A half-equation shows just one of the two changes in a redox reaction, with the electrons written in. Two half-equations, balanced for electrons and added together, give the full ionic equation.
This lesson is part of SPM Chemistry redox equilibrium. It builds on identifying oxidation and reduction.
How do you write one half-equation?
Start with the species and what it becomes, then add electrons until the charge is equal on both sides.
- Oxidation, electrons lost: Mg → Mg²⁺ + 2e⁻.
- Reduction, electrons gained: Ag⁺ + e⁻ → Ag.
Check the charge on each side. For Mg → Mg²⁺ + 2e⁻ the left side is 0 and the right side is +2 + (−2) = 0.
Worked example: magnesium in silver nitrate
Mg displaces silver from silver nitrate solution.
- Oxidation: Mg → Mg²⁺ + 2e⁻ (loses 2 electrons).
- Reduction: Ag⁺ + e⁻ → Ag (gains 1 electron).
- Equalise electrons: multiply the silver half-equation by 2, giving 2Ag⁺ + 2e⁻ → 2Ag.
- Add and cancel 2e⁻ on both sides: Mg + 2Ag⁺ → Mg²⁺ + 2Ag.
Check the charge: left 0 + 2(+1) = +2. Right +2 + 0 = +2. It balances.
Worked example with different electron numbers
Aluminium displaces copper from copper(II) sulfate.
- Oxidation: Al → Al³⁺ + 3e⁻.
- Reduction: Cu²⁺ + 2e⁻ → Cu.
- The lowest common multiple of 3 and 2 is 6, so multiply the first by 2 and the second by 3.
- 2Al → 2Al³⁺ + 6e⁻ and 3Cu²⁺ + 6e⁻ → 3Cu.
- Add: 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu.
Charge check: left 3 × (+2) = +6, right 2 × (+3) = +6.
The mistake that costs marks
The common slip is putting the electrons on the wrong side, for example Mg + 2e⁻ → Mg²⁺. The charge test catches it straight away.
| Line | Left charge | Right charge | Verdict |
|---|---|---|---|
| Mg + 2e⁻ → Mg²⁺ | −2 | +2 | Not balanced |
| Mg → Mg²⁺ + 2e⁻ | 0 | 0 | Balanced |
If the charges do not match, move the electrons or change how many there are.
Check yourself
Zinc is placed in lead(II) nitrate solution and lead forms. Write the two half-equations and the ionic equation.
Answer
Oxidation: Zn → Zn²⁺ + 2e⁻.
Reduction: Pb²⁺ + 2e⁻ → Pb.
Both have two electrons, so add directly: Zn + Pb²⁺ → Zn²⁺ + Pb.
Charge check: left +2, right +2.
What to study next
Half-equations are used in both electrolysis and cells. Continue with predicting electrolysis products and then explaining simple chemical cells. The redox practice set tests all of them.
The mole and stoichiometry steps tool helps with the amount calculations that sometimes follow. If you want a teacher to check your half-equations, see online one-to-one Chemistry tuition.