In which of the following reactions ozone acts as a reducing agent?

Options

A. `BaO_(2)+O_(3)rarr BaO+2O_(2)`
B. `2HCl+O_(3)rarr Cl_(2)+H_(2)O+O_(2)`
C. `PbS+4O_(3)rarr PbSO_(4)+4O_(2)`
D. `2KI+O_(3)+H_(2)Orarr 2KOH+I_(2)+O_(2)`

Answer

To determine in which of the given reactions ozone (O3) acts as a reducing agent, we need to analyze the oxidation states of the elements involved in each reaction. A reducing agent is a substance that donates electrons and reduces another species while being oxidized itself.

### Step-by-Step Solution:

1. **Identify the Reactions**: We have four reactions to consider:
- Reaction 1: BaO2 + O3 → BaO + 2 O2
- Reaction 2: 2 HCl + O3 → Cl2 + H2O + O2
- Reaction 3: PbS + 4 O3 → PbSO4 + 4 O2
- Reaction 4: 2 KI + O3 + H2O → 2 KOH + I2 + O2

2. **Analyze Each Reaction**:
- **Reaction 1**: BaO2 + O3 → BaO + 2 O2
- In BaO2, Ba has an oxidation state of +4 (since O is -2, and there are 2 O atoms, total -4).
- In BaO, Ba has an oxidation state of +2.
- The oxidation state of Ba decreases from +4 to +2, indicating that Ba is reduced. Therefore, O3 is acting as a reducing agent here.

- **Reaction 2**: 2 HCl + O3 → Cl2 + H2O + O2
- In HCl, Cl has an oxidation state of -1. In Cl2, Cl has an oxidation state of 0.
- The oxidation state of Cl increases from -1 to 0, indicating that Cl is oxidized. O3 is not acting as a reducing agent here.

- **Reaction 3**: PbS + 4 O3 → PbSO4 + 4 O2
- In PbS, Pb has an oxidation state of +2. In PbSO4, Pb has an oxidation state of +2 as well.
- The oxidation state of Pb does not change, so O3 is not acting as a reducing agent here.

- **Reaction 4**: 2 KI + O3 + H2O → 2 KOH + I2 + O2
- In KI, K has an oxidation state of +1 and I has -1. In I2, I has an oxidation state of 0.
- The oxidation state of I increases from -1 to 0, indicating that I is oxidized. O3 is not acting as a reducing agent here.

3. **Conclusion**: The only reaction where ozone acts as a reducing agent is:
- **BaO2 + O3 → BaO + 2 O2** (O3 reduces Ba from +4 to +2).

### Final Answer:
Ozone acts as a reducing agent in the reaction: **BaO2 + O3 → BaO + 2 O2**.