Disproportionation

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When chlorine gas reacts with cold dilute sodium hydroxide, the element undergoes a reaction in which it is simultaneously oxidised and reduced; what is this called?

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A disproportionation reaction is a special redox process in which a single element in one oxidation state is simultaneously oxidised to a higher state and reduced to a lower state. When chlorine (oxidation state 0) reacts with cold dilute sodium hydroxide, part of the chlorine is reduced to chloride (Cl^-, state -1) and part is oxidised to hypochlorite (ClO^-, state +1): Cl_2 + 2NaOH → NaCl + NaOCl + H_2O. Because the same element ends up in both a higher and a lower oxidation state, this is the defining example of disproportionation. Comproportionation is the reverse process, where two different oxidation states combine to a single intermediate state. Neutralisation is an acid-base reaction with no change in oxidation number. Combustion involves reaction with oxygen and is not relevant here. This reaction is the standard NCERT illustration of disproportionation in p-block chemistry. Plausibility check: chlorine starts at 0 and finishes at both -1 and +1, the unmistakable signature of disproportionation.

Disproportionation

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