Catalysts offer an alternative route for a reaction with a lower activation energy, thereby making the reaction proceed at a higher rate. Catalysts do not change the equilibrium of a reaction; they can only increase the rate. Without a catalyst, the reaction would therefore still proceed in the same direction, just slower.

A plot of the transition of from the high substrate energy state to low product energy state, when the transition state has a higher energy level then both the substrate and a product. The use of catalyst lowers the transitions state energy level.

Figure 1: A reaction from a substrate to a product is a transition from one energy state to another. A transition state exists between the substrate and product. This state has a higher energy level than both the substrate and product. A catalyst will lower this energy level, so that the transition energy is reached more easily, and this results in a faster reaction.