1. catalysis
By prof. Abhinav Shukla
Iit.abhinav@gmail.com
9096905695

2. Catalyst