Question

Sulphur dioxide is a strong reducing agent. However, it can also act as an oxidizing agent. Which of the following reactions shows its oxidizing nature?
A) Bleaching flower petals
B) Decolourising of acidified $KMn{O_4}$ solution
C) Reaction with ${H_2}S$ to give sulphur
D) Turning acidified dichromate paper green

Answer 1

Reduction is the reaction in which oxygen is removed, hydrogen or electron is gained while oxidation is the reaction in which oxygen is gained, hydrogen or electron is removed. The agents which cause reduction are called the reducing agent and the agent which causes oxidation is called an oxidizing agent.

Complete answer:
Bleaching is a reduction process. Decolourising of acidified $KMn{O_4}$ solution occurs when it gets reduced from manganate ion to manganese. Turning acidified dichromate paper green is also a reduction reaction. Hence, options A), B), and D) are incorrect.
The reaction of sulphur dioxide with ${H_2}S$ to give sulphur will be - ${}^{ + 4}S{O_2} + 2{H_2}{S^{ - 2}} \to 2{S^0} + 2{H_2}O$
Here, the sulphur dioxide is acting as an oxidizing agent and converts sulphur in ${H_2}S$ from $- 2$ to $0$ in S.
Hence, the correct option is C) Reaction with ${H_2}S$ to give sulphur. In this reaction, sulphur dioxide shows its oxidizing nature.

Note:
Sulphur dioxide exists in gaseous form and has a pungent smell. It is a strong reducing agent. It occurs as in nature as the product of volcanic activity and the by-product of copper extraction and is also released from the burning of sulphur-containing fossil fuels.