How does the presence of a catalyst affect the enthalpy change of a reaction?
A catalyst has precisely no effect on the the enthalpy change of a reaction.
Catalysts work by reducing the activation energy of a reaction, by providing an alternative reaction pathway. Because the activation energy is reduced, more reactant particles have the requisite energy to combine effectively, and the reaction rates increases so that equilibrium is achieved the sooner.
Catalysts do not alter the enthalpy change,