOH, F, NaOH, F22 andand AlAl • Biological redoxBiological redox reactionsreactions The heme groupThe heme group

9. 99 Electrochemical CellsElectrochemical Cells • An apparatus thatAn apparatus that allows a redox reactionallows a redox reaction to occur by transferringto occur by transferring electrons through anelectrons through an external connector.external connector. • Product favoredProduct favored reaction --->reaction ---> voltaic orvoltaic or galvanic cellgalvanic cell ---->----> electric currentelectric current • Reactant favoredReactant favored reaction --->reaction ---> electrolyticelectrolytic cellcell ---> electric current---> electric current used to cause chemicalused to cause chemical Batteries areBatteries are voltaic cellsvoltaic cells

10. 1010 AnodeAnode CathodCathod ee Basic ConceptsBasic Concepts of Electrochemical Cellsof Electrochemical Cells Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons

11. 1111 CHEMICAL CHANGE --->CHEMICAL CHANGE ---> ELECTRIC CURRENTELECTRIC CURRENT Zn metal Cu2+ ions With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.” With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.” •Zn is oxidizedZn is oxidized and is the reducing agentand is the reducing agent Zn(s) ---> ZnZn(s) ---> Zn2+2+ (aq) + 2e-(aq) + 2e- •CuCu2+2+ is reducedis reduced and is the oxidizing agentand is the oxidizing agent CuCu2+2+ (aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)

12. 1212 •To obtain a usefulTo obtain a useful current, we separate thecurrent, we separate the oxidizing and reducingoxidizing and reducing agents so that electronagents so that electron transfer occurs thru antransfer occurs thru an external wire.external wire. Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons CHEMICAL CHANGE --->CHEMICAL CHANGE ---> ELECTRIC CURRENTELECTRIC CURRENT This is accomplished in aThis is accomplished in a GALVANICGALVANIC oror VOLTAICVOLTAIC cell.cell. A group of such cells is called aA group of such cells is called a batterybattery.. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

13. 1313 Z n Z n2+ ions C u C u2+ ions wire salt bridge electrons Z n Z n2+ ions C u C u2+ ions wire salt bridge electrons ••Electrons travel thru external wire.Electrons travel thru external wire. •Salt bridgeSalt bridge allows anions and cations to moveallows anions and cations to move between electrode compartments.between electrode compartments. ••Electrons travel thru external wire.Electrons travel thru external wire. •Salt bridgeSalt bridge allows anions and cations to moveallows anions and cations to move between electrode compartments.between electrode compartments. Zn --> ZnZn --> Zn2+2+ + 2e-+ 2e- CuCu2+2+ + 2e- --> Cu+ 2e- --> Cu <--Anions<--Anions Cations-->Cations--> OxidationOxidation AnodeAnode NegativeNegative OxidationOxidation AnodeAnode NegativeNegative RedReductionuction CatCathodehode PositivePositive RedReductionuction CatCathodehode PositivePositive RED CATRED CAT

14. 1414 Terms Used for Voltaic CellsTerms Used for Voltaic Cells

15. 1515 CELL POTENTIAL, ECELL POTENTIAL, E • For Zn/Cu cell,For Zn/Cu cell, potentialpotential isis +1.10 V+1.10 V at 25 ˚Cat 25 ˚C and when [Znand when [Zn2+2+ ] and [Cu] and [Cu2+2+ ] = 1.0 M.] = 1.0 M. • This is theThis is the STANDARD CELLSTANDARD CELL POTENTIAL, EPOTENTIAL, Eoo • ——a quantitative measure of the tendency ofa quantitative measure of the tendency of reactants to proceed to products when allreactants to proceed to products when all are in their standard states at 25 ˚C.are in their standard states at 25 ˚C. Z n Z n2+ ions C u C u2+ ions wire salt bridge electrons Z n Z n2+ ions C u C u2+ ions wire salt bridge electrons

