Chap.20 Voltaic Cells (Galvanic Cells) The energy released in a spontaneous redox reaction can be used to perform electrical work △G=△H-T△S Voltaic or Galvanic cells are devices in which electron transfer occurs via an external circuit rather than directly between reactants. Switch 1.10 The actual charge on the Voltmeter electrode are zero. anode Na Cu Anode:the NO cathode electrode at Zn2+ which the NO Cathode:the electrode oxidation Zn(s)-Zn2i (ag)+2e Cu2 (ng)+2e Culs) at which the reduction occurs Movement of cations occurs Movement of anions
Chap.20 Voltaic Cells (Galvanic Cells) The energy released in a spontaneous spontaneous redox reaction reaction can be used to perform electrical work. Voltaic or Galvanic cells are devices in which electron transfer occurs via an external circuit rather than directly between reactants. The actual charge on the ∆ = ∆ − ∆ G H T S Cathode: the electrode at which the reduction occurs Anode: the electrode at which the oxidation occurs The actual charge on the electrode are zero
Cell EMF The difference in electrical potential between the anode and cathode is .Electromotive Force (EMF) .Cell Potential (Ece Cell Voltage (Ece Electromotive Force (EMF):The force required to push electrons through the external circuit anode 1/2H,(g）+Fe3+(aq)→H+(aq)+Fe2+(ag) nn metal =Ecathode-Eamnode-E redox couple ZnS0, CUSOA
• Electromotive Force (EMF) • Cell Potential (Ecell) •Cell Voltage (Ecell) Cell EMF The difference in electrical potential between the anode and cathode is : E E E cell cathode anode ° ° ° = − Electromotive Force (EMF): The force required to push electrons through the external circuit 3 2 2 / / cell cathode anode Fe Fe H H E E E E E + + + ° ° ° ° ° = − = − 3 2 2 1 2 ( ) ( ) ( ) ( ) H g Fe aq H aq Fe aq + + + + → + redox couple
Cell EMF Standard reduction potential Voltmeter anode Zinc The difference in potential energy per anode K Copper cathode Salt bridge electrical charge between two electrodes is measured in units of volts. Cotton SO plugs 224 ZnSOsolution CuSO,solution The charge ofone coulomb (1 C)falling though a potential difference of one volt (1 V)releases one joule (1J)ofenergy. 1V×1C=1J The maximum work that an electron can do is equal to its charge times the difference in electrical potential through which it falls
Cell EMF The difference in potential energy potential energy per electrical charge between two electrodes is measured in units of volts. E E E cell cathode anode ° ° ° = − Standard Standard reduction reduction potential 1 J 1 V 1 C = The charge of one coulomb (1 C) falling though a potential difference of one volt (1 V) releases one joule (1J) of energy. • The maximum work that an electron can do is equal to its charge times the difference in electrical potential through which it falls. 1 V 1C 1J × =
Cell EMF Standard reduction potential (Era):the voltage associated with a reduction reaction at an electrode when all solutes present at I M and all gases at I atm. Standard potential (Erd )a measure Voltmeter ofelectron-pulling power of a single electrode Zinc anode CI K Copper cathode Salt bridge Zn"(aq)+2eZn(s)E=-0.76V Cu(aq)+2eCu(s)Eo=+0.34V Cotton plugs Zn2 SO =B -E° ZnSOa solution CuSOa solution anode In a voltaic cell,the electrodes pulling in opposite directions,so the overall pulling power of a cell,the cell standard EMF,is the difference of the standard potential of two electrodes
