Question

Give reactions to show that SO₂ gas acts as a reducing agent.

Answer 1

Sulphur dioxide (SO₂) acts as a reducing agent in the presence of moisture.

(i) Sulphur dioxide, SO₂ reduces halogens (X⁰) to haloacids (X¹⁻).

\[\ce{\overset{0}{I}_2 + 2H2O + SO2 -> 2\overset{-1}{HI} + H2SO4}\]

(ii) SO₂ gas when passed through acidified solution of KMnO₄ it is decolourised due to reduction (Mn⁷⁺ to Mn²⁺).

\[\ce{2K\overset{7+}{M}nO4 + 2H2O + 5SO2 -> K2SO4 + 2M\overset{2+}{n}SO4 + 2H2SO4}\]

(iii) Sulphur dioxide (SO₂) reduces Fe³⁺ to Fe²⁺ 

\[\ce{2\overset{3+}{F}eCl3 + SO2 + 2H2O -> 2\overset{2+}{F}eCl2 + H2SO4 + 2HCl}\]