In each half-cell of an electrochemical cell, which process occurs at the anode and which at the cathode?

• A. Oxidation occurs at the cathode; reduction occurs at the anode
• B. Oxidation occurs at both electrodes
• C. Reduction occurs at both electrodes
• D. Oxidation occurs at the anode; reduction occurs at the cathode

Answer: (D)

Oxidation occurs at the anode and reduction occurs at the cathode. In every half-cell, the electrode where electrons are produced is the anode, so the species there loses electrons. Those electrons travel through the external circuit to the other electrode, where they are consumed by a reduction reaction. This flow direction holds for galvanic cells (where electrons flow spontaneously from anode to cathode) and electrolytic cells (where an external source drives the same electron flow). For example, in a zinc–copper cell, zinc at the anode is oxidized to Zn2+ while electrons leave the electrode; at the cathode, Cu2+ ions gain electrons and are reduced to copper metal. Oxidation state rises at the anode and falls at the cathode.