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The standard reduction potentials for Zn²⁺/Zn, Ni²⁺/Ni and Fe²⁺/Fe are -0.76, -0.23 and -0.44 V respectively. The reaction X + Y²⁺ → X²⁺ + Y will be spontaneous when:

1. X = Ni, Y = Fe
2. X = Ni, Y = Zn
3. X = Fe, Y = Zn
4. X = Zn, Y = Ni

Correct answer: X = Zn, Y = Ni

Solution

For X + Y2+ -> X2+ + Y to be spontaneous, X must be more easily oxidized, i.e. have a more negative reduction potential than Y. With Zn (-0.76) and Ni (-0.23), X = Zn, Y = Ni gives a positive cell potential and is spontaneous.

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