Mistake in Pub. Soln on Carmen - 20.40(b) in 12th-14th ed.

[Zellmer, Robert]

There is a mistake in the solutions manual on Carmen (12th, 13th, 14th ed.) for
20.40(b).

20.40(b)  The solutions manual uses an electrode not given in the problem.
                To get the smallest (+) emf from the 1/2-cells given would
                be using the first two half-cells listed,

    Anode: AuBr4^- (aq)  +  3 e-  ---->  Au(s)  +  4 Br- (aq)

    Cat:    Eu^3+ (aq)  +  1 e-  ----> Eu^2+ (aq)

    To balance the electrons you need to multiply the cathode rxn by 3

     Cat:    3 (Eu^3+ (aq)  +  1 e-  ----> Eu^2+ (aq))

    Then you get:

    Anode: AuBr4^- (aq)  +  3 e-  ---->  Au(s)  +  4 Br^- (aq)

    Cat:    3 Eu^3+ (aq)  +  3 e-  ---->  3 Eu^2+ (aq)

    Add them to get the overall rxn.

    AuBr4^- (aq)  +  Eu^3+ (aq) ---->  Au(s)  +  4 Br^- (aq)  +  3 Eu^2+ (aq)

            E^o = E^o (cathode) - E^o (anode)

                    = (-.43 V) - (-.858 V)

                    = + 0.43 V

Dr. Zellmer
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