In which of the following reactions does water act as a reducing agent?

Options

A. `2Na+2H_( 2)Oto2NaOH+H_(2)`
B. `2Cl_(2)+2H_(2)Oto4HCl+O_(2)`
C. `CaC_(2)+2H_(2)OtoCa(OH)_(2)C_(2)H_(2)`
D. `H_(2)O+NH_(3)toNH_(4)OH`

Answer

To determine in which reaction water acts as a reducing agent, we need to analyze the oxidation states of the elements involved in the reactions provided. A reducing agent is a substance that donates electrons in a chemical reaction, causing another substance to be reduced.

### Step-by-Step Solution:

1. **Identify the Reactions**:
We need to look at the reactions provided in the question. For this example, let's consider two reactions:
- Reaction 1: \( \text{Na} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2 \)
- Reaction 2: \( \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{O}_2 \)

2. **Determine Oxidation States**:
- In Reaction 1:
- Sodium (Na) starts at 0 and goes to +1 in NaOH.
- Hydrogen in water (H2O) is +1 and remains +1 in NaOH, but the H2 produced is 0.
- Oxygen in water is -2 and remains -2 in NaOH.
- The oxidation state of H in H2 changes from +1 to 0, indicating reduction.
- In Reaction 2:
- Chlorine (Cl) starts at 0 and goes to -1 in HCl.
- The oxidation state of oxygen in water is -2 and changes to 0 in O2, indicating oxidation.

3. **Identify Oxidation and Reduction**:
- In Reaction 1, sodium is oxidized (loses electrons), and water is not acting as a reducing agent since it does not donate electrons.
- In Reaction 2, chlorine is reduced (gains electrons), and water is oxidized (loses electrons), indicating that water is acting as a reducing agent.

4. **Conclusion**:
Based on the analysis, water acts as a reducing agent in Reaction 2: \( \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{O}_2 \).

### Final Answer:
Water acts as a reducing agent in Reaction 2.