1.9.3States of Matter

Writing Half-Equations — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 30 Unit quiz

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Part 1 of 5Oxidation and Reduction in Electrolysis

In electrolysis, oxidation and reduction are defined by the transfer of electrons at the electrodes.

Oxidation

Loss of electrons;
- at the anode, negative ions lose electrons and are oxidised to form neutral atoms or molecules.

Reduction

Gain of electrons;
- at the cathode, positive ions gain electrons and are reduced to form neutral atoms or molecules.

OIL RIG

A memory aidOxidation Is Loss, Reduction Is Gain, of electrons, which works for every electrode reaction.

Balancing

Both atoms and charges must balance, and the electrons gained at the cathode must equal the electrons lost at the anode.

Position of electrons

Cathode half-equations have electrons on the reactant side, while anode half-equations have electrons on the product side.

Common Half-Equations

Acidic solutions

Acidic solutions contain few hydroxide ions, so water molecules are discharged at the anode to give oxygen.

Sodium chloride

Water is discharged at the cathode in sodium chloride solution, because sodium is too reactive to be discharged.

Copper sulfate

Copper is less reactive than hydrogen, so copper ions are discharged at the cathode instead of hydrogen ions.

Electrolyte	Cathode (reduction)	Anode (oxidation)
Molten lead(II) bromide	Pb²⁺ + 2e⁻ → Pb	2Br⁻ → Br₂ + 2e⁻
Concentrated aqueous sodium chloride	2H₂O + 2e⁻ → H₂ + 2OH⁻	2Cl⁻ → Cl₂ + 2e⁻
Dilute sulfuric acid	2H⁺ + 2e⁻ → H₂	2H₂O → O₂ + 4H⁺ + 4e⁻
Aqueous copper(II) sulfate	Cu²⁺ + 2e⁻ → Cu	2H₂O → O₂ + 4H⁺ + 4e⁻

Identify the ions in the electrolyte, including H⁺ and OH⁻ from water if the electrolyte is aqueous.

Decide which ion is discharged at each electrode, using the rules for metal ions and halide ions.

Write the species discharged at that electrode, showing its charge and the number of electrons transferred.

Balance the atoms first, then balance the charges by adding electrons to the side that needs them.

Make the electrons equal at both electrodes and add the two half-equations to give the overall equation.

Question Write the half-equations and the overall equation for the electrolysis of molten aluminium oxide, Al₂O₃.

Al₂O₃ contains Al³⁺ and O²⁻ ions, so Al³⁺ moves to the cathode and O²⁻ moves to the anode.
CathodeAl³⁺ + 3e⁻ → Al.
Anode2O²⁻ → O₂ + 4e⁻.
Multiply the cathode equation by 4 and the anode equation by 3, so that 12 electrons appear on each side.
Add the equations4Al³⁺ + 6O²⁻ → 4Al + 3O₂.

Answer

Half-equation An equation showing only the electrons and species involved at one electrode.
Oxidation The loss of electrons by a species.
Reduction The gain of electrons by a species.
Discharge The reaction of an ion at an electrode to form a neutral element or molecule.
Electron balance The equal number of electrons gained at the cathode and lost at the anode.

Using the oxygen-gain definition of oxidation for electrode reactions;
- electrolysis is defined by electron transfer.
Putting the electrons on the wrong side of an electrode equation, such as 2Cl⁻ + 2e⁻ → Cl₂.
Using H⁺ in the cathode equation for sodium chloride solution;
- there water is reduced to hydrogen and OH⁻ forms.
Balancing atoms but forgetting the charges on each side of a half-equation.

Last updated: 28 October 2026

Chemistry 0620 & Edexcel OL Product Prediction Rules

At Cathode (-): Least reactive cation discharged. If metal is more reactive than hydrogen (Na⁺, K⁺, Mg²⁺), H₂ gas is produced: 2H⁺ + 2e^- → H₂. If Cu²⁺ or Ag⁺, the metal coats the cathode.
At Anode (+): If concentrated halide present (Cl^-, Br^-, I^-), halogen is formed (e.g. 2Cl^- → Cl₂ + 2e^-). If sulfate (SO₄^2-) or nitrate (NO₃^-), O₂ gas is discharged: 4OH^- → O₂ + 2H₂O + 4e^-.