A catalyst only

Options

A. decreases activation energy
B. increases activation energy
C. both (a) and (b)
D. comes to equilibrium

Explanation

To answer the question "A catalyst only...", we need to understand the role of a catalyst in chemical reactions. Here’s a step-by-step breakdown of the explanation:

### Step 1: Understanding Activation Energy
- **Definition**: Activation energy (Ea) is the minimum energy required for reactants to convert into products during a chemical reaction. It is represented as the energy barrier that must be overcome for the reaction to proceed.

### Step 2: Role of a Catalyst
- **Function**: 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, which has a lower activation energy compared to the uncatalyzed reaction.

### Step 3: Graphical Representation
- **Graph Explanation**: If we plot a graph with the reaction coordinate on the x-axis and energy on the y-axis, we can visualize the energy changes during the reaction.
- The graph shows the energy of the reactants, the transition state (the peak), and the energy of the products.
- The difference in energy between the reactants and the transition state is the activation energy (Ea).

### Step 4: Effect of a Positive Catalyst
- **Positive Catalyst**: When a positive catalyst is present, it lowers the activation energy. This is represented by a new pathway on the graph that has a lower peak (lower activation energy), allowing the reaction to proceed faster.

### Step 5: Effect of a Negative Catalyst
- **Negative Catalyst**: Conversely, a negative catalyst increases the activation energy. This is represented by a pathway with a higher peak (higher activation energy), which slows down the reaction rate.

### Step 6: Conclusion
- **Summary**: A catalyst can either decrease or increase the activation energy depending on whether it is a positive or negative catalyst. Therefore, the correct answer to the question is that a catalyst can do both: decrease activation energy (positive catalyst) or increase activation energy (negative catalyst).

### Final Answer
A catalyst can decrease activation energy or increase it, depending on whether it is a positive or negative catalyst.

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