Question

Indicate which of the substance/ion in the following reaction is an oxidising agent and which is a reducing agent?
$2S{O}_2+O_2\to 2S{O}_3$.

Answer 1

$2S{O}_2+O_2\to 2S{O}_3$.
Oxidising agent- $O_2$ The oxidation number of S decreases from 0 to -2. Thus, O itself is reduced.

Reducing agent- $S{O}_2$. The oxidation number of S increases from +4 to +6. Thus, S itself is oxidised.

Note: Gain of oxygen is oxidation. Thus, $S{O}_2$ is oxidised to $S{O}_3$.

Any species that is oxidised acts as reducing agent.

Any species that is reduced acts as oxidising agent.