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Types of Redox Reactions: Disproportionation and More

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Every redox reaction fits one of four types: combination, decomposition, displacement, or disproportionation. Classifying a reaction tells you its pattern of electron flow and often lets you predict the products.

This classification is an exam favourite because it tests two skills at once: assigning oxidation numbers and recognising reaction patterns. The trickiest type is disproportionation, where a single element is simultaneously oxidised and reduced.

Key Concepts

  • Combination: A + B → AB. Example: 2Mg + O2 → 2MgO (Mg: 0 → +2 oxidised; O: 0 → −2 reduced).
  • Decomposition: AB → A + B, the reverse pattern. Example: 2KClO3 → 2KCl + 3O2 (Cl: +5 → −1; O: −2 → 0).
  • Displacement: a more reactive element displaces a less reactive one. Metal example: Zn + CuSO4 → ZnSO4 + Cu. Halogen example: Cl2 + 2KBr → 2KCl + Br2. The activity series predicts which displacement occurs.
  • Disproportionation: the same element in one reactant is both oxidised and reduced. Example: 2H2O2 → 2H2O + O2 (oxygen: −1 → −2 and −1 → 0). Cold-alkali example: Cl2 + 2NaOH → NaCl + NaOCl + H2O (chlorine: 0 → −1 and 0 → +1).

How It Works

  1. Assign oxidation numbers to every atom in reactants and products.
  2. Find which atoms change number: a rise means oxidation, a fall means reduction.
  3. If one element is oxidised and a different one reduced, look at the overall pattern: two reactants joining (combination), one reactant splitting (decomposition), or one element swapping places (displacement).
  4. If the same element both rises and falls in oxidation number, it is disproportionation — this check comes first, because disproportionations can superficially resemble decompositions.

Worked Example

Classify 2Pb(NO3)2 → 2PbO + 4NO2 + O2:

  1. Oxidation numbers: in Pb(NO3)2, Pb = +2, N = +5, O = −2; in products, Pb in PbO = +2, N in NO2 = +4, O in O2 = 0.
  2. Nitrogen: +5 → +4 (reduced); oxygen: −2 → 0 (oxidised). Lead is unchanged.
  3. One reactant breaks into three products → decomposition, and since oxidation numbers change, it is a redox decomposition.

Common Misconceptions

  • "Disproportionation is just decomposition." No — decomposition describes the pattern (one → many), disproportionation the electron flow (same element both ways). 2H2O2 → 2H2O + O2 is both.
  • "Combustion isn't redox." Burning is redox: the fuel is oxidised and O2 is reduced to −2.
  • "Displacement only happens with metals." Halogens displace each other too — a more reactive halogen displaces a less reactive one from its salt.

Real-World Applications

  • Bleaching powder and NaOCl are made by the disproportionation of chlorine in alkali.
  • Metal extraction: carbon displaces metals from their oxides in smelting (displacement).
  • Hydrogen peroxide storage: its slow disproportionation is why H2O2 bottles are dark and vented.

When you meet a redox equation, assign oxidation numbers first and ask one question: did the same element go both up and down? That single check separates disproportionation from everything else, and the overall pattern names the rest.

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