Which of the following can act as reducing agent ?

Explanation

To determine which of the given options can act as a reducing agent, we need to analyze each compound and identify their oxidation states. A reducing agent is a substance that donates electrons to another substance, reducing that substance while itself being oxidized.

Let's evaluate each option step by step:

### Step 1: Understand the Definition of a Reducing Agent
A reducing agent is a substance that reduces another substance by donating electrons and, in the process, gets oxidized itself.

### Step 2: Analyze Option A - K₂Cr₂O₇
1. Determine the oxidation state of chromium (Cr) in K₂Cr₂O₇.
- Potassium (K) has an oxidation state of +1.
- Let the oxidation state of Cr be x.
- The formula gives: 2(+1) + 2x + 7(-2) = 0
- This simplifies to: 2 + 2x - 14 = 0 → 2x - 12 = 0 → 2x = 12 → x = +6.
2. Since +6 is the maximum oxidation state of chromium, K₂Cr₂O₇ cannot act as a reducing agent.
- **Conclusion**: K₂Cr₂O₇ is an oxidizing agent.

### Step 3: Analyze Option B - CaCl₂
1. Determine the oxidation state of calcium (Ca) in CaCl₂.
- Chlorine (Cl) has an oxidation state of -1.
- Let the oxidation state of Ca be x.
- The formula gives: x + 2(-1) = 0 → x - 2 = 0 → x = +2.
2. Since +2 is the maximum oxidation state of calcium, CaCl₂ cannot act as a reducing agent.
- **Conclusion**: CaCl₂ is an oxidizing agent.

### Step 4: Analyze Option C - PbO₂
1. Determine the oxidation state of lead (Pb) in PbO₂.
- Oxygen (O) has an oxidation state of -2.
- Let the oxidation state of Pb be x.
- The formula gives: x + 2(-2) = 0 → x - 4 = 0 → x = +4.
2. Since +4 is the maximum oxidation state of lead, PbO₂ cannot act as a reducing agent.
- **Conclusion**: PbO₂ is an oxidizing agent.

### Step 5: Analyze Option D - H₂S
1. Determine the oxidation state of sulfur (S) in H₂S.
- Hydrogen (H) has an oxidation state of +1.
- Let the oxidation state of S be x.
- The formula gives: 2(+1) + x = 0 → 2 + x = 0 → x = -2.
2. Sulfur can have multiple oxidation states, and it can be oxidized to +6. Thus, H₂S can act as a reducing agent as it can donate electrons.
- **Conclusion**: H₂S is a reducing agent.

### Final Conclusion
Among the options provided:
- Option A (K₂Cr₂O₇) is an oxidizing agent.
- Option B (CaCl₂) is an oxidizing agent.
- Option C (PbO₂) is an oxidizing agent.
- Option D (H₂S) is a reducing agent.

**The correct answer is option D: H₂S.**

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