Question

Indicate which of the substance/ion in the following reactions is an oxidising agent and which is a reducing agent?
$S{O}_2+2H_{2}S\to 3S+2H_{2}O$.

Answer 1

$S{O}_2+2H_{2}S\to 3S+2H_{2}O$.
Oxidising agent- $S{O}_2$ The oxidation number of S decreases from +4to 0. Thus, S itself is reduced.
Reducing agent- $H_{2}S$. The oxidation number of S increases from -2 to 0. Thus, S itself is oxidised.

Note:
Any species that is oxidised acts as reducing agent.
Any species that is reduced acts as oxidising agent.
Gain of oxygen (or loss of hydrogen) is oxidation.
Loss of oxygen (or gain of hydrogen) is reduction.