← Oxidation-reduction processes

4 Balancing half-equations

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Mnemonic: balancing half-equations

“Atoms, Oxygen, Hydrogen, Charge” = “All Other Hens Count.” Balance the main atom → add H₂O for oxygen → add H⁺ for hydrogen → add e⁻ to balance charge. Always in that order.

For oxidation numbers: “Hydrogen is plus one, Oxygen is minus two, unless they’re being weird” (H is −1 in metal hydrides; O is −1 in peroxides).

Method (acidic solution)

• 1. Balance the atom being oxidised/reduced.
• 2. Balance O by adding H₂O.
• 3. Balance H by adding H⁺.
• 4. Balance charge by adding electrons (e⁻).
• To combine: multiply each half-equation so electrons cancel, then add.

Worked example MnO₄⁻ → Mn²⁺ Balance the reduction half-equation for permanganate in acid. Mn balanced: MnO₄⁻ → Mn²⁺. Balance O with H₂O: MnO₄⁻ → Mn²⁺ + 4H₂O. Balance H with H⁺: MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O. Balance charge with e⁻: left charge = −1+8 = +7, right = +2, add 5e⁻ to left. ✓ MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

Practise just this