Question

Indicate which of the substance/ion in the following reaction is an oxidising agent and which is a reducing agent?
$S{n}^{2+}+2H{g}^{2+}\to H{g}_2^{2+}+S{n}^{4+}$.

Answer 1

$S{n}^{2+}+2H{g}^{2+}\to H{g}_2^{2+}+S{n}^{4+}$.
Oxidising agent- $H{g}^{2+}$ The oxidation number of Hg decreases from +2 to +1. Thus, Hg itself is reduced.
Reducing agent- $S{n}^{2+}$. The oxidation number of Sn increases from +2 to +4. Thus, Sn itself is oxidised.

Note:
Any species that is oxidised acts as reducing agent.
Any species that is reduced acts as oxidising agent.