What happens at the anode and the cathode? What are the reactions at each electrode?
In an electrochemical cell electrons flow from the anode to the cathode as they are transferred from one substance to another. A chemical reaction in which electrons are transferred is called oxidation-reduction. The substance being oxidized is losing electrons, and those electrons are gained by the substance being reduced. Therefore oxidation takes place at the anode, where electrons are lost, and reduction takes place at the cathode where electrons are gained.
The image below is an example of an electrochemical cell. The two beakers represent the anode and cathode half-cells. The beaker on the left contains the zinc anode, which is oxidized. Electrons from the zinc travel through the external circuit to the cathode, where Cu2+ ions in solution gain the electrons and plate onto the copper strip as reduced Cu. The KNO3 salt bridge is a source of positive and negative electrons that flow into each half-cell to balance the charge. For example, NO3- ions go into the anode half-cell to balance the lost electrons. The voltmeter shows that electrons are flowing in the circuit.
Here are the half reactions taking place at each electrode:
anode (oxidation): `Zn -> Zn^(2+) + 2 e-`
cathode (reduction): `Cu^(2+) + 2e- -> Cu`