`A + B to` products is an elementary reaction. When excess of A is taken in this reaction,then the molecularity and order are respectively

Options

A. 2 and 2
B. 2 and 1
C. 1 and 2
D. 1 and 1

Explanation

To solve the question regarding the molecularity and order of the reaction \( A + B \to \text{products} \) when excess of \( A \) is taken, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
- The reaction \( A + B \to \text{products} \) is stated to be an elementary reaction. This means it occurs in a single step and the stoichiometry directly reflects the molecularity.

2. **Determine Molecularity**:
- Molecularity is defined as the number of reactant molecules involved in an elementary reaction. In this case, we have one molecule of \( A \) and one molecule of \( B \) reacting together.
- Therefore, the molecularity of the reaction is \( 1 + 1 = 2 \).

3. **Write the Rate Law Expression**:
- The rate law for an elementary reaction can be written as:
\[ \text{Rate} = k[A]^1[B]^1 \]
- When \( A \) is in excess, the concentration of \( A \) does not change significantly during the reaction, and thus we can consider it constant.

4. **Adjust the Rate Law for Excess \( A \)**:
- When \( A \) is in excess, we can express the rate law as:
\[ \text{Rate} = k'[B]^1 \]
- Here, \( k' = k[A] \) (where \( [A] \) is treated as a constant).

5. **Determine the Order of the Reaction**:
- The order of a reaction is defined as the sum of the powers of the concentration terms in the rate law expression. In this case, since we have only \( B \) in the rate law after considering \( A \) in excess, the order is:
\[ \text{Order} = 1 \]

6. **Final Answer**:
- Thus, the molecularity of the reaction is \( 2 \) and the order of the reaction is \( 1 \).

### Summary:
- **Molecularity**: 2
- **Order**: 1