which of the following reaction characteristics at constant temperature are changing by addition of a catalyst to a reaction
(i) Activation energy
(ii) Equilibrium constant
(iii) Reaction entropy
(iv) Reaction enthalpy

Options

A. (i) only
B. (iii) only
C. (i) and (ii) only
D. All of these

Answer

To determine which reaction characteristics change with the addition of a catalyst at constant temperature, we will analyze each option provided in the question.

### 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 reaction pathway with a lower activation energy.

2. **Analyzing Activation Energy (i)**:
- The activation energy (Ea) is the minimum energy required for a reaction to occur. When a catalyst is added, it lowers the activation energy required for the reaction to proceed.
- **Conclusion**: Activation energy changes with the addition of a catalyst.

3. **Analyzing Equilibrium Constant (ii)**:
- The equilibrium constant (K) is determined by the ratio of the concentrations of products to reactants at equilibrium. A catalyst does not change the position of equilibrium; it only helps the system reach equilibrium faster.
- **Conclusion**: The equilibrium constant does not change with the addition of a catalyst.

4. **Analyzing Reaction Entropy (iii)**:
- Entropy is a measure of disorder in a system. The addition of a catalyst does not affect the overall change in entropy of the reactants and products. It may affect the entropy of intermediates, but not the final states.
- **Conclusion**: Reaction entropy does not change with the addition of a catalyst.

5. **Analyzing Reaction Enthalpy (iv)**:
- Enthalpy (ΔH) is the total heat content of a system. The addition of a catalyst does not change the enthalpy of the reactants or products; it only affects the energy barrier (activation energy) for the reaction.
- **Conclusion**: Reaction enthalpy does not change with the addition of a catalyst.

### Final Answer:
The only characteristic that changes with the addition of a catalyst is **(i) Activation energy**.