Transition metals and their compounds act as catalysts. Catalysts increase rates of chemical reactions by lowering the required amount of activation energy by providing an alternative route.
RECAP: Activation energy = the amount of energy required to start a reaction.
Catalysts can be classified into two groups; homogeneous and heterogeneous. Homo from the Latin meaning "the same", and hetero meaning "different".
Heterogeneous catalysts are in a different state from the reactants. The reaction occurs at the surface of the catalyst. Adsorption of the reactants takes place at the active site on the surface of the catalyst. A reaction between modified reactants takes place, then the product is desorbed.
* Depends on the strength of adsorption
* When weak adsorption occurs, the modified reactant is not sufficiently held on the surface, which equates to a lowered chance of reaction.
* Strong adsorption occurs when the modified reactant is held too strongly to the active site and cannot desorb.
* The reaction rate can be negatively affected if the product is not readily desorbed.
* A good catalyst achieves balance between adsorption and desorbtion.
Preparation of catalysts
* Increasing the surface area leads to increased efficiency.