A catalyst :

Options

A. increase the average kinetic energy of reactiong molecules
B. decreases the activation energy
C. can alters the reaction mechanism
D. Can change pre-exponential factor

Answer

**Step-by-Step Solution:**

1. **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 facilitates the reaction by providing an alternative pathway with a lower activation energy.

2. **Effect on Activation Energy**: The primary function of a catalyst is to decrease the activation energy required for a reaction to occur. This allows the reactants to convert into products more quickly.

3. **Graphical Representation**: To visualize the effect of a catalyst, we can draw a graph plotting potential energy against the progress of the reaction.
- Without a catalyst, the activation energy (Ea) is higher, resulting in a slower reaction rate.
- With a catalyst, the activation energy is lowered, which leads to an increased reaction rate.

4. **Reactions at Equilibrium**: In reactions that reach equilibrium, a catalyst affects both the forward and backward reactions equally. It does not change the position of the equilibrium but helps the system reach equilibrium faster.

5. **Conclusion**: The correct statement regarding a catalyst is that it decreases the activation energy, thereby increasing the rate of the reaction.

**Final Answer**: A catalyst decreases the activation energy and increases the rate of reaction.

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