Cell EMF Standard Standard reduction reduction potential ( ) : the voltage associated with a reduction reaction at an electrode when all solutes present at 1 M and all gases at 1 atm. Standard potential ( ) : a measure of electron electron-pulling power pulling power of a single electrode Ered ° Ered ° In a voltaic cell, the electrodes pulling in opposite directions, so the overall pulling power of a cell, the cell standard EMF, is the difference of the standard potential of two electrodes. E E E cell cathode anode ° ° ° = − 2 Zn aq e Zn s ( ) 2 ( ) + + → 2 Cu aq e Cu s ( ) 2 ( ) + + → 2 / 0.76 Zn Zn E V + ° = − 2 / 0.34 Cu Cu E V + ° = +
Page:1128 Standard Reduction Potentials at 25C* Half-Reaction E'(V) The more positive Eed the F2(g)+2e 2F(aq) +2.87 O,(g)+2H ag)+2e- O(g)+H.O +2.07 stronger the oxidizing agent Co(ag)+ →co2+(ag) +1.82 H202(ag)+H*(aq)+2e →2H0 +1.77 PbO2(s)+4H (aq)+SO (ag)+2e →P%S04()+2H0+1.70 on the left side. Ce+(ag)+ →Ce3+(ag) +1.61 MnO (aq) 8H'(ag)+5e →Mn2+(ag)+4H0 +1.51 Au(aq)+ Au(s) +1.50 Cl(g)+2e +2C1(ag) +136 CrzO月(ag) 14H(ag)+6e- →2Cr23+(aq)+7H20 +1.33 MnOx(s)+ H (ag)2e Mn2(aq)2H2O +1.23 The more positive the E O2(g)+4H ag)4e ◆2H20 +1.23 Bra()+2e →2Br(ag) +1.07 NO3(ag)+ H*(aq)3e →NO(g)+2H2O +0.96 the greater the electron- 2Hg2+(ag)+ 2 →Hg*(a +0.92 Hg(ag)+ →2Hg(0 +0.85 Ag"(ag)+ ◆Ag(s) +0.80 pulling power of the half Fe(aq)+ →Fe2t(ag) +0.77 O2(g)+2H" ag)2e- H2Oag) +0.68 MnO (ag)+2H2O 3e →MnO2(s)+4OH(ag) +059 reducing reaction. 12(s)+2e 21(q) +0.53 02(g)+2H2 +4e- 40H(aq) +0.40 Cu(ag)+ Cu(s) +0.34 AgCl(s)+ →Ags)+ClT(aq) +0.22 2= 0.20 oxidation state reduction state 0.15 0.13 The more negative the E 0.00 oxidizing agent reducing agent 0.13 0.14 the greater the electron- Ni2(aq)2e →Nis) -0.25 Co2(ag)2e →Co(s) -0.28 PbSOa(s)+2e >Pb(s)+ (aq) -0.31 donating power of the half Cd2*(aq)+2e →Cds) -0.40 Fe2(aq)2e Fe(s) -0.44 C3+(ag)+3e →Crts) -0.74 reducing reaction. Zn2(aq)+2e Zn(s) -0.76 2H0+2e H-(g)+20(aq) -0.83 Mn2+(aq)2e Mn(s) -1.18 Al+(aq)+3e →AI(s) -1.66 Be2(aq)+2e →Be(s) -1.85 Mg2"(aq)+2e →Mg() -2.37 Na (ag)+e Na(s) -2.71 Ca(ag)+2e The more negative of E, Ca(s) -2.87 Sr2*(aq)+2e →Srs) 2.89 Ba(aq)+2e →Ba(s) -2.90 the stronger the reducing K*(ag)+e K(s) -2.93 Li(aq)+e Li(s) -3.05 agent on the right side. "For all half-reactions the conc species and the pressure is I atm for gases.These are the standard-state values
The more positive E°red , the stronger the oxidizing agent on the left side. The more positive the E°red , the greater the electronpulling power of the half reducing reaction. Page:1128 The more negative of E°red , the stronger the reducing agent on the right side. The more negative the E°red, the greater the electrondonating power of the half reducing reaction. oxidizing agent reducing agent oxidation state / reduction state
