A reaction involving two different reactants can never be:

Options

A. (a) Second order reaction
B. (b) Bimolecular reaction
C. (c) Unimolecular reaction
D. (d) First order reaction

Answer

To solve the question "A reaction involving two different reactants can never be:", we need to analyze the concepts of molecularity and order of reaction.

### Step-by-Step Solution:

1. **Understanding Molecularity**:
- Molecularity refers to the number of reactant molecules that collide to form products in a reaction.
- It is classified as:
- **Unimolecular**: Involves one reactant molecule (e.g., A → products).
- **Bimolecular**: Involves two reactant molecules (e.g., A + B → products).
- **Termolecular**: Involves three reactant molecules (less common).

2. **Identifying the Reactants**:
- The question specifies that we have two different reactants, which we can denote as A and B.

3. **Determining the Molecularity**:
- Since we have two different reactants (A and B), the reaction must involve a collision between these two molecules.
- Therefore, the molecularity of the reaction is **bimolecular**.

4. **Understanding Reaction Order**:
- The order of a reaction is the sum of the powers of the concentration terms in the rate law expression.
- For a bimolecular reaction involving A and B, the rate law can be expressed as:
\[ \text{Rate} = k[A]^m[B]^n \]
- Here, \(m\) and \(n\) can be 1, leading to an overall order of 2 (second order), or one of the reactants can be in excess, leading to a pseudo-unimolecular reaction (first order).

5. **Conclusion**:
- Since the reaction involves two different reactants, it cannot be unimolecular.
- Therefore, the correct answer to the question is that a reaction involving two different reactants can never be **unimolecular**.

### Final Answer:
A reaction involving two different reactants can never be **unimolecular**.

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