Catalysts do not enter into the stoichiometric expression for a reaction because they are neither produced nor consumed in the reaction. As a consequence, the presence of a catalyst does not affect the underlying chemical thermodynamics of the reaction. The catalyst does not alter the internal energy change, enthalpy change, entropy change, free energy change or position of equilibrium.

This is such an important point that almost every MCAT will have you demonstrate in some question that you can separate chemical kinetics concepts, such as catalysis, from chemical thermodynamics questions, such as the position of equilibrium.

It is true that there is definitely a kinetics perspective on the equilibrium state. At equilibrium, after all, the forward and reverse reaction rates are equal. But a catalyst increases the rate of both the forward and reverse reactions. A catalyst does not alter the free energy change. A catalyst cannot alter the position of equilibrium.

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