Which of the following is not a first order reaction

Options

A. Hydrolysis of an ester in acidic medium
B. Decomposition of `N_(2)O_(5)`
C. decomposition of calcium carbide
D. Oxidation of nitric oxide.

Explanation

To determine which of the following reactions is not a first-order reaction, we will analyze each option based on the information provided.

### Step-by-Step Solution:

1. **Understanding First-Order Reactions**:
- A first-order reaction is one where the rate of reaction is directly proportional to the concentration of one reactant. The rate law can be expressed as:
\[ \text{Rate} = k[A]^1 \]
- Here, \( k \) is the rate constant, and \( [A] \) is the concentration of the reactant.

2. **Analyzing the First Option: Hydrolysis of an Ester**:
- The reaction involves an ester and water in an acidic medium, producing a carboxylic acid and an alcohol.
- Since water is in excess, the rate of reaction depends only on the concentration of the ester.
- Therefore, this is a first-order reaction.

3. **Analyzing the Second Option: Decomposition of N2O5**:
- The decomposition of N2O5 can be represented as:
\[ 2N2O5 \rightarrow 4NO2 + O2 \]
- The rate of this reaction is proportional to the concentration of N2O5.
- Thus, this is also a first-order reaction.

4. **Analyzing the Third Option: Decomposition of Calcium Carbide (CaC2)**:
- The reaction can be represented as:
\[ CaC2 \rightarrow CaO + CaO2 \]
- The rate of this reaction depends on the concentration of calcium carbide.
- Hence, this is a first-order reaction as well.

5. **Analyzing the Fourth Option: Oxidation of Nitric Oxide (NO)**:
- The reaction involves nitric oxide (NO) reacting with oxygen to form nitrogen dioxide (NO2):
\[ 2NO + O2 \rightarrow 2NO2 \]
- The rate law for this reaction is:
\[ \text{Rate} = k[NO]^2[O2]^1 \]
- Since the rate depends on the square of the concentration of NO, this is not a first-order reaction.

### Conclusion:
The reaction that is **not** a first-order reaction is the **oxidation of nitric oxide (option d)**.