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In which one of the following reactions, +2 state in a reactant is reduced to +1 state?

  • A.\(\mathrm{Mg + S \rightarrow MgS}\)
  • B.\(\mathrm{2HgCl_{2} + SnCl_{2} \rightarrow Hg_{2}Cl_{2} + SnCl_{4}}\)
  • C.\(\mathrm{CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O}\)
  • D.\(\mathrm{2K_{4}[Fe(CN)_{6}] + H_{2}O_{2} \rightarrow 2K_{3}[Fe(CN)_{6}] + 2KOH}\)
  • E.\(\mathrm{2H_{2}S + O_{2} \rightarrow S + 2H_{2}O}\)

Answer: B

The reaction is \(\mathrm{2HgCl_{2} + SnCl_{2} \rightarrow Hg_{2}Cl_{2} + SnCl_{4}}\).

In HgCl2 mercury is +2. In Hg2Cl2 (calomel, with an Hg-Hg bond) each mercury is +1. So the +2 state is reduced to +1, while tin is oxidised from +2 to +4.

The others involve different changes: in Mg + S, Mg goes 0 to +2; in the combustion of methane carbon goes -4 to +4; in the ferrocyanide reaction iron goes +2 to +3, an oxidation; and in H2S + O2 sulphur goes -2 to 0. None of them reduces +2 to +1.

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