For example when the reaction
2HI(g) H2(g) + I2(g)
is uncatalysed, its activation
energy is 185 kJ mol-1 in the forward direction and 164 kJ
mol-1 in the reverse direction.
When a platinum catalyst is used, the activation energy in the forward direction is reduced to 59 kJ mol-1.
Which catalyst gives the greatest increase in the rate?
In two ways:
It forms bonds with one or more of the reactants and so reduces the energy needed by the reactant molecules in order to complete the reaction
It brings the reactants together and holds them in a way that makes reaction more likely. When molecules come together in the correct orientation for reaction there is a big reduction in entropy. If a catalyst already holds the molecules next to each other then the entropy change for the reaction step will be far less negative than it would be without the catalyst, and the reaction is more likely. Note that the catalyst does not affect the overall entropy change for the reaction.
In other words catalysts change the path of a reaction; they change its mechanism.