Half Equations and Electrode Reactions in Electrolysis — AQA GCSE Chemistry (Higher Tier)

Half equations show what happens to electrons at each electrode during electrolysis.

What is a half equation?

A half equation shows the gain or loss of electrons at one electrode. Electrons are written as e⁻.

• At the cathode (negative), positive ions gain electrons (reduction).
• At the anode (positive), negative ions lose electrons (oxidation).

Writing half equations

Cathode (reduction) examples:

• 2H⁺ + 2e⁻ → H₂
• Cu²⁺ + 2e⁻ → Cu
• Na⁺ + e⁻ → Na

Anode (oxidation) examples:

• 2Cl⁻ → Cl₂ + 2e⁻
• 4OH⁻ → O₂ + 2H₂O + 4e⁻
• 2Br⁻ → Br₂ + 2e⁻

Balancing half equations

Make sure both the atoms and the charges balance. The number of electrons gained at the cathode must equal the number lost at the anode in the overall process.

Linking to OIL RIG

Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain. So the cathode (gain) is reduction and the anode (loss) is oxidation.

Exam tips

• Cathode = reduction = electrons on the left (gained).
• Anode = oxidation = electrons on the right (lost).
• Balance atoms and charges.
• Use OIL RIG to decide which electrode does which.