Which of the following does not effect half-life of first order reaction?

Options

A. Initial concentration
B. Catalyst
C. Temperature
D. Pressure

Explanation

To determine which factor does not affect the half-life of a first-order reaction, we need to analyze the relationship between half-life and various factors.

### Step-by-Step Solution:

1. **Understanding Half-Life in First-Order Reactions**:
- The half-life (T_half) of a first-order reaction is defined as the time required for the concentration of a reactant to decrease to half of its initial concentration.

2. **Formula for Half-Life**:
- For a first-order reaction, the half-life is given by the formula:
\[ T_{1/2} = \frac{0.693}{k} \]
- Here, \( k \) is the rate constant of the reaction.

3. **Dependence on Initial Concentration**:
- From the formula, we can see that the half-life (T_half) is independent of the initial concentration of the reactant. This means that no matter how much the initial concentration changes, the half-life remains constant for a first-order reaction.

4. **Factors Affecting Half-Life**:
- The half-life does depend on the rate constant \( k \). The rate constant can be influenced by:
- Temperature: As temperature increases, the rate constant typically increases, which decreases the half-life.
- Catalysts: The presence of a catalyst can also increase the rate constant, thereby affecting the half-life.
- Pressure: For gaseous reactions, changes in pressure can affect the rate constant.

5. **Conclusion**:
- Therefore, the factor that does not affect the half-life of a first-order reaction is the **initial concentration** of the reactant.

### Final Answer:
The initial concentration of the reactant does not affect the half-life of a first-order reaction.