What are the displacement reactions? Give one example.

Select the oxidising agent and reducing agent in the reaction:

$4\mathrm{Na} + {\mathrm{O}}_2 \to 2{\mathrm{Na}}_2\mathrm{O}$

When sulphur dioxide is passed through a saturated solution of hydrogen sulphide, the following reaction occurs:

${\mathrm{SO}}_2 +2{\mathrm{H}}_2\mathrm{S} \to 2{\mathrm{H}}_2\mathrm{O} + 3\mathrm{S}$

Predict the reducing agent and oxidising agent in this reaction.