In which of the following reaction boron does not act as reducing agent ?

Options

A. `B + CO_(2) to`
B. `B + Mg to `
C. `B + SiO_(2) to`
D. `B + HNO_(3) to`

Answer

To determine in which reaction boron does not act as a reducing agent, we need to analyze each reaction provided in the options. A reducing agent is a substance that donates electrons in a chemical reaction, causing another substance to be reduced (gain electrons) while itself gets oxidized (loses electrons).

### Step-by-Step Solution:

1. **Understand the Role of Reducing Agents**:
- A reducing agent is characterized by a decrease in its oxidation state during a chemical reaction. If boron is oxidized (its oxidation state increases), it is acting as a reducing agent.

2. **Analyze Each Reaction**:
- **Option 1**: Boron + Carbon Dioxide (B + CO2)
- Reaction: \( B + CO_2 \rightarrow B_2O_3 + C \)
- In this reaction, boron is oxidized from 0 to +3 (in B2O3), while carbon is reduced from +4 (in CO2) to 0 (in C). Thus, boron acts as a reducing agent here.

- **Option 2**: Boron + Magnesium (B + Mg)
- Reaction: \( 3Mg + B_2 \rightarrow Mg_3B_2 \)
- In this case, boron does not change its oxidation state; it remains at +3 in Mg3B2. Therefore, boron does not act as a reducing agent in this reaction.

- **Option 3**: Boron + Silicon Dioxide (B + SiO2)
- Reaction: \( 2B + SiO_2 \rightarrow B_2O_3 + Si \)
- Here, boron is oxidized from 0 to +3, while silicon is reduced from +4 to 0. Thus, boron acts as a reducing agent.

- **Option 4**: Boron + Nitric Acid (B + HNO3)
- Reaction: \( B + HNO_3 \rightarrow H_3BO_3 + NO_2 \)
- In this reaction, boron is oxidized from 0 to +3 in H3BO3, while nitrogen is reduced from +5 (in HNO3) to +4 (in NO2). Thus, boron acts as a reducing agent.

3. **Conclusion**:
- After analyzing all the reactions, we find that in **Option 2** (Boron + Magnesium), boron does not act as a reducing agent because its oxidation state does not change.

### Final Answer:
**Boron does not act as a reducing agent in Option 2 (B + Mg).**