The Effect of a Catalyst
Reversible chemical reactions will eventually achieve equilibrium. At equilibrium, the rate of the formation of products is equal to the rate of the formation of reactants. Reaction progress towards this equilibrium state can be accelerated through the addition of catalysts or heat to the system.
Catalysts are compounds that can accelerate the progress of a reaction without being consumed. Common examples of catalysts include acid catalysts and enzymes. A schematic depiction of the effect of a catalyst on a reaction coordinate can be seen in the figure below. Catalysts allow reactions to proceed by a different reaction pathway involving a lower-energy transition state. By lowering the energy of the rate-limiting step (the transition state), catalysts reduce the required energy of activation and allow a reaction to proceed and, in the case of a reversible reaction, reach equilibrium more rapidly.
However, catalysts do not affect the equilibrium state of a reaction. In the presence of a catalyst, the same amounts of reactants and products will be formed as would form in the uncatalyzed reaction. As with the addition of heat, the addition of a catalyst speeds the rate of both the forward and reverse reactions, resulting in no change to the chemical equilibrium. That is to say, catalysts affect the kinetics but not the thermodynamics of a reaction. If the addition of catalysts could alter the equilibrium state of the reaction, this would violate the second rule of thermodynamics and allow for the creation of a perpetual-motion machine.