Question

The order of a reaction whose rate constant \(k = 0.693\times10^{-6}\,\text{L mol}^{-1}\text{s}^{-1}\) is:

• A. 0
• B. 2
• C. 1
• D. 3

Hint:

The unit \(\text{L mol}^{-1}\text{s}^{-1}\) is the most commonly asked unit for a second-order reaction in competitive examinations.

Answer 1

The Correct Option is B

Concept: The unit of the rate constant depends upon the order of reaction. For an \(n^{th}\) order reaction, \[ [k] = (\text{concentration})^{1-n} (\text{time})^{-1} \] Hence, identifying the unit of \(k\) immediately reveals the order of reaction.

Step 1: Write the given unit of rate constant.
\[ k = 0.693\times10^{-6} \text{L mol}^{-1}\text{s}^{-1} \] Thus, \[ [k] = \text{L mol}^{-1}\text{s}^{-1} \]

Step 2: Recall standard units.
Zero order: \[ \text{mol L}^{-1}\text{s}^{-1} \] First order: \[ \text{s}^{-1} \] Second order: \[ \text{L mol}^{-1}\text{s}^{-1} \] Third order: \[ \text{L}^{2}\text{mol}^{-2}\text{s}^{-1} \]

Step 3: Compare the units.
The given unit exactly matches the unit of a second-order reaction. Therefore, \[ \boxed{\text{Order}=2} \]