A substance that increases the rate of a chemical reaction is called a catalyst, and the process of increase of reaction rate is called catalysis.
Hydrogen and oxygen form water very slowly at ordinary temperature but proceeds more rapidly in the presence of Pt.
, decomposes to give O2
, but its reaction rate can be enhanced in the presence of MnO2.
, is commercially prepared from HCl by its reaction with O2
, but CuCl2
, catalyses the reaction.
Catalysis and energy of activation.
A catalyst increases the reaction rate by decreasing the energy of activation of a reaction. A new reaction path is provided to the reaction. In this way, a greater number of molecules can cross the lower energy barrier. Fig (11.10). The uncatalyzed reaction is showing activation energy Ea1, while catalysed reaction has an energy of activation Ea’1. The values of energy of activation for reverse reaction steps (If the reaction is reversible) are Ea’2 and Ea’2 respectively.
Types Of Catalysis.
There Are Two Types of Catalysis.
When the catalyst and the reactants are in the same phase and the reacting system is homogeneous throughout, then it is called catalysis. Actually, the catalyst is distributed uniformly throughout the system.
, is prepared commercially by the lead chamber process, then NO and NO2
gases act as catalysts. The reactants SO2(g)
along with the catalysts NO(g)
are all gases.
Hydrolysis of esters to give acid and alcohol is homogeneous catalysis, because the reactants and dilute H2
, are all in the solution state.
When the catalyst and reactants are in different phases, then it is called heterogeneous catalysis.
Nitric acid is prepared by the Birkland Eyde process by the oxidation of NH3
, to NO. The burning of NH3
, to NO is catalysed by Pt.
Hydrogenation of unsaturated organic compounds like alkenes
, cyclo-alkenes, benzene
, and sometimes aldehydes and ketones add hydrogen in the presence of ‘Ni(s)’
to give saturated compounds.
- Vegetable oils are hydrogenated in the presence of Ni to give vanaspati ghee.
- Manufacture of H2SO4, by contact process needs V2O5(s), or Pt metal as solid the catalyst for the combination of SO2(g) and O2(g).
- Methyl alcohol is prepared in the whole world from CO(g) and H2(g) by using ZnO(s) and Cr2O3(s) as solid catalysts.
- NH3, synthesis by Haber’s process is carried out with Fe2O3(s) and Mo2O3(s) as catalysts.
Activation of a Catalyst.
Those substances which promote the activity of a catalyst are called promoters or activators. Tetraethyl lead Pb(C2H5)4 is added to petrol because it saves petrol from pre-ignition. This compound of lead is used to improve the octane number of petrol.
The catalytic activity of Ni in the hydrogenation of vegetable oils is promoted by using copper and tellurium.
Iron is used as a catalyst in Haber’s process. The small quantities of high melting oxides, like Al2O3, Cr2O3 or trace earth oxides increase the efficiency of iron.
Sometimes the rate of a reaction can be retarded by an added substance. This is said to be a negative catalyst or inhibitor.
That substance that is formed in the course of reaction some time acts as a catalyst.
The hydrolysis of ethyl acetate is catalysed by acetic acid
which is formed during the reaction and acts as an autocatalyst.
Copper reacts with nitric acid but the reaction is slow in the beginning. The reaction rate increases due to the formation of nitrous acid, during the progress of the reaction.
The reaction of oxalic acid in acidified KMnO4 is slow at the beginning, but after sometimes, the MnSO4 produced by reaction makes it faster.