Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases `H_(2)O_(2)` acts as a reducing agent in acid medium ?

Options

A. `Kl`
B. `Cr_(2)O_(7)^(2-)`
C. `SO_(3)^(2-)`
D. `MnO_(4)^(-)`

Answer

To determine in which case hydrogen peroxide (H₂O₂) acts as a reducing agent in an acidic medium, we need to analyze the nature of the reacting species provided in the options.

### Step-by-Step Solution:

1. **Understanding the Role of H₂O₂**:
- Hydrogen peroxide can act as both an oxidizing agent and a reducing agent depending on the other substances it reacts with. In acidic medium, it typically acts as a reducing agent when it reacts with a strong oxidizing agent.

2. **Identifying Strong Oxidizing Agents**:
- Among the given options, we need to identify which species are strong oxidizing agents. The options provided are:
- A) KI
- B) Cr₂O₇²⁻ (Dichromate)
- C) SO₃²⁻ (Sulfite)
- D) MnO₄⁻ (Permanganate)

3. **Analyzing Each Option**:
- **KI**: Potassium iodide does not act as a strong oxidizing agent.
- **Cr₂O₇²⁻**: Dichromate is a strong oxidizing agent, but it is not as strong as MnO₄⁻ in acidic medium.
- **SO₃²⁻**: Sulfite is a reducing agent and does not act as a strong oxidizing agent.
- **MnO₄⁻**: Permanganate ion is a well-known strong oxidizing agent, especially in acidic conditions.

4. **Conclusion**:
- Since H₂O₂ acts as a reducing agent when reacting with a strong oxidizing agent, and MnO₄⁻ is the only strong oxidizing agent among the options, we conclude that H₂O₂ acts as a reducing agent in the presence of MnO₄⁻ in acidic medium.

5. **Final Answer**:
- Therefore, the correct answer is **D) MnO₄⁻**.