State one condition under which a bimolecular reaction may be kinetically of first order reactions.

Options

A. When both reactants have same concentration
B. when one of the reacting species is in large excess
C. when the reaction is at equilibrium
D. when the activation energy of reaction is less

Answer

To determine the condition under which a bimolecular reaction may be kinetically of first order, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Reaction Orders**:
- A bimolecular reaction typically involves two reactants and is expected to depend on the concentration of both reactants. The rate law for a bimolecular reaction is generally expressed as:
\[ \text{Rate} = k[A][B] \]
where \( A \) and \( B \) are the concentrations of the reactants, and \( k \) is the rate constant.

2. **Identifying Pseudo Unimolecular Reactions**:
- A bimolecular reaction can appear to be first order under certain conditions. This occurs in what are known as pseudo unimolecular reactions. In these reactions, one of the reactants is present in a large excess compared to the other.

3. **Condition for Kinetic First Order**:
- When one of the reactants is in a large excess, its concentration remains relatively constant throughout the reaction. This means that the change in concentration of this reactant is negligible compared to the other reactant.

4. **Simplifying the Rate Equation**:
- If we denote the reactant in excess as \( B \) and the other reactant as \( A \), the rate law can be simplified:
\[ \text{Rate} = k[A][B] \approx k'[A] \]
where \( k' = k[B] \) is a new rate constant that incorporates the constant concentration of \( B \). Thus, the reaction behaves as a first-order reaction with respect to \( A \).

5. **Example**:
- A classic example is the hydrolysis of an ester in water. If water is in large excess, the reaction can be represented as:
\[ \text{Ester} + \text{Water} \rightarrow \text{Acid} + \text{Alcohol} \]
Here, since water is in excess, the reaction appears to be first order with respect to the ester.

### Conclusion:
The condition under which a bimolecular reaction may be kinetically of first order is when one of the reactants is present in large excess.