State a condition under which a bimolecular reaction is kinetically first order reaction.

Explanation

To determine the condition under which a bimolecular reaction behaves as a first-order reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Reaction Order**:
- A bimolecular reaction typically involves two reactants and is second-order overall. However, under certain conditions, it can appear to be first-order.

2. **Identify the Condition**:
- The key condition for a bimolecular reaction to be kinetically first-order is the presence of one reactant in excess.

3. **Explain the Concept of Excess Reactant**:
- When one reactant is present in a significantly larger amount compared to the other, its concentration remains relatively constant throughout the reaction.

4. **Example of a Pseudo First-Order Reaction**:
- Consider the acid-catalyzed hydrolysis of ethyl acetate:
- The reaction can be represented as:
\[ \text{Ethyl Acetate} + \text{Water} \xrightarrow{\text{Acid}} \text{Acetic Acid} + \text{Ethanol} \]
- In this case, if water is present in excess, the concentration of water does not change significantly during the reaction.

5. **Rate Law Derivation**:
- The rate law for this reaction can be expressed as:
\[ \text{Rate} = k[\text{Ethyl Acetate}][\text{Water}] \]
- If water is in excess, its concentration can be considered constant, leading to:
\[ \text{Rate} = k'[\text{Ethyl Acetate}] \]
where \( k' = k[\text{Water}] \) (a new constant).

6. **Conclusion**:
- Thus, the reaction behaves as a first-order reaction with respect to ethyl acetate, even though it is fundamentally bimolecular. This is why it is referred to as a pseudo first-order reaction.

### Final Statement:
The condition under which a bimolecular reaction behaves as a first-order reaction is when one reactant is present in excess, allowing the reaction rate to depend solely on the concentration of the other reactant.

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