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A half equations is used to represent the reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:

• electrons are shown as e^-
• the numbers of atoms of each element must be the same on both sides
• the total charge on each side must be the same (usually zero)

Cathode reactions

Positively charged ions gain electrons at the cathode. These are half equations for some reactions at the cathode:

Na⁺ + e^- → Na

Pb²⁺ + 2e^- → Pb

2H⁺ + 2e^- → H₂

Example

Balance the half equation for the formation of aluminium during electrolysis: Al³⁺ + e^- → Al. The balanced half equation is: Al³⁺ + 3e^- → Al (because three negatively charged electrons are needed to balance the three positive charges on the aluminium ion).

Anode reactions

Negatively charged ions lose electrons at the anode. These are half equations for some reactions at the anode:

2Cl^- → Cl₂ + 2e^-

2O^2- → O₂ + 4e^-

Example

Write a balanced half equation for the formation of oxygen from hydroxide ions.

4OH^- → 2H₂O + O₂ + 4e^-

Oxidation and Reduction

Remember from Unit 2.2 that oxidation is loss of electrons and reduction is gain of electrons. Reduction reactions occur at the cathode and oxidation reactions occur at the anode during electrolysis.

For the half equation: Al³⁺ + 3e^- → Al, this is described as a reduction reaction as aluminium ions are gaining electrons and the gain of electrons is reduction.

For the half equation 2O^2- → O₂ + 4e^-, this is described an oxidation reaction as the oxide ions are losing electron and loss of electrons is oxidation.