Assertion (A) : The rate constant of a pseudo unimolecular reaction has the units of a second order reaction. Reason (R ): A pseudo unimolecular reaction is a reaction of second order in whiich one of the reactant is present in large excess.

Options

A. If both `(A)` and `(R )` are correct, and `(R )` is the correct explnation of `(A)`.
B. If both `(A)` and `(R )` are correct, but `(R )` is noth the correct explanation of `(A)`.
C. If `(A)` is correct, but `(R )` is incorrect.
D. If `(A)` is incorrect, but `(R )` is correct.

Answer

To solve the question regarding the assertion and reason about pseudo unimolecular reactions, we can break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "The rate constant of a pseudo unimolecular reaction has the units of a second-order reaction."

- **Explanation**: A pseudo unimolecular reaction typically involves two reactants, but one of them is present in such a large excess that its concentration remains effectively constant during the reaction. This allows us to simplify the rate law to depend primarily on the concentration of the other reactant.

### Step 2: Analyze the Reason
The reason states that "A pseudo unimolecular reaction is a reaction of second order in which one of the reactants is present in large excess."

- **Explanation**: This statement is misleading. While a pseudo unimolecular reaction involves two reactants, it behaves as a first-order reaction with respect to the reactant that is not in excess. The overall reaction order is not second-order; it is first-order because the concentration of the excess reactant is constant.

### Step 3: Determine the Rate Law
For a pseudo unimolecular reaction, if we assume the reaction is:
\[ A + B \rightarrow \text{Products} \]
where B is in large excess, the rate law can be expressed as:
\[ \text{Rate} = k' [A] \]
where \( k' = k[B] \) (with \( k \) being the rate constant and \( [B] \) being the concentration of the excess reactant).

### Step 4: Identify the Units of the Rate Constant
The units of the rate constant \( k' \) for a first-order reaction are:
\[ \text{Units of } k' = \text{concentration}^{-1} \cdot \text{time}^{-1} = \text{M}^{-1} \cdot \text{s}^{-1} \]
However, since \( k' \) incorporates the concentration of the excess reactant, the effective units of \( k' \) will still reflect the second-order nature of the original reaction when considering both reactants.

### Step 5: Conclusion
- The assertion is **correct** because the rate constant of a pseudo unimolecular reaction does indeed have units that can be associated with a second-order reaction when considering the original reaction context.
- The reason is **incorrect** because it mischaracterizes the order of the reaction.

### Final Answer
The correct answer is **Option C**: The assertion is correct, but the reason is incorrect.

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