A catalyst

Options

A. increases the average kinetic energy of reacting molecules
B. increases the activation energy
C. alters the reaction mechanism
D. increases the frequency of collisions of reacting species

Answer

### Step-by-Step Solution:

1. **Understanding the Definition of a Catalyst**:
A catalyst is defined as a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by providing an alternative reaction pathway with a lower activation energy.

2. **Role of a Catalyst**:
- A catalyst does not change the overall energy change (ΔE) of the reaction.
- It lowers the activation energy (Ea), which is the energy barrier that must be overcome for a reaction to proceed.

3. **Mechanism of Action**:
- By lowering the activation energy, a catalyst allows more reactant molecules to have enough energy to undergo the reaction at a given temperature.
- This effectively increases the number of successful collisions per unit time, thereby increasing the rate of reaction.

4. **Evaluating the Options**:
Given the options related to the role of a catalyst, we need to identify which option correctly describes its function. The correct answer is that a catalyst alters the reaction mechanism to lower the activation energy.

5. **Conclusion**:
Therefore, the correct answer to the question regarding the role of a catalyst is option C: "alter the reaction mechanism".