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REDOX & Electrochemistry - LSU Chemistry

REDOX & Electrochemistry - LSU Chemistry


REDOX 24Example: Zn metal reacts with HCl, but Cu metaldoesn’t. Calculate the cell potentials for these rxnsto see if they fit the experimental data.Zn(s) + 2H + (aq)Zn 2+ (aq) + H 2 (g)Splitting this rxn into the two half cell rxns, wehave:Zn(s) Zn 2+ (aq) + 2e - +0.762H + (aq) + 2e - H 2 (g) 0.00Zn(s) + 2H + (aq)Zn 2+ (aq) + H 2(g)+0.76VNote that the Zn half-cell rxn is flipped aroundfrom how it is written in the Standard ReductionTable, since it is acting as a reducing agent here.The calculated positive cell potential indicates aspontaneous rxn.

For the copper rxn, we have:REDOX 25Cu(s) + 2H + (aq)Cu 2+ (aq) + H 2 (g)Cu(s) Cu 2+ (aq) + 2e - −0.342H + (aq) + 2e - H 2 (g) 0.00Cu(s) + 2H + (aq)Cu 2+ (aq) + H 2(g)−0.34VOnce again, note that the copper half-cell rxn isflipped around from how it is written in theStandard Reduction Table, since it is acting as areducing agent here. So here we calculate that thecell potential is negative, indicating a nonspontaneousrxn – or one that should not occurnormally, which fits the experimental data.This is because copper metal is a more inertmaterial relative to zinc metal. One could also notethe position of copper above the hydrogen half cellrxn, which indicates that the reverse rxn of Cu +2with H 2 gas would be spontaneous:Cu 2+ (aq) + H 2(g)Cu(s) + 2H + (aq)+0.34V

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