In which of the following reactions, hydrogen acts as an oxidising agent?

Options

A. `H_2 + I_2 to 2HI`
B. `2Li + H_2 to 2LiH`
C. `N_2 + 3H_2 to 2NH_3`
D. `H_2 + S to H_2S`

Explanation

To determine in which of the given reactions hydrogen acts as an oxidizing agent, we need to analyze the oxidation states of the elements involved in each reaction. An oxidizing agent is a substance that causes another substance to be oxidized (lose electrons) while it itself gets reduced (gains electrons).

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to examine the reactions provided in the question. Let's assume we have the following reactions:
- Reaction 1: H2 + I2 → 2 HI
- Reaction 2: H2 + Li → 2 LiH
- Reaction 3: H2 + N2 → 2 NH3
- Reaction 4: H2 + S → H2S

2. **Determine Oxidation States**:
- **Reaction 1 (H2 + I2 → 2 HI)**:
- Oxidation state of H in H2 = 0 (elemental state)
- Oxidation state of I in I2 = 0 (elemental state)
- In HI, H = +1 and I = -1
- Change: H goes from 0 to +1 (oxidized), I goes from 0 to -1 (reduced).
- **Conclusion**: H acts as a reducing agent here.

- **Reaction 2 (H2 + Li → 2 LiH)**:
- Oxidation state of H in H2 = 0
- Oxidation state of Li in elemental state = 0
- In LiH, Li = +1 and H = -1
- Change: Li goes from 0 to +1 (oxidized), H goes from 0 to -1 (reduced).
- **Conclusion**: H acts as an oxidizing agent here.

- **Reaction 3 (H2 + N2 → 2 NH3)**:
- Oxidation state of H in H2 = 0
- Oxidation state of N in N2 = 0
- In NH3, N = -3 and H = +1
- Change: N goes from 0 to -3 (reduced), H goes from 0 to +1 (oxidized).
- **Conclusion**: H acts as a reducing agent here.

- **Reaction 4 (H2 + S → H2S)**:
- Oxidation state of H in H2 = 0
- Oxidation state of S in elemental state = 0
- In H2S, S = -2 and H = +1
- Change: S goes from 0 to -2 (reduced), H goes from 0 to +1 (oxidized).
- **Conclusion**: H acts as a reducing agent here.

3. **Final Conclusion**:
- Hydrogen acts as an oxidizing agent only in the reaction with lithium (Reaction 2: H2 + Li → 2 LiH). In all other reactions, hydrogen acts as a reducing agent.

### Answer:
The correct answer is **Reaction 2: H2 + Li → 2 LiH**.