Half equations at the electrodes

A half equation shows what happens to one ion at one electrode. It must balance in two ways: the atoms must balance, and the total charge on each side must be equal. Electrons are written in the equation.

At the cathode, positive ions gain electrons, so the electrons are on the left. Copper ions gain two electrons: Cu²⁺ + 2e^- → Cu. The charge on the left is 2+ plus 2-, which is zero, and the copper atom has no charge. Hydrogen ions give: 2H⁺ + 2e^- → H₂.

At the anode, negative ions lose electrons, so the electrons are on the right. Chloride ions give chlorine: 2Cl^- → Cl₂ + 2e^-. Two chloride ions form one chlorine molecule and release two electrons. When hydroxide ions are discharged, oxygen forms: 4OH^- → 2H₂O + O₂ + 4e^-.