Chapter 16 · Oxidation and Reduction
16.4Balancing Redox Equations
13 min · two checks
Predict
Why can’t you always balance a redox equation by inspection?
The idea
- Balance a redox equation in acid by the half-reaction method.
- Check atoms and charge at the end.
Split the reaction into oxidation and reduction half-reactions. Balance atoms other than O and H. In acidic solution, balance O by adding H₂O, then H by adding H⁺, then charge by adding electrons. Multiply the halves so the electrons lost equal the electrons gained. Add them and cancel anything that appears on both sides. The electrons must disappear. If they do not, the multipliers are wrong.
In basic solution, do the acid balance first, then add OH⁻ to both sides to neutralize every H⁺, turning H⁺ + OH⁻ into water, and cancel extra water. A check: atoms balance, and the total charge is the same on both sides. This method is the one to trust for permanganate, dichromate, and anything else that looks tangled. Inspection is fine for simple combination reactions such as 2 Mg + O₂ → 2 MgO.
Keep these
- Balance each half, then equalize electrons.
- Acid: O with H₂O, H with H⁺, charge with e⁻.
- Electrons must cancel. Charge and atoms both have to balance.
Worked path
Balance the skeleton Fe²⁺ → Fe³⁺ and MnO₄⁻ → Mn²⁺ in acid, as halves, then combine.
Observe
Oxidation: Fe²⁺ → Fe³⁺ + e⁻. Reduction: MnO₄⁻ needs 8 H⁺ and 5 e⁻ to become Mn²⁺ + 4 H₂O.