Half-Reaction Page:1128 E(V) Ep.:Give an F(g)+2e→2F(ag) +2.87 increasing Ox(g)+2H"(aq)+2e-02(g)+H2O +2.07 Co“(ag)+e→Co2*(ag +1.82 order of the H2Oz(ag)+2H*(ag)+2e-2H2O +177 following PbOx(s)+4H"(ag)+SO(ag)+2e-PbSO(s)+2H2O +1.70 oxidizing Cet(aa)+eCe(ao) MnO (aq)8H"(aq)+5e →Mn2(ag)+4Hs0 agents: AU(@g+3e→AuS F1.0 C(g)+2e→2C1(aq) +136 FeCl,KMnO(H*) Cr2-(ag)14H*(ag)+6e--2Cr(aq)+7H2O +1.33 I2,K2C5O,(H+) Mno-(s)+4H(ag+2eMn2*(aq)+2H20 +123 0(g)+4H(aq)+4e→2H,0 +1.23 BT+2e→2Br(a网 NOj(aq)+4H(aq)3e-NO(g)+2H2O +0.96 2Hg2*(aq)+2e→Hgi(aq） +0.92 Ep.:Give an Hg3(aq)+2e→2Hg0 +0.85 increasing Ag'(ag)+e→Ag(s) +0.80 order of the Fe3+(ag)+e→Fe2+(ag) +0.77 O(g)+2H"(ag)+2eHO(ag) +0.68 following MnO (ag)+2H2O +3e-MnOz(s)+40H (ag) +0.59 reducing agents: 4■ 0(g)+2H,0+4e→40H(ag) +0.40 KI,FeCl,,Cu,Ag Cu(aq)+2e→Cu(S) AgCl(s)+e-Ag(s)+Cl(aq) +0.22 S0(aq)+4H'(aq)+2e→S0(g)+2H,0 +0.20 Cu2*(ag)+e→Cu(ag) +0.15 sn(ag)+2e→Sn2+(ag) +0.13 2H'(ag)+2e→HR) 0.00
3 4 2 2 2 7 , ( ) , ( ) FeCl KMnO H I K Cr O H + + Ep.: Give an increasing order of the following oxidizing agents: Ep.: Give an Page:1128 2 KI FeCl Cu Ag , , , Ep.: Give an increasing order of the following reducing agents:
Standard reduction Potentials at 25C 2H(ag)+2e→Hg) 0.00 Pb2*(aq)+2e →Pb(s) -0.13 The better oxidizer sn2*(aq)+2e→Sn -0.14 Nr2*(aq)+2e→Ni() CoCL2 or Cr(NO)3 -0.25 Co2*(aq)+2e →Cs) -0.28 PbSO (s)+2e →Pbs)+s0i(ag) -031 Cd+(aq)+2e→Cds) -0.40 Fe2(aq)+2c→Fe) -0.44 Cr3(ag)+3e→Cr -0.74 Zn2ag)+2e→Zn9 -076 2H,0+2e→Hg)+20H(ag) -0.83 Mn2t(ag)+2e→Mns) -1.18 AI3*(aq)+3e→Als) -1.66 Be2(aq)+2e→Bc9 -1.85 Mg2+(aq)+2e→Mg9 -237 Na'(aq)+e→Nas) -2.71 The better reducer Ca2(ag)+2e→Cas) -2.87 Ni(s)or Zn(母 sr2+(aq)+2e→Srt9 -2.89 Ba2(aq)+2e→Bas) -2.90 K(ag)+e→K(s) -293 Li(aq)+e→Li(s) -3.05
Standard reduction Potentials at 25ºC The better oxidizer ? CoCl2 or Cr(NO 3 ) 3 The better reducer ? Ni(s) or Zn(s)
Chap.20 Standard Reduction Potentials Appendix E Electrochemistry Oxidation state Reduction state Most positive values of Ered The strongest F2(g) 2e 2F (aq) oxidizing agent 2H+(aq)+2e H2(g) Increasing strength of reducing agent Lit(aq)+e- Li(s) The strongest Most negative values of Ered reducing agent
Standard Reduction Potentials Oxidation state Reduction state Chap. 20 Electrochemistry The strongest oxidizing agent Appendix E The strongest reducing agent
Standard Reduction Potentials Electrical potentials:an intensive property Zn2+(aq)+2e->Zn(s),Eed=-0.76 V 2Zn2+(aq)+4e>2Zn(s),E red=-0.76 V E:the potential per V electrical charge The charge ofone coulomb (1 C)falling though a potential difference of one volt(I V)releases one joule (1J)ofenergy. Zn(s)Zn2t(aq)+2e,Ex=+0.76 V
Zn2+(aq) + 2e- → Zn(s), E°red = - 0.76 V Standard Reduction Potentials Electrical potentials: an intensive property 2Zn2+(aq) + 4e- → 2Zn(s), E°E°red red= ? V = - 0.76 V Eo:the potential per electrical charge Zn(s) →Zn2+(aq) + 2e-, E°ox = + 0.76 V 1 J 1 V 1 C = The charge of one coulomb (1 C) falling though a potential difference of one volt (1 V) releases one joule (1J) of energy
Spontaneity of Redox Reactions 自发性 氧化还原 △G:the maximum amount of energy for useful work △H-T△S=△G=W non-exp ansion △H-T△S=△G=Welecrical work △G=Welecrical work Gibbs free energy Cell potential
Spontaneity of Redox Reactions ∆ − ∆ = ∆ = H T S G Wnon ansion -exp ∆G : the maximum amount of energy for useful work ∆ − ∆ = ∆ = H T S G Welectrical work 自发性 氧化还原 Gibbs free energy electrical work ? ? ∆G E → → W cell ∆ = G Welectrical work ? ? Cell potential