Assertion : Complex reaction takes place in different steps and the slowest step determines the rate of reaction.
Reason : Order and molecularity of a reaction are always equal.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that complex reactions occur in multiple steps and that the slowest step (known as the rate-determining step) dictates the overall rate of the reaction.
- This is a fundamental concept in chemical kinetics. In a multi-step reaction, the step that takes the longest time to complete will limit the speed of the entire reaction.

2. **Example of a Complex Reaction:**
- Consider the reaction between nitrogen dioxide (NO2) and carbon monoxide (CO), which produces nitric oxide (NO) and carbon dioxide (CO2).
- The overall reaction can be represented as:
\[ 2 \text{NO}_2(g) + \text{CO}(g) \rightarrow 2 \text{NO}(g) + \text{CO}_2(g) \]
- Experimental data shows that the rate of this reaction is proportional to the square of the concentration of NO2, indicating that the rate-determining step is independent of CO concentration.

3. **Mechanism of the Reaction:**
- The reaction can be broken down into two steps:
- **Step 1 (Slow Step):**
\[ \text{NO}_2(g) + \text{NO}(g) \rightarrow \text{NO}(g) + \text{NO}_3(g) \]
- **Step 2 (Fast Step):**
\[ \text{NO}_3(g) + \text{CO}(g) \rightarrow \text{CO}_2(g) + \text{NO}_2(g) \]
- The first step is the slow step and thus determines the rate of the overall reaction.

4. **Rate Expression:**
- The rate of the overall reaction can be expressed as:
\[ \text{Rate} = k[\text{NO}_2]^2 \]
- This indicates that the reaction is second order with respect to NO2.

5. **Understanding the Reason:**
- The reason states that the order and molecularity of a reaction are always equal.
- This is not true for complex reactions. The order of a reaction is determined experimentally and can be different from the molecularity, which refers to the number of molecules involved in the rate-determining step.

6. **Counterexample:**
- For example, if two moles of NO react to form N2O2:
- The rate expression would be:
\[ \text{Rate} = k[\text{NO}]^2 \]
- Here, the reaction is second order (order = 2), but the molecularity is unimolecular (molecularity = 1) because only one type of molecule is involved in the rate-determining step.

7. **Conclusion:**
- The assertion is correct: complex reactions do occur in steps, and the slowest step determines the rate.
- The reason is incorrect: order and molecularity are not always equal.
- Therefore, the correct answer is that the assertion is true, but the reason is false.