In which of the following reactions `H_(2)O_(2)` acts as a reducing agent ?

Options

A. `H_(2)O_((l)) + NH_(3(aq)) hArr OH_((aq))^(-) + NH_(4(aq))^(+)`
B. `H_(2)O_((l)) + H_(2)S_((aq)) hArr H_(3)O_((aq))^(+) + HS_((aq))^(-)`
C. `H_(2)O_((l)) + H_(2)O_((l)) hArr H_(3)O_((aq))^(+) + OH_((aq))^(-)`
D. `H_((aq))^(+) + OH_((aq))^(-) hArr H_(2)O_((l))`

Explanation

To determine in which of the given reactions \( H_2O_2 \) acts as a reducing agent, we will analyze each reaction step by step. A reducing agent is a substance that donates electrons and is oxidized in the process, leading to a decrease in its oxidation state.

### Step 1: Analyze the first reaction
**Reaction:** \( H_2O_2 + NH_3 \rightarrow OH^- + NH_4^+ \)

1. **Assign oxidation states:**
- \( H_2O_2 \): H = +1, O = -1
- \( NH_3 \): N = -3, H = +1
- \( OH^- \): O = -2, H = +1
- \( NH_4^+ \): N = -3, H = +1

2. **Check for changes in oxidation states:**
- In \( H_2O_2 \), O remains at -1, and in \( OH^- \), O is -2 (reduction).
- N in \( NH_3 \) remains -3 in \( NH_4^+ \) (no oxidation).

**Conclusion:** \( H_2O_2 \) is not acting as a reducing agent here.

### Step 2: Analyze the second reaction
**Reaction:** \( H_2O_2 + H_2S \rightarrow H_3O^+ + HS^- \)

1. **Assign oxidation states:**
- \( H_2O_2 \): H = +1, O = -1
- \( H_2S \): H = +1, S = -2
- \( H_3O^+ \): H = +1, O = -2
- \( HS^- \): H = +1, S = -2

2. **Check for changes in oxidation states:**
- O in \( H_2O_2 \) remains -1, and in \( H_3O^+ \), it is -2 (reduction).
- S in \( H_2S \) remains -2 in \( HS^- \) (no oxidation).

**Conclusion:** \( H_2O_2 \) is not acting as a reducing agent here.

### Step 3: Analyze the third reaction
**Reaction:** \( H_2O_2 + H_2O \rightarrow H_3O^+ + OH^- \)

1. **Assign oxidation states:**
- \( H_2O_2 \): H = +1, O = -1
- \( H_2O \): H = +1, O = -2
- \( H_3O^+ \): H = +1, O = -2
- \( OH^- \): O = -2, H = +1

2. **Check for changes in oxidation states:**
- O in \( H_2O_2 \) remains -1, and in \( H_3O^+ \) and \( OH^- \), it is -2 (reduction).
- No oxidation occurs.

**Conclusion:** \( H_2O_2 \) is not acting as a reducing agent here.

### Step 4: Analyze the fourth reaction
**Reaction:** \( H^+ + OH^- \rightarrow H_2O_2 \)

1. **Assign oxidation states:**
- \( H^+ \): H = +1
- \( OH^- \): O = -2, H = +1
- \( H_2O_2 \): H = +1, O = -1

2. **Check for changes in oxidation states:**
- H in \( H^+ \) remains +1.
- O in \( OH^- \) is -2, and in \( H_2O_2 \), it is -1 (oxidation).
- No reduction occurs.

**Conclusion:** \( H_2O_2 \) is not acting as a reducing agent here.

### Final Conclusion
After analyzing all reactions, we find that in none of the reactions does \( H_2O_2 \) act as a reducing agent.