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2018 O Level Chem P1 Q21 — Redox / Disproportionation Solution

Redox / Disproportionation 2018 O Level Pure Chemistry · Paper 1 · Q21

Question

The reaction in which the same element is both oxidised and reduced is called a disproportionation reaction. Which equation is an example of a disproportionation reaction?

A3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O
BCl₂ + 2NaOH → NaCl + NaOCl + H₂O
CFe₂(SO₄)₃ + 2KI → 2FeSO₄ + K₂SO₄ + I₂
D2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Correct answer: B

Video Solution

Explanation

Answer: B (\(\text{Cl}_2 + 2\text{NaOH} \rightarrow \text{NaCl} + \text{NaOCl} + \text{H}_2\text{O}\)). In a disproportionation reaction the same element is both oxidised and reduced.

Chlorine starts at 0 in \(\text{Cl}_2\). In \(\text{NaCl}\) it is \(-1\) (reduced); in \(\text{NaOCl}\) it is \(+1\) (oxidised). The same element, chlorine, is both oxidised and reduced.

In A copper is oxidised while nitrogen is reduced; in C iron is reduced while iodine is oxidised; in D nitrogen is reduced while oxygen is oxidised. These involve different elements, so only B is disproportionation. Answer B.

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