Question

The rate constant for the reaction, $2\text{N}_2\text{O}_{5(\text{g})} \longrightarrow 2\text{N}_2\text{O}_{4(\text{g})} + \text{O}_{2(\text{g})}$ is $4.98 \times 10^{-4} \text{ s}^{-1}$ . What is the order of reaction?

• A. 2
• B. 1
• C. Zero
• D. 3

Hint:

Unit of $k$ is $s^{-1}$? It's ALWAYS first order. Unit is $M^{-1}s^{-1}$? It's second order.

Answer 1

The Correct Option is B

Step 1: Concept
The order of a reaction can often be determined by looking at the units of the rate constant ($k$).

Step 2: Meaning
For a reaction of order $n$, the units of $k$ are $(\text{mol L}^{-1})^{1-n} \text{ s}^{-1}$.

Step 3: Analysis
The given rate constant is $4.98 \times 10^{-4} \text{ s}^{-1}$. The unit $\text{s}^{-1}$ (per second) is characteristic of a first-order reaction, where $1-n = 0$, so $n = 1$.

Step 4: Conclusion
The decomposition of $\text{N}_2\text{O}_5$ is a well-known first-order reaction. Final Answer: (B)