What gets reduced in an electrolytic cell made with nickel and copper electrodes?
A. Ni(s)
B. Cu2+
C. Cu(s)
D. Ni2+

D. Ni²⁺  

Explanation:

We know at once that the answer cannot be A or C, because Ni and Cu are already in their lowest oxidation states.

The correct answer must be either B or D.

An electrolytic cell is the opposite of a galvanic cell. In the former, the reaction proceeds spontaneously. In the latter, you must force the reaction to occur.  

One strategy to solve this problem is:

1. Standard reduction potentials

                                E°/V

Cu²⁺ + 2e⁻ ⟶ Cu; 0.3419

Ni²⁺ + 2e⁻ ⟶ Ni;  -0.257

2. Galvanic Cell

We reverse the direction of the more negative half cell and add.

                                       E°/V

Ni ⟶ Ni²⁺ + 2e⁻;           0.257

Cu²⁺ + 2e⁻ ⟶ Cu;         0.3419

Ni + Cu²⁺ ⟶ Cu + Ni²⁺; 0.599

This is the spontaneous direction.

Cu²⁺ is reduced to Cu.

3. Electrochemical cell

                                        E°/V

Ni²⁺ + 2e⁻ ⟶ Ni;           -0.257

Cu ⟶ Cu²⁺ + 2e⁻;         -0.3419

Cu + Ni²⁺ ⟶ Ni + Cu²⁺; -0.599

This is the non-spontaneous direction.

Ni²⁺ is reduced to Ni in the electrolytic cell.