The reaction, A + B `to` P can never be

Options

A. Second order reaction
B. First order reaction
C. Unimolecular reaction
D. Biomolecular reaction

Answer

To determine which type of reaction the equation A + B → P can never be, we need to analyze the definitions of molecularity and order of a reaction.

### Step-by-Step Solution:

1. **Understanding the Reaction**: The reaction given is A + B → P, where A and B are the reactants and P is the product.

2. **Defining Molecularity**: Molecularity refers to the number of reactant species involved in an elementary reaction. In this case, there are two reactants (A and B), which means the molecularity of this reaction is 2.

3. **Identifying Reaction Types**:
- **Unimolecular Reaction**: This type involves only one reactant species. Since we have two reactants (A and B), this reaction cannot be unimolecular.
- **Bimolecular Reaction**: This type involves two reactant species. Since we have A and B, this reaction can be classified as bimolecular.
- **First Order Reaction**: A reaction can be first order if the rate depends on the concentration of one reactant. This is possible here since we can have a rate law that includes either A or B.
- **Second Order Reaction**: A reaction can also be second order if the rate depends on the concentrations of both reactants. This is also possible here.

4. **Conclusion**: Since the reaction A + B → P involves two reactants, it can be classified as bimolecular. It can also be first or second order depending on the rate law. However, it can never be unimolecular because unimolecular reactions only involve one reactant species.

### Final Answer:
The reaction A + B → P can never be a **unimolecular reaction**.