Catalysis - unit 5

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  • Created by: Kelly
  • Created on: 27-03-12 13:36

Catalysis

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 Catalysis

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.

(http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Hydrogenation_on_catalyst.png/220px-Hydrogenation_on_catalyst.png)

Catalytic Efficiency

* 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.

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