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Chemical Ideas: 10.5
Catalyst: A substance which speeds up a reaction but can also be recovered chemically unchanged at
Catalysis: Process of speeding up a chemical reaction by using a catalyst.
Catalysts do not undergo any permanent chemical change, but they may change physically. For
example, the surface of a solid catalyst may become roughened and crumble, this suggests that the
catalyst is taking some part in the reaction, but is regenerated. Only small amounts of a catalyst are
needed. The catalyst does not affect the amount of product formed, only the rate at which it is
Types of Catalysts
If the reactants and the catalyst are in the same physical state (e.g. both are in an aqueous solution)
then the reaction is said to involve homogenous catalysis. Enzyme-catalysed reactions in cells take
place in aqueous solution are said to be homogenous.
Many industrial processes involve heterogeneous catalysis, where the reactants and the catalyst
are in different physical states e.g. a mixture of gases or liquids reacting with a solid catalyst.
When a solid catalyst is used to increase the rate of reaction between gases or liquids, the reaction
takes place on the surface of the solid.
The reactants form bonds with atoms on the surface of the catalyst;
they are adsorbed onto the surface. As a result the bonds in the
reactant molecules are weakened and break. New bonds form
between the reactants, held close together on the surface, to
form the products. This in turn weakens the bonds to the catalyst
surface and the product molecules are released.
It is important that the catalyst has a large surface area for contact
with reactants. Solid catalysts are used in a finely divided form or
as a fine wire mesh. Sometimes catalysts are supported on a
porous material to increase its surface area and prevent it from
crumbling, this happens in the catalytic converters fitted to car
Zeolites (crystalline aluminosilicate) are used in industry as
heterogeneous catalysts e.g. the cracking of petroleum
Many of the heterogeneous catalysts used in industrial processes are transition metals.
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Chemical Ideas: 10.5
Catalysts can be poisoned so that they can no longer function properly. In heterogeneous catalysis
the `poison' molecules are adsorbed more strongly to the catalyst than the reactant molecules. The
catalyst cannot catalyse a reaction of the poison and so becomes inactive with poison molecules
blocking the active sites on the surface. This is why leaded petrol cannot be used in cars fitted with a
catalytic converter as lead is strongly adsorbed to the surface of the catalyst.…read more