Electrolysis half equations: a step-by-step guide for GCSE Chemistry

What happens at each electrode

• Positive ions move to the negative electrode, the cathode. There they gain electrons: that’s reduction.
• Negative ions move to the positive electrode, the anode. There they lose electrons: that’s oxidation.
• Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Writing a half equation in three steps

• 1. Write the ion and what it becomes. Chloride ions make chlorine gas, which goes around in pairs: Cl⁻ → Cl₂.
• 2. Balance the atoms: 2Cl⁻ → Cl₂.
• 3. Balance the charges with electrons (e⁻). The left has a charge of 2−, the right 0, so two electrons go on the right: 2Cl⁻ → Cl₂ + 2e⁻.

Worked examples

• Molten lead bromide. Cathode: Pb²⁺ + 2e⁻ → Pb. Anode: 2Br⁻ → Br₂ + 2e⁻.
• Molten aluminium oxide. Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻.
• Brine (sodium chloride solution). Cathode: 2H⁺ + 2e⁻ → H₂ (sodium is more reactive than hydrogen). Anode: 2Cl⁻ → Cl₂ + 2e⁻.
• Copper sulfate solution. Cathode: Cu²⁺ + 2e⁻ → Cu. Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (sulfate isn’t a halide, so oxygen forms).

Common mistakes

• Forgetting that gases like chlorine, oxygen and hydrogen are diatomic (Cl₂, O₂, H₂).
• Putting the electrons on the wrong side: at the cathode they’re on the left (gained), at the anode on the right (lost).
• Forgetting the rule for solutions: hydrogen forms at the cathode if the metal is more reactive than hydrogen, and oxygen forms at the anode unless there are halide ions.

Questions people ask

What is a half equation?

An equation that shows what happens at one electrode in electrolysis: ions gaining electrons (at the cathode) or losing them (at the anode).

Are half equations Higher tier only?

For AQA, writing half equations is Higher tier content, but every student needs to know which products form at each electrode.