16. 1616 Calculating Cell VoltageCalculating Cell Voltage • Balanced half-reactions can be addedBalanced half-reactions can be added together to get overall, balancedtogether to get overall, balanced equation.equation. Zn(s) ---> ZnZn(s) ---> Zn2+2+ (aq) + 2e-(aq) + 2e- CuCu2+2+ (aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) ---------------------------------------------------------------------------------------- CuCu2+2+ (aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+ (aq) + Cu(s)(aq) + Cu(s) Zn(s) ---> ZnZn(s) ---> Zn2+2+ (aq) + 2e-(aq) + 2e- CuCu2+2+ (aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) ---------------------------------------------------------------------------------------- CuCu2+2+ (aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+ (aq) + Cu(s)(aq) + Cu(s) If we know EIf we know Eoo for each half-reaction, wefor each half-reaction, we could get Ecould get Eoo for net reaction.for net reaction.

17. 1717 TABLE OF STANDARDTABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS 2 Eo (V) Cu2+ + 2e- Cu +0.34 2 H+ + 2e- H 0.00 Zn 2++ 2e- Zn -0.76 oxidizing ability of ion reducing ability of element To determine an oxidationTo determine an oxidation from a reduction table, justfrom a reduction table, just take the opposite sign of thetake the opposite sign of the reduction!reduction!

18. 1818 Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell Zn(s) ---> ZnZn(s) ---> Zn2+2+ (aq) + 2e-(aq) + 2e- EEoo = +0.76 V= +0.76 V CuCu2+2+ (aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V= +0.34 V ------------------------------------------------------------------------------------------------------------------------------ CuCu2+2+ (aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+ (aq) + Cu(s)(aq) + Cu(s) EEoo = +1.10 V= +1.10 V Cathode,Cathode, positive,positive, sink forsink for electronselectrons Anode,Anode, negative,negative, source ofsource of electronselectrons Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons Zn Zn2+ ions Cu Cu2+ ions wire salt bridge elect rons ++

19. 1919

20. 2020 V olts Cd Salt Bridge Cd2+ Fe Fe2+ V olts Cd Salt Bridge Cd2+ Fe Fe2+ Cd --> CdCd --> Cd2+2+ + 2e-+ 2e- oror CdCd2+2+ + 2e- --> Cd+ 2e- --> Cd Fe --> FeFe --> Fe2+2+ + 2e-+ 2e- oror FeFe2+2+ + 2e- --> Fe+ 2e- --> Fe EEoo for a Voltaic Cellfor a Voltaic Cell All ingredients are present. Which way doesAll ingredients are present. Which way does reaction proceed?reaction proceed?

21. 2121 From the table, you seeFrom the table, you see •• Fe is a better reducingFe is a better reducing agent than Cdagent than Cd •• CdCd2+2+ is a better oxidizingis a better oxidizing agent than Feagent than Fe2+2+ V olts Cd Salt Bridge Cd2+ Fe Fe2+ V olts Cd Salt Bridge Cd2+ Fe Fe2+ EEoo for a Voltaic Cellfor a Voltaic Cell

22. 2222 More AboutMore About Calculating Cell VoltageCalculating Cell Voltage Assume IAssume I-- ion can reduce water.ion can reduce water. 2 H2O + 2e- ---> H2 + 2 OH- Cathode 2 I- ---> I2 + 2e- Anode ------------------------------------------------- 2 I- + 2 H2O --> I2 + 2 OH- + H2 2 H2O + 2e- ---> H2 + 2 OH- Cathode 2 I- ---> I2 + 2e- Anode ------------------------------------------------- 2 I- + 2 H2O --> I2 + 2 OH- + H2Assuming 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 V Minus E˚ means rxn. occurs in opposite directionMinus E˚ means rxn. occurs in opposite direction (the connection is backwards or you are(the connection is backwards or you are recharging the battery)recharging the battery)

