Keep track of every electron.
Oxidation is electron loss; reduction is electron gain. The reducing agent loses electrons and is oxidised. The oxidising agent gains electrons and is reduced. An oxidation number increases on oxidation.
In acidic solution, balance atoms other than H and O, then O with H₂O, H with and charge with electrons. Multiply half-equations to cancel equal electron counts before adding. Check atoms and charge at the end.
For MnO₄⁻ → Mn²⁺, oxygen gives four waters on the right, then eight on the left. Five electrons on the left balance charge. In Mn₂O₃ the average Mn oxidation state is +3, so the compound is manganese(III) oxide.
Follow the electrons
Metal/Cu galvanic cell under standard conditions. Ion and electron movement is schematic. The salt bridge carries ions; the external wire carries electrons.
Assumed knowledge
Oxygen usually has oxidation number −2; oxidation numbers sum to the species charge.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Identify oxidation, reduction and their agents.
- Balance redox half-equations in acidic solution.
- Use oxidation numbers in transition-metal names.