Question

HBr + H₂SO₄ SO₂ + Br₂ + H₂O

What is the oxidation number of the oxidizing agent?

What is the oxidation number of the reducing agent?

Answer 1

\(\text{2HBr+H₂SO₄ → SO₂+Br₂+2H₂O}\)

Use your oxidation state rules and find the oxidation states for every element/polyatomic in both the products and the reactants. Then, you can figure out what gets oxidized and reduced.

Reactants
H: 2
Br: -2
H: +2
S: +6
O: -8

Products
S: +2
O: -4
Br: 0
H: +4
O: -4


As you can see, the oxidation state for S has gone DOWN from +6 to +2, and the oxidation state for Br has gone UP from -2 to 0.
Hence, S is the one that is being oxidized, and Br is the one that is being reduced. But that is not your answer because the oxidizing and reducing agents are opposite. Br is the one that does the oxidizing, so Br is the oxidizing agent, and S is the one that does the reducing, so S is the reducing agent.