How is a catalyst is used in a reaction?

A catalyst provides an alternative reaction pathway which has a lower activation energy and therefore a higher proportion of reactant particles have energy greater than or equal to the new activation energy. This increases the frequency of successful collisions and therefore increases the rate of reaction.

A catalyst is used to speed up the rate of a reaction by providing an alternatve pathway with a lower actvation enery for the reaction to occur.

Eg. #V_2O_5# in the Contact process.
Most heterogenous catalysts go through the same stages.

The reactants are first adsorbed onto the surface of the catalyst at active sites. Adsorption is the temporary attraction of a molecule to a solid surface.

Next, when they are adsorbed onto the metal, the bonds within the reactant molecules weaken as as some of its electrons are used to form bonds with the catalyst.

This allows the reaction to take place faster because energy for bond breaking is less.

OR

The reactant adsorbed onto the metal may be held in the correct orientation for the reaction to occur.
This increases the number of favourable collisions #therefore# increased rate.

The product molecules then break away from the metal surface leavng the active sites available for other molecules.

Overall effect is a decrease in activation energy.