Why does a catalyst speed up a chemical reaction?

A catalyst lowers the activation energy of a reaction. It does this by providing an alternative, lower energy pathway for the reaction. An example of this - a catalyst may form a complex with the reactants which means the are in the correct orientation to react, increasing the likelihood of a reaction and the reaction rate. Draw diagram of a reaction without a catalysts Draw diagram with a catalyst Show that overall enthalpy change is the same.