Which half-reaction occurs at the cathode when copper ions are reduced?

• A. Cu2+ + 2e- → Cu
• B. Cu → Cu2+ + 2e-
• C. Cu2+ + e- → Cu+
• D. Cu + 2e- → Cu2-

Answer: (A)

At the cathode, reduction happens: ions in solution gain electrons to become neutral metal. Copper ions with a 2+ charge accept two electrons and sit as solid copper on the electrode. That’s exactly Cu2+ + 2e- → Cu, which shows the right amount of electrons and the formation of metallic copper deposited at the cathode. The other possibilities involve either oxidation (loss of electrons) or forming a copper ion with only +1 charge, both of which don’t describe what happens when copper is being deposited at the cathode under typical plating conditions.