Chemguide: Core Chemistry 14 - 16

The effect of adding a catalyst on an equilibrium

This brief page explains why adding a ctalyst to an equilibrium reaction has no effect on the position of equilibrium.

What a catalyst does to an equilibrium

In previous pages of this section we have talked about the equilibrium central to the manufacture of ammonia.

N₂(g) + 3H₂(g)    2NH₃(g)

We have looked at the effect of temperature and pressure on the position of equilibrium, but haven't as yet mentioned the fact that an iron catalyst is used.

What effect does this have on the position of equilibrium? The answer is "none"!

A catalyst has no effect on the position of equilibrium. Its function is to speed up the rate at which the equilibrium is reached.

In the absence of an iron catalyst, the reaction would take very much longer to reach the equilibrium position. In an industrial process like this the gases flow continuously through a reactor vessel, and the reaction has to happen very fast. In the absence of a catalyst, the gases would be through the reactor before they had time to react.

Why doesn't the catalyst have any effect on the position of equilibrium? Because it speeds up the forward reaction and back reaction equally.