In the reaction `A + B rArr "Products"`, if B is taken in excess, then it is an example of

Options

A. Second order reaction
B. Zero order reaction
C. Fractional order reaction
D. First order reaction

Explanation

To determine the order of the reaction \( A + B \rightarrow \text{Products} \) when B is taken in excess, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Reaction**: The reaction involves two reactants, A and B, combining to form products. The stoichiometry of the reaction is 1:1.

2. **Writing the Rate Law**: The general form of the rate law for this reaction can be expressed as:
\[ \text{Rate} = k[A]^m[B]^n \]
where \( k \) is the rate constant, and \( m \) and \( n \) are the orders of the reaction with respect to A and B, respectively.

3. **Determining the Order**: Since the stoichiometry of A and B is 1:1, we initially assume that \( m = 1 \) and \( n = 1 \). Therefore, the overall order of the reaction would be:
\[ \text{Overall Order} = m + n = 1 + 1 = 2 \]
This indicates that the reaction is second order under normal conditions.

4. **Considering B in Excess**: When B is in excess, its concentration remains relatively constant during the reaction. This means that changes in the concentration of B will not significantly affect the rate of the reaction.

5. **Revising the Rate Law**: When B is in excess, we can simplify the rate law. The concentration of B can be treated as a constant, leading to a modified rate law:
\[ \text{Rate} = k'[A] \]
where \( k' = k[B]^n \) (with \( n \) being constant).

6. **Determining the New Order**: With B treated as a constant, the rate law now depends only on the concentration of A. This means the reaction behaves as a first-order reaction with respect to A:
\[ \text{Overall Order} = 1 \]

7. **Conclusion**: Therefore, when B is taken in excess, the reaction \( A + B \rightarrow \text{Products} \) behaves as a first-order reaction with respect to A.

### Final Answer:
The reaction \( A + B \rightarrow \text{Products} \) with B taken in excess is an example of a **first-order reaction**.

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