OCR AS Chemistry F331: Catalysts

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Catalysts
A pair of reacting molecules must collide, with a combined energy greater than the activation
enthalpy for the reaction in order to make a successful collision. Catalysts are used in order to
overcome the activation energy barrier more easily.
A catalyst is a substance which speeds up the rate of a reaction but can be recovered chemically
unchanged at the end.
The process of speeding up a chemical reaction using a catalyst is called catalysis.
Catalysts do not undergo any permanent chemical change, though sometimes they may be changed
physically.
Only small amounts of a catalyst are needed usually.
The catalyst does not affect the amount of product formed, only the rate at which it is formed.
Types of Catalysts
Homogenous catalysts: when the reactants and catalyst are in the same physical state
Heterogeneous catalysts: when the reactants and catalyst are in different physical states
It is important that the catalyst has a large surface area for contact with reactants.
For this reason solid catalysts are used in finely divided form or as a fine wire mesh.
Sometimes the catalyst is supported on porous material to increase its surface area and prevent it
from crumbling.
Industrial Process Catalyst
Reforming of petroleum fractions to produce high Platinum finely dispersed on the surface of
octane petrol components aluminium oxide
e.g. hexane cyclohexane
Hydrogenation of unsaturated oils to give Nickel powder
saturated fats
e.g. vegetable oil + H2 margarine
Manugacture of ammonia from nitrogen and Finely divided iron
hydrogen in the Haber process
N2+3H2+2NH3
Zeolites are widely used in industry as heterogeneous catalysts ­ e.g. in the cracking of petroleum
fractions.
Many of the heterogeneous catalysts used in industrial processes are transition metals or transition
metal compounds.

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Catalyst Poisoning
Catalysts can be poisoned so they no longer function properly.
Many substances which are poisonous to humans operate by blocking an enzyme-catalysed reaction.
In heterogeneous catalysis, the `poison' molecules are adsorbed more strongly to the catalyst
surface 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 its surface.…read more

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