The fact that catalysts don't affect the position of an equilibrium can be problematic for some students because they can't predict this using Le Chatelier's Principle.

Students need to realise that a catalyst provides an alternative route for the reaction. The activation enthalpy of both forward and back reactions are lowered by the same amount, so the rates of reaction of both forward and back reactions are increased by the same amount. There is therefore no change in the position of the equilibrium.

Again an energy diagram can be used to illustrate these points.

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Students can be asked to use one of the activities that were used to get over the idea of a dynamic equilibrium in topic 2 to model how a catalyst affects the forward and back reactions. 

Look back at the princesses and toads activity. What would students have to do to modify Scenario 2 to model the effect of adding a catalyst?

Students would have to turn over the cards more quickly but both students would need to turn the cards at the same (quicker) rate.

What effect would this modification have on the outcome of the process?

It would have no effect – the outcome would be the same. This will reinforce the idea that a catalyst increases the rate of the forward and back reactions by the same amount so it means that equilibrium is established more quickly but the position of equilibrium isn't changed.