Explanation:
When a catalyst is changed in a reaction system, only the activation energy of the reaction is affected. The other factors such as enthalpy of reaction, free energy of the reaction, and equilibrium constant remain unchanged.

Enthalpy of reaction:
The enthalpy of reaction is the heat energy exchanged during a chemical reaction. It depends on the difference in energy between the products and the reactants. The presence of a catalyst does not affect the enthalpy of reaction as it only provides an alternative pathway for the reaction to occur without being consumed or produced in the overall reaction.

Activation energy:
The activation energy is the minimum energy required for a reaction to occur. It represents the energy barrier that must be overcome for the reactants to convert into products. A catalyst can lower the activation energy by providing an alternative reaction pathway with a lower energy barrier. This allows more reactant particles to have sufficient energy to overcome the barrier, leading to an increased reaction rate.

Free energy of the reaction:
The free energy of the reaction is the energy available to do work. It is related to the difference in energy between the reactants and the products. The presence of a catalyst does not change the free energy of the reaction, as it only provides an alternative pathway for the reaction to occur without affecting the overall energy change.

Equilibrium constant:
The equilibrium constant is a ratio of the concentrations of the products to the concentrations of the reactants at equilibrium. It is determined by the temperature and is independent of the presence of a catalyst. Changing the catalyst does not affect the equilibrium constant as it only influences the rate at which the reaction reaches equilibrium, not the position of the equilibrium.

In summary, when only the catalyst is changed in a reaction system, the only factor that changes is the activation energy of the reaction. The enthalpy of reaction, free energy of the reaction, and equilibrium constant remain unaffected.