23. 2323 Charging a BatteryCharging a Battery When you charge a battery, you areWhen you charge a battery, you are forcing the electrons backwards (fromforcing the electrons backwards (from the + to the -). To do this, you willthe + to the -). To do this, you will need a higher voltage backwards thanneed a higher voltage backwards than forwards. This is why the ammeter inforwards. This is why the ammeter in your car often goes slightly higheryour car often goes slightly higher while your battery is charging, and thenwhile your battery is charging, and then returns to normal.returns to normal. In your car, the battery charger isIn your car, the battery charger is called an alternator. If you have acalled an alternator. If you have a dead battery, it could be thedead battery, it could be the battery needs to be replaced ORbattery needs to be replaced OR the alternator is not charging thethe alternator is not charging the battery properly.battery properly.

24. 2424 Dry Cell BatteryDry Cell Battery Anode (-)Anode (-) Zn ---> ZnZn ---> Zn2+2+ + 2e-+ 2e- Cathode (+)Cathode (+) 2 NH2 NH44 ++ + 2e- --->+ 2e- ---> 2 NH2 NH33 + H+ H22

25. 2525 Alkaline BatteryAlkaline Battery Nearly same reactions asNearly same reactions as in common dry cell, butin common dry cell, but under basic conditions.under basic conditions. Anode (-):Anode (-): Zn + 2 OHZn + 2 OH-- ---> ZnO + H---> ZnO + H22O + 2e-O + 2e- Cathode (+):Cathode (+): 2 MnO2 MnO22 + H+ H22O + 2e- --->O + 2e- ---> MnMn22OO33 + 2 OH+ 2 OH--

26. 2626 Mercury BatteryMercury Battery Anode:Anode: Zn is reducing agent under basic conditionsZn is reducing agent under basic conditions Cathode:Cathode: HgO + HHgO + H22O + 2e- ---> Hg + 2 OHO + 2e- ---> Hg + 2 OH--

27. 2727 Lead Storage BatteryLead Storage Battery Anode (-)Anode (-) EEoo = +0.36 V= +0.36 V Pb + HSOPb + HSO44 -- ---> PbSO---> PbSO44 + H+ H++ + 2e-+ 2e- Cathode (+)Cathode (+) EEoo = +1.68 V= +1.68 V PbOPbO22 + HSO+ HSO44 -- + 3 H+ 3 H++ + 2e-+ 2e- ---> PbSO---> PbSO44 + 2 H+ 2 H22OO

28. 2828 Ni-Cad BatteryNi-Cad Battery Anode (-)Anode (-) Cd + 2 OHCd + 2 OH-- ---> Cd(OH)---> Cd(OH)22 + 2e-+ 2e- Cathode (+)Cathode (+) NiO(OH) + HNiO(OH) + H22O + e- ---> Ni(OH)O + e- ---> Ni(OH)22 + OH+ OH--

29. 2929 HH22 as a Fuelas a Fuel Cars can use electricity generated by HCars can use electricity generated by H22/O/O22 fuel cells.fuel cells. HH22 carried in tanks or generated fromcarried in tanks or generated from hydrocarbonshydrocarbons

30. 3030 Balancing EquationsBalancing Equations for Redox Reactionsfor Redox Reactions Some redox reactions have equations that must be balanced bySome redox reactions have equations that must be balanced by special techniques.special techniques. MnOMnO44 -- + 5 Fe+ 5 Fe2+2+ + 8 H+ 8 H++ ---> Mn---> Mn2+2+ + 5 Fe+ 5 Fe3+3+ + 4 H+ 4 H22OO Mn = +7Mn = +7 Fe = +2Fe = +2 Fe = +3Fe = +3Mn = +2Mn = +2

31. 3131 Balancing EquationsBalancing Equations Consider theConsider the reduction of Agreduction of Ag++ ionsions with copper metal.with copper metal. Cu + AgCu + Ag++ --give--> Cu--give--> Cu2+2+ + Ag+ Ag

