In which of the following reactions hydrogen is acting as an oxidising agent?

Explanation

To determine in which of the following reactions hydrogen acts as an oxidizing agent, we need to analyze the oxidation states of hydrogen in each reaction. An oxidizing agent is a substance that gains electrons (is reduced) and, in doing so, causes another substance to be oxidized.

### Step-by-Step Solution:

1. **Understand Oxidation States**:
- The oxidation state of hydrogen (H) in its elemental form (H₂) is 0.
- When hydrogen forms compounds, its oxidation state can change. If it becomes more positive, it is oxidized; if it becomes more negative, it is reduced.

2. **Analyze Each Reaction**:
- **Option A**: H₂ + I₂ → 2HI
- In H₂, the oxidation state of H is 0.
- In HI, the oxidation state of H is +1.
- Here, hydrogen is oxidized (its oxidation state increases), so it cannot be an oxidizing agent.

- **Option B**: H₂ + 2Li → 2LiH
- In H₂, the oxidation state of H is 0.
- In LiH, the oxidation state of H is -1.
- Here, hydrogen is reduced (its oxidation state decreases), indicating that it acts as an oxidizing agent.

- **Option C**: N₂ + 3H₂ → 2NH₃
- In H₂, the oxidation state of H is 0.
- In NH₃, the oxidation state of H is +1.
- Here, hydrogen is oxidized, so it cannot be an oxidizing agent.

- **Option D**: H₂ + S → H₂S
- In H₂, the oxidation state of H is 0.
- In H₂S, the oxidation state of H is +1.
- Here, hydrogen is oxidized, so it cannot be an oxidizing agent.

3. **Conclusion**:
- The only reaction where hydrogen acts as an oxidizing agent is **Option B** (H₂ + 2Li → 2LiH), as its oxidation state decreases from 0 to -1.

### Final Answer:
**Option B** is the reaction in which hydrogen acts as an oxidizing agent.

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