Types of Redox Reactions

Once you can compute oxidation numbers, you can classify every redox reaction into one of five types, based on the pattern of oxidation-number change:

1. Combination reactions -- two substances combine to form a single new compound. Example: C + O₂ → CO₂, where carbon's oxidation number rises from 0 to +4 (oxidation) and oxygen's falls from 0 to −2 (reduction).
2. Decomposition reactions -- the reverse of combination: a single compound breaks down into two or more products, with the oxidation numbers of different elements within that same original substance changing in opposite directions. Example: 2KClO₃ →(Δ) 2KCl + 3O₂, where chlorine falls from +5 to −1 (reduction) and oxygen rises from −2 to 0 (oxidation).
3. Displacement reactions -- an ion or atom in a compound is replaced by an ion or atom of another element. These split into two sub-types:
   • Metal displacement: e.g. CuSO₄(aq) + Zn(s) → Cu(s) + ZnSO₄(aq) -- a zinc strip placed in blue copper sulphate solution slowly decolourises the solution as Zn (0 → +2, oxidised) displaces Cu²⁺ (+2 → 0, reduced), coating the strip with metallic copper.
   • Non-metal displacement: e.g. Zn + 2HCl → ZnCl₂ + H₂ -- zinc (0 → +2, oxidised) displaces hydrogen (+1 → 0, reduced) from hydrochloric acid.
4. Disproportionation reactions (auto-redox) -- the same element, in the same starting compound, is simultaneously oxidised and reduced. Examples: 2H₂O₂ → 2H₂O + O₂ (oxygen in H₂O₂, at −1, ends up partly at −2 in H₂O — reduced — and partly at 0 in O₂ — oxidised); 3Cl₂ + 6KOH → 5KCl + KClO₃ + 3H₂O (chlorine, starting at 0, ends up partly at −1 in KCl — reduced — and partly at +5 in KClO₃ — oxidised). …