Disproportionation

Disproportionation is the simultaneous oxidation and reduction of an element in a single reaction.For example:2 Cu⁺ (aq) → Cu²⁺ (aq) + Cu (s) copper is reduced from +1 to 0 and oxidised from +1 to +2.To spot disproportionation, work out the oxidation number of the element in the reactant and in each product. If the same element starts in one oxidation state and ends up in two different ones, one higher and one lower, the reaction is a disproportionation.In the copper example, one Cu⁺ ion loses an electron and the other gains it:Cu⁺ → Cu²⁺ + e⁻    (oxidation)
Cu⁺ + e⁻ → Cu    (reduction)So the same species acts as both the oxidising agent and the reducing agent.Another example is the decomposition of hydrogen peroxide:2 H₂O₂ (aq) → 2 H₂O (l) + O₂ (g)Oxygen is −1 in H₂O₂. It is reduced to −2 in water and oxidised to 0 in oxygen gas.Chlorine also disproportionates when it reacts with water:Cl₂ (aq) + H₂O (l) → HCl (aq) + HClO (aq)Chlorine goes from 0 to −1 in HCl and to +1 in HClO, chloric(I) acid.