Read Which Oxidation States Can Disproportionate
Disproportionation is only possible for an intermediate oxidation state — an element must be able to go both up and down from where it sits, and E° values decide whether that reaction is actually favourable.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Only intermediate oxidation states disproportionate
An element at its lowest or highest accessible oxidation state cannot disproportionate — it has nowhere to go in one direction. Bromine disproportionating to Br⁻ and BrO₃⁻ in hot alkali is possible precisely because Br₂ (oxidation state 0) sits between Br⁻ (-1) and BrO₃⁻ (+5).
Equivalent weight in a disproportionation reaction
When one species both oxidises and reduces itself, its equivalent weight in that overall reaction is based on the total electrons transferred per formula unit across both halves of the disproportionation, not just one direction.
Recognising redox-driven qualitative tests
Classic qualitative observations — a brown colour at an interface, a colour change on mixing two reagents — are frequently redox reactions in disguise: identify the oxidising and reducing species first, and the colour or precipitate observed usually follows from a known product of that specific redox pair.
OlympIQ exam lens
- ✓Disproportionation needs an intermediate oxidation state — check that the element can go both up and down from its starting state.
- ✓Equivalent weight in a disproportionation reaction accounts for electrons transferred in both directions, not one.
- ✓A classic qualitative colour test is very often a specific, nameable redox reaction — identify oxidant and reductant before predicting the observation.
Don't call a reaction disproportionation just because oxidation states change — check specifically that the SAME element ends up at both a higher and a lower oxidation state among the products.
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