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Two half-cell standard reduction potentials are: Cd2+(aq) + 2e- -> Cd(s), E° = -0.40 V and Ag+(aq) + e- -> Ag(s), E° = 0.80 V. For the cell reaction 2Ag+(aq) + Cd(s) -> 2Ag(s) + Cd2+(aq), the standard Gibbs free energy change is:

1. 115.8 kJ
2. -115.8 kJ
3. -231.6 kJ
4. 231.6 kJ

Correct answer: -231.6 kJ

Solution

Silver is reduced (cathode) and cadmium is oxidized (anode). E°cell = 0.80 - (-0.40) = 1.20 V. Two electrons are transferred (n = 2). dG° = -nFE° = -(2)(96500)(1.20) = -231600 J = -231.6 kJ. The negative sign confirms the reaction is spontaneous.

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