Which of the following statement is incorrect?

Options

A. A second order reaction must be a bimolecular elementry reaction
B. A bimolecular elementry reaction must be a second order reacton
C. Zero order reaction must be a complex reaction
D. First order reaction may be complex or elementary reaction

Answer

To determine which statement is incorrect, we will analyze each option one by one.

### Step 1: Analyze the first option
**Statement:** A second-order reaction must be a biomolecular elementary reaction.

**Analysis:** This statement is incorrect. A second-order reaction can arise from:
- Two reactants in an elementary reaction (biomolecular).
- A single reactant undergoing a reaction that is second order (e.g., A → products, where the rate depends on the concentration of A squared).

Thus, a second-order reaction does not have to be biomolecular; it can also be from complex reactions.

### Step 2: Analyze the second option
**Statement:** A biomolecular elementary reaction must be a second-order reaction.

**Analysis:** This statement is correct. In an elementary reaction, the order of the reaction is equal to its molecularity. Since a biomolecular reaction involves two reactants, the overall order is 2, making it a second-order reaction.

### Step 3: Analyze the third option
**Statement:** A zero-order reaction must be a complex reaction.

**Analysis:** This statement is correct. A zero-order reaction cannot be elementary because an elementary reaction requires a stoichiometric relationship between reactants. Zero-order reactions typically occur when a reactant is in excess or when the reaction is limited by a surface or catalyst, which makes them complex.

### Step 4: Analyze the fourth option
**Statement:** A first-order reaction may be a complex or an elementary reaction.

**Analysis:** This statement is correct. A first-order reaction can indeed be either an elementary reaction (where the rate depends on the concentration of one reactant) or a complex reaction (where the rate law can be derived from the mechanism).

### Conclusion
After analyzing all the options, the only incorrect statement is the **first option**: "A second-order reaction must be a biomolecular elementary reaction."