A catalyst only

Options

A. Decrease activation energy
B. Increases activation energy
C. Both (a) and (b)
D. comes to equilibrium

Answer

**Step-by-Step Solution:**

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

2. **Activation Energy Concept:**
- Activation energy is the minimum energy required for a reaction to occur. It is the energy barrier that must be overcome for reactants to be transformed into products.

3. **Effect of a Positive Catalyst:**
- A positive catalyst decreases the activation energy of a reaction. By lowering the energy barrier, it allows more reactant molecules to have sufficient energy to undergo the reaction, thus increasing the rate of the reaction.

4. **Effect of a Negative Catalyst:**
- Conversely, a negative catalyst (or inhibitor) increases the activation energy of a reaction. This means that it raises the energy barrier, making it harder for reactants to convert into products, thereby decreasing the rate of the reaction.

5. **Conclusion:**
- Based on the above points, we can conclude that a catalyst can either decrease or increase the activation energy depending on whether it is a positive catalyst or a negative catalyst. Therefore, the answer to the question is that a catalyst can both decrease activation energy and, in some cases, increase it.

**Final Answer:**
- The correct answer is that a catalyst can both decrease activation energy (positive catalyst) and increase activation energy (negative catalyst). Thus, the answer is option C: both A and B.

---