3. DEFINITION
The phenomenon of altering the rate
of a reaction with the help of catalyst is
known as catalysis.
That is a catalyst may increase or decrease
the rate of a reaction
4. Catalysts are of two types,
1. Positive catalyst
These are substances that help hasten a chemical
reaction. Some typical examples include biological catalysts
like Amylase, Maltase, Protease, and Lipase; as well as
popular chemicals like sulfuric acid and manganese dioxide.
2. Negative Catalyst
These are substances that decreases the rate of a
chemical reaction. Lead tetraethyl as antiknock compound.
Glycerol in decomposition of H2O2. Ethanol in the oxidation
of chloroform
5. Homogeneous Catalysis
A catalytic process in which the catalyst and the
reactants are present in the same phase
Eg. Oxidation of SO2 to SO3 with
nitric oxide catalyst
2 SO2(g) + O2(g) ⇌ 2 SO3
6. Heterogeneous catalysts
In heterogeneous catalysis catalysts are in a
different phase than the reactants. For example,
the catalyst might be in the solid phase while the
reactants are in a liquid or gas phase
Eg:
.
7. PROMOTORS
• A substance which though itself not a catalyst ,
promotes the activity of a catalyst is called a
promoter.
• Eg: molybdenum or aluminium oxide promotes the
activity of iron catalyst in tne Haber process for the
manufacture of ammonia
8. ANTICATALYST
A substance which destroys the activity of the
catalyst is called anticatalyst or catalytic poison
and the process is called catalytic poisoning.
Eg: Platinum catalyst used in the oxidation of
sulphur dioxide is poisoned by arsenic oxide
9. AUTOCATALYSIS
When one of the products of a reaction itself act as a
catalyst for that reaction, the phenomenon is called
autocatalysis.
Eg:When oxalic acid is oxidised by acidified
potassium permanganate, mangonous sulphate
produced during the reaction act as a catalyst for
the reaction
