Oxidation always occurs at the anode, while reduction always occurs at the cathode. This rule applies to both voltaic cells and electrolytic cells at IB Chemistry SL and HL, although the signs of the electrodes differ.
The Reasoning
An electrochemical cell separates the two parts of a redox reaction:
- At the anode, a species loses electrons: oxidation.
- At the cathode, a species gains electrons: reduction.
- Electrons are removed at the anode and supplied to the cathode through the external circuit.
- Cations move toward the cathode, while anions move toward the anode.
In a voltaic cell, a salt bridge completes the circuit. Its ions migrate to prevent charge buildup: anions enter the anode half-cell as cations form, while cations enter the cathode half-cell as cations are removed.
For example, in a zinc-copper voltaic cell:
Anode:
