Which of the following statement (s) is/are correct?

Options

A. Zero order reactions are complex reactions.
B. A reaction having first order may be either elementary or complex reaction.
C. A reaction having second order reaction must have molecularity `= 2`.
D. A reaction with molecularity `=2` must be a second order reaction.

Explanation

To determine which statements are correct regarding the order of reactions and molecularity, let's analyze each statement step by step.

### Step 1: Analyze Statement A
**Statement A**: A zero-order reaction cannot be elementary; it must be a complex reaction.

- **Explanation**:
- A zero-order reaction is characterized by a constant rate that is independent of the concentration of the reactants.
- For elementary reactions, the order of the reaction is equal to the molecularity. Since molecularity cannot be zero, zero-order reactions cannot be elementary. Therefore, they must be complex reactions.

**Conclusion**: Statement A is **correct**.

### Step 2: Analyze Statement B
**Statement B**: A reaction having first order may be either elementary or complex.

- **Explanation**:
- A first-order reaction can arise from a single elementary step (where molecularity equals order) or from a series of steps (complex reaction) that overall yield a first-order rate law.
- Therefore, it is true that a first-order reaction can be either elementary or complex.

**Conclusion**: Statement B is **correct**.

### Step 3: Analyze Statement C
**Statement C**: A reaction having second order must have molecularity 2.

- **Explanation**:
- While it is true that for elementary reactions, the order of the reaction equals the molecularity, this does not hold for complex reactions.
- A second-order reaction can have a molecularity greater than 2 if it involves multiple steps. For example, a complex reaction can have a second-order rate law while having a molecularity of 3 or more.

**Conclusion**: Statement C is **incorrect**.

### Step 4: Analyze Statement D
**Statement D**: A reaction with molecularity 2 must be a second-order reaction.

- **Explanation**:
- Molecularity refers to the number of reactant particles involved in an elementary step. However, a reaction can have molecularity 2 and still be first-order if it involves a single step that is first-order in terms of rate law.
- For example, a reaction can be first-order overall but involve two molecules in a complex mechanism.

**Conclusion**: Statement D is **incorrect**.

### Final Conclusion
The correct statements are:
- **Statement A**: Correct
- **Statement B**: Correct
- **Statement C**: Incorrect
- **Statement D**: Incorrect

Thus, the correct options are **A and B**.

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