Which factor affects the velocity constant k of a reaction ?

Options

A. Change in the concentration of the reactant
B. change of temperature
C. change in the concentration of the product
D. None of the above

Answer

To determine which factors affect the velocity constant \( k \) of a reaction, we can follow these steps:

### Step 1: Understand the Velocity Constant \( k \)
The velocity constant \( k \) is a proportionality constant in the rate equation of a chemical reaction. It relates the rate of the reaction to the concentrations of the reactants.

### Step 2: Consider a Reversible Reaction
Let’s consider a general reversible reaction:
\[ A + B \rightleftharpoons C + D \]

### Step 3: Write the Rate Law
According to the rate law, the rate \( r \) of the reaction can be expressed as:
\[ r = k[A][B] \]
Where:
- \( [A] \) and \( [B] \) are the concentrations of the reactants.
- \( k \) is the velocity constant.

### Step 4: Identify Factors Affecting \( k \)
The velocity constant \( k \) is affected by:
1. **Temperature**: The value of \( k \) changes with temperature due to the effect of temperature on the kinetic energy of molecules.
2. **Catalysts**: The presence of a catalyst can increase the rate of reaction without being consumed, thereby affecting the value of \( k \).

### Step 5: Analyze the Options
Now, we analyze the options provided in the question:
- Change in the concentration of reactants: This does not affect \( k \); it affects the rate of the reaction.
- Change in temperature: This does affect \( k \).
- Change in concentration: This is similar to the first point and does not affect \( k \).
- None of these: This option is incorrect.

### Conclusion
The correct answer is that the velocity constant \( k \) is affected by temperature and the presence of a catalyst.

### Final Answer
The factor that affects the velocity constant \( k \) of a reaction is **temperature** and **catalysts**.

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