A16 Reactions of Sulfur Dioxide

Description
Sulfur dioxide acts as both an oxidizing and a reducing agent. Sulfur dioxide reduces both a potassium permanganate solution and a potassium dichromate solution. A color change is observed as each solution is poured into the gas.

Sulfur dioxide oxidizes hydrogen sulfide gas to sulfur in aqueous droplets. Note that sulfur with two different oxidation numbers(-2 and +4) combine to form sulfur of an intermediate oxidation number(0).

Equation
2 MnO₄^- + 5 SO₂ + H⁺ + 2 H₂O --> 2 Mn²⁺ + 5 HSO₄^-

Cr₂O₇²^- + 3 SO₂ + 5 H⁺--> 2 Cr³⁺ + 3 HSO₄^- + H₂O

2 H₂S + SO₂--> 3 S + 2 H₂O

Reference
B. Z. Shakhashiri, "Chemical Demonstrations", Vol. 2, The University of Wisconsin Press, Madison ,Wisconsin, (1985) p.184.