The reaction equation is:
HBr + H₂SO₄ → SO₂ + Br₂ + H₂O
An oxidizing agent is a substance that itself becomes reduced and oxidizes the other chemical species in the reaction mixture. A reducing agent is similar, except that it becomes oxidized and reduces the other substance.
In the reaction, the valency of bromine changes from -1 to 0, so it is oxidized, making it the reducing agent. The valency of sulfur changes from +6 to +4, so it is reduced, making it the oxidizing agent.
