Half Equation
Summary: A half equation shows either oxidation (loss of electrons) or reduction (gain of electrons) separately. It includes the electrons transferred and must be balanced for both atoms and charge. Tags: igcse chemistry redox electrolysis keyword Created: 2026-07-14 Last Updated: 2026-07-14
Definition
A half equation is a chemical equation that shows either the oxidation or the reduction part of a redox reaction separately, explicitly showing the electrons (e⁻) lost or gained.
Key Rule: OIL RIG
- Oxidation Is Loss of electrons → e⁻ on the RIGHT side
- Reduction Is Gain of electrons → e⁻ on the LEFT side
How to Write a Half Equation
- Balance the atom being oxidised/reduced
- Balance oxygen atoms by adding H₂O
- Balance hydrogen atoms by adding H⁺
- Balance charge by adding electrons (e⁻)
- Check: total charge must be equal on both sides
Common IGCSE Examples
| Half Equation | Type |
|---|---|
| Na⁺ + e⁻ → Na | Reduction |
| 2Cl⁻ → Cl₂ + 2e⁻ | Oxidation |
| Cu²⁺ + 2e⁻ → Cu | Reduction |
| Zn → Zn²⁺ + 2e⁻ | Oxidation |
| 2H⁺ + 2e⁻ → H₂ | Reduction |
| 4OH⁻ → O₂ + 2H₂O + 4e⁻ | Oxidation |
| Fe → Fe²⁺ + 2e⁻ | Oxidation (rusting) |
| O₂ + 2H₂O + 4e⁻ → 4OH⁻ | Reduction (rusting) |
Combining Half Equations
- Make the number of electrons equal in both (multiply equations if needed)
- Add the two equations
- Cancel the electrons (they appear on both sides)
See Also
- Ionic Equations and Half Equations — Full topic page
- Redox Reactions — Oxidation and reduction
- OIL RIG — Memory aid for redox direction
- Electrolysis — Half equations at electrodes
- IGCSE-Chem-Index