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 of the statements regarding chemical kinetics is correct, we will analyze each option step by step.

### Step 1: Analyze the First Statement
**Statement:** Zero order reactions are complex reactions.

- **Definition of Zero Order Reaction:** A zero order reaction is one where the rate of reaction is independent of the concentration of the reactants. This means that even if the concentration of the reactants changes, the rate remains constant.

- **Complex Reaction:** A complex reaction involves multiple steps, which may include electron transfer steps and other chemical processes.

- **Example:** The decomposition of ammonia (2NH3 → N2 + 3H2) is a zero order reaction. In this reaction, increasing the concentration of ammonia does not change the rate of reaction, indicating that it is indeed a zero order reaction. Since it involves multiple steps, it can also be classified as a complex reaction.

**Conclusion for Step 1:** The first statement is correct.

### Step 2: Analyze the Second Statement
**Statement:** Reactions having first order may be either elementary or complex reactions.

- **Definition of First Order Reaction:** A first order reaction is one where the rate is directly proportional to the concentration of one reactant.

- **Elementary Reaction:** An elementary reaction is a single step reaction that proceeds in one step.

- **Complex Reaction:** As defined earlier, a complex reaction involves multiple steps.

- **Conclusion:** A first order reaction can indeed be either elementary (one step) or complex (multiple steps).

**Conclusion for Step 2:** The second statement is correct.

### Step 3: Analyze the Third Statement
**Statement:** Reaction having second order reaction must be a molarity.

- **Definition of Molecularity:** Molecularity refers to the number of reactant molecules involved in an elementary reaction step.

- **Second Order Reaction:** A second order reaction can involve either one reactant (bimolecular) or two reactants (unimolecular) but does not necessarily mean that the molecularity is equal to the order of the reaction.

- **Example:** For a reaction like A + B → products, if it is second order, it can be bimolecular (Molecularity = 2). However, a reaction like 2A → products is also second order but has a molecularity of 2.

- **Conclusion:** The order of a reaction does not have to equal the molecularity. For instance, a zero order reaction can have a molecularity of 2.

**Conclusion for Step 3:** The third statement is incorrect.

### Final Conclusion:
- **Correct Statements:** The first and second statements are correct.
- **Incorrect Statement:** The third statement is incorrect.