Collision Theory and Catalyst
Adding a catalyst
A catalyst is a substance that speeds up a reaction without being used up itself, by providing the reaction an alternative reaction pathway with a lower activation energy. Some reactions have catalysts that can speed them up, but for many reactions there is no catalyst that works. During a chemical reaction, a catalyst can be temporally consumed, but once the reaction is complete all of the catalyst is replaced by the reaction mechanism.
A catalyst can be classified as:

1. Heterogeneous catalysts

     These are solid catalysts that are not in the same phase as the other reactants.

1. Homogeneous catalysts

     Homogeneous catalysts are in the same phase as the reactants they work on.

Therefore, collision theory explains the affect of a catalyst on the rate of a reaction by:

1. A catalyst provides a surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting.
2. A catalyst provides an alternative reaction pathway with a lower activation energy (the minimum amount of energy needed for a reaction to take place). This means that the particles can react with less energy than they needed before the catalyst was added.

A word of caution!
Be very careful if you are asked about this in an exam. The correct form of words is

"A catalyst provides an alternative route for the reaction with a lower activation energy."
It does not "lower the activation energy of the reaction". There is a subtle difference between the two statements that is easily illustrated with a simple analogy.

Suppose you have a mountain between two valleys so that the only way for people to get from one valley to the other is over the mountain. Only the most active people will manage to get from one valley to the other.

Now suppose a tunnel is cut through the mountain. Many more people will now manage to get from one valley to the other by this easier route. You could say that the tunnel route has a lower activation energy than going over the mountain.
But you haven't lowered the mountain! The tunnel has provided an alternative route but hasn't lowered the original one.

The original mountain is still there, and some people will still choose to climb it.
In the chemistry case, if particles collide with enough energy they can still react in exactly the same way as if the catalyst wasn't there. It is simply that the majority of particles will react via the easier catalysed route.