32. 3232 Balancing EquationsBalancing Equations Step 1:Step 1: Divide the reaction into half-reactions, oneDivide the reaction into half-reactions, one for oxidation and the other for reduction.for oxidation and the other for reduction. OxOx Cu ---> CuCu ---> Cu2+2+ RedRed AgAg++ ---> Ag---> Ag Step 2:Step 2: Balance each element for mass. AlreadyBalance each element for mass. Already done in this case.done in this case. Step 3:Step 3: Balance each half-reaction for charge byBalance each half-reaction for charge by adding electrons.adding electrons. OxOx Cu ---> CuCu ---> Cu2+2+ ++ 2e-2e- RedRed AgAg++ ++ e-e- ---> Ag---> Ag

33. 3333 Balancing EquationsBalancing Equations Step 4:Step 4: Multiply each half-reaction by a factor soMultiply each half-reaction by a factor so that the reducing agent supplies as many electronsthat the reducing agent supplies as many electrons as the oxidizing agent requires.as the oxidizing agent requires. Reducing agentReducing agent Cu ---> CuCu ---> Cu2+2+ + 2e-+ 2e- Oxidizing agentOxidizing agent 22 AgAg++ ++ 22 e- --->e- ---> 22 AgAg Step 5:Step 5: Add half-reactions to give the overallAdd half-reactions to give the overall equation.equation. Cu + 2 AgCu + 2 Ag++ ---> Cu---> Cu2+2+ + 2Ag+ 2Ag The equation is now balanced for bothThe equation is now balanced for both charge and mass.charge and mass.

34. 3434 Balancing EquationsBalancing Equations Balance the following in acid solution—Balance the following in acid solution— VOVO22 ++ + Zn ---> VO+ Zn ---> VO2+2+ + Zn+ Zn2+2+ Step 1:Step 1: Write the half-reactionsWrite the half-reactions OxOx Zn ---> ZnZn ---> Zn2+2+ RedRed VOVO22 ++ ---> VO---> VO2+2+ Step 2:Step 2: Balance each half-reaction forBalance each half-reaction for mass.mass. OxOx Zn ---> ZnZn ---> Zn2+2+ RedRed VOVO22 ++ ---> VO---> VO2+2+ ++ HH22OO2 H2 H++ ++ Add HAdd H22O on O-deficient side and add HO on O-deficient side and add H++ on other side for H-balance.on other side for H-balance.

35. 3535 Balancing EquationsBalancing Equations Step 3:Step 3: Balance half-reactions for charge.Balance half-reactions for charge. OxOx Zn ---> ZnZn ---> Zn2+2+ ++ 2e-2e- RedRed e-e- + 2 H+ 2 H++ + VO+ VO22 ++ ---> VO---> VO2+2+ + H+ H22OO Step 4:Step 4: Multiply by an appropriate factor.Multiply by an appropriate factor. OxOx Zn ---> ZnZn ---> Zn2+2+ ++ 2e-2e- RedRed 22e-e- ++ 44 HH++ ++ 22 VOVO22 ++ --->---> 22 VOVO2+2+ ++ 22 HH22OO Step 5:Step 5: AddAdd balancedbalanced half-reactionshalf-reactions Zn + 4 HZn + 4 H++ + 2 VO+ 2 VO22 ++ ---> Zn---> Zn2+2+ + 2 VO+ 2 VO2+2+ + 2 H+ 2 H22OO

36. 3636 Tips on Balancing EquationsTips on Balancing Equations • Never add ONever add O22, O atoms, or, O atoms, or OO2-2- to balance oxygen.to balance oxygen. • Never add HNever add H22 or H atoms toor H atoms to balance hydrogen.balance hydrogen. • Be sure to write the correctBe sure to write the correct charges on all the ions.charges on all the ions. • Check your work at the endCheck your work at the end to make sure mass andto make sure mass and charge are balanced.charge are balanced. • PRACTICE!PRACTICE!

Ch18 electrochemistry

Ch18 electrochemistry

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