At room temperature, the reaction between `NO` and `O_(2)` to give `NO_(2)` is fast, while that between `CO` and `O_(2)` is slow. It is due to:

Options

A. CO is smaller in size than that of NO
B. CO is poisonous
C. The activation energy for the reactions `2NO + O_2 to 2NO_2` is less then `2CO + O_2 to 2CO_2`
D. None of these

Answer

To solve the question regarding the difference in reaction rates between NO and O2 forming NO2, and CO and O2 forming CO2, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Reaction Rates**:
- The rate of a chemical reaction can vary significantly based on several factors, including the nature of the reactants, the presence of catalysts, temperature, and activation energy.

2. **Analyzing the Reactants**:
- The reactions in question are:
- \( \text{NO} + \text{O}_2 \rightarrow \text{NO}_2 \) (fast reaction)
- \( \text{CO} + \text{O}_2 \rightarrow \text{CO}_2 \) (slow reaction)
- We need to determine why one reaction is faster than the other.

3. **Considering Activation Energy**:
- Activation energy (Ea) is the minimum energy required for a reaction to occur. A lower activation energy typically results in a faster reaction rate.
- The rate constant \( k \) of a reaction is given by the Arrhenius equation:
\[ k = A e^{-\frac{E_a}{RT}} \]
where \( A \) is the pre-exponential factor, \( R \) is the gas constant, and \( T \) is the temperature in Kelvin.

4. **Comparing Activation Energies**:
- For the reaction \( \text{NO} + \text{O}_2 \rightarrow \text{NO}_2 \), it is likely that the activation energy is lower compared to the reaction \( \text{CO} + \text{O}_2 \rightarrow \text{CO}_2 \).
- This means that the reaction involving NO can proceed more readily than the one involving CO.

5. **Conclusion**:
- The faster reaction between NO and O2 is due to the lower activation energy required for the reaction to occur compared to the reaction between CO and O2.

### Final Answer:
The difference in reaction rates is due to the activation energy for the reaction \( \text{NO} + \text{O}_2 \rightarrow \text{NO}_2 \) being less than that for \( \text{CO} + \text{O}_2 \rightarrow \text{CO}_2 \).