Redox and electrochemistry slide 2425 80 end of

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Redox and Electrochemistry
Slide 24/25-80End of Chapter 24/25: Redox and ElectrochemistrySlides following are for informational purposes onlyRedox and Electrochemistry
Slide 24/25-81Redox and ElectrochemistryFaradays Laws-3So, if we have a cell where the anode and cathode are as follows:and we allow 0.500 A (500 mA) to pass for 2 minutes (120 sec) thru that cell, we will find n= 6.22 x 10-4moles of electrons were moved thru the cell. This means:2++Pb( ) Pb() + 2Cu () + Cu( )saqeandaqes⎯⎯→⎯⎯→–4–41 Pb207.2 Pbamt of Pb() used6.22102 1 Pb0.064 1 Cu63.5 Cuamt of Cu() made6.22101 1 Cu0.040 molgsmol emol emolgANDmolgsmol emol emolg=×××==×××=
Slide 24/25-82Determinationof Unknown Eohalf-cellUnknown half-cell voltages can be determined by either using the SHE ORrunning a galvanic cell where we knowone the voltage of one of the half-cells. Consider:A voltaic cell houses the reaction between aqueous bromine and zinc metal:Br2(aq) + Zn(s) Zn2+(aq) + 2Br -(aq) Eocell= 1.83 VCalculate Eobromine, given Eozinc= -0.76 V.Approach this problem like this: first, the reaction must be spontaneous as written since the Eocellis (+); based on this zinc is being oxidized and is the anode. Therefore, the bromine half-reaction must be the cathode. Eobrominecan be found using Eocell= Eocathode- Eoanode.Anode: Zn(s) Zn2+(aq) + 2e-E= +0.76 VEoZnas Zn2+(aq) + 2e-Zn(s) is -0.76 VEocell= Eocathode- Eoanode = 1.83 = Eobromine- (-0.76)Eobromine= 1.83 + (-0.76) = 1.07 VRedox and Electrochemistry
Slide 24/25-83Redox and ElectrochemistryPredicting Electrolysis Products (3)What products form during electrolysis of aqueous AgNO3? Assume a 0.4 to 0.6 V overpotential for waterSimilar analysis for the cathode predicts Ag(s) will be formed:For the anode however, the N in NO3-is alreadyin its mostoxidizedform (+5) so water will have to be oxidized to produce O2at the anode (even with overpotential).–0.80 V+–0.42 V + –0.6 V (OP)22Ag () + Ag( )2HO() + 2H() + 2OH()EEaqeslegaq==⎯⎯⎯→⎯⎯⎯⎯⎯⎯→oo
Slide 24/25-84Redox and ElectrochemistryHalf Cell Voltage DeterminationCan we use half cells/half cell potentials to determine other half cell potentials? For example: what if we knew:andBut needed:– Cant just add upvoltages (electrons wont cancel!)Need to use !Gº instead:323CrCr0.42 VCr3Cr ()–0.72 VeEesE++++=+=⎯⎯→⎯⎯→oo2Cr2Cr ()?? VesE++=⎯⎯→ostepstepstepGnFEΔ=oo
Slide 24/25-85Redox and ElectrochemistryHalf Cell Voltage DeterminationSet up half reactions to give you what you want:231113222233312CrCr0.42 VCr3Cr ()–0.72 VCr2Cr ()?? VeEGn FEesEGn FEesEGn FE+++++= +Δ+=Δ+=Δ⎯⎯→⎯⎯→⎯⎯→ooooooooo312331122112233–1.74 V0.87 V2GGGn FEn FEn FEn En EEnΔ=ΔΔ=+=++==oooooooooSo:
Slide 24/25-86Redox and ElectrochemistryTemperature Effects on E°Since cell potential depends on !G° the temperature dependence of E° can be simply determined:112221112221122112lnln11ln11TTTTTTGRTKnFEandGRTKnFEKHSinceKRTTEEHWe getTTnFTTΔ==Δ==Δ=Δ=ooo

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