DIFFERENT TYPES OF CATALYSIS
A catalyst is a substance that generally increases the rate of reaction but remains chemically
unchanged at the end of the reaction. Catalysis can be divided into two main types: heterogeneous and
homogeneous. In a heterogeneous reaction, the catalyst is in a different phase from the reactants as
there as in a an homogeneous reaction , the catalyst is in the same phase as the reactants.
Heterogeneous catalysis involves the use of a catalyst in a different phase from the reactants. Typical
examples involve a solid catalyst with the reactants as either liquids or gases.
Most of heterogeneous catalysis goes through the same stages:
The first stage is called : adsorption in this process, the particles of reactants stick to the surface of
the catalyst on special sites called : active sites (these sites help the substances to react ) .
There is a specific sort of interaction between the surface of the catalyst and the reactant molecules
that makes them more reactive.Once the reaction happens the products of the reaction are desorbed.
Desorption means that the product molecules break away leaving the active sites available for a new
set of molecules to attach to and react.
A good catalyst needs to adsorb the reactant molecules strongly in order for them to react , but not so
strongly that the product molecules stick more or less permanently to the surface. Silver, for example,
is not a good catalyst because it does not form strong enough attachments with reactant molecules.
Metals like platinium and nickel make good catalysts because they adsorb strongly enough to activate
the reactants, but not so strongly that the products cannot break away.
The simplest example of this is the reaction between ethene and hydrogen in the presence of a nickel
catalyst. The equation of the reaction is:
Ni
CH2 = CH2 + H2
CH3CH3
One important industrial use is in the hydrogenation of vegetable oils to make margarine, which
involves reacting a carbon-carbon double bond in the vegetable oil with hydrogen in the presence of a
nickel catalyst. Ethene molecules are adsorbed on the surface of the nickel, the double bond between
the carbon atoms breaks and the electrons are used to bond them to the nickel surface.
In homogeneous catalysis, the catalyst is in the same phase as the reactants, which means that all the
substances will be present as a gas or contained in a single liquid phase. The reaction between
peroxodisulphate ions S2O82- and iodide ions I- is a good example.
Peroxodisulphate ions are very powerful oxidising agents. Iodide ions are very easily oxidised to
iodine.
S2O82+
2I2 S2O42- +
I2
The reaction needs a collision between two negative ions but the repulsion between the two particles
inpedes them from colliding. This reaction is very slow so , to increase the rate we generally use
sulfuric acid ( 2H3O++ SO42- ) in aqueous solution . It does not react with the reactants and it remains
at the end of the reaction.
Autocatalysis is a particular type of reaction: the reaction is catalysed by one of the products. One of
the simplest examples is the oxidation of a solution of oxalic acid (ethanedioic acid ) by an acidified
solution of potassium permanganate. This reaction is very slow at room temperature so it is usually
carried out at a temperature of approximatly 60°C.The reaction is catalysed by manganese II ions
Mn2+. When the reaction starts there aren’t any of these ions, they are produced by the reaction
because the MnO4- ions are tranformed in Mn2+. So, more and more catalyst is produced as the
reaction proceeds therefore the reaction speeds up.
ANG CHICIN 106737951
Page 1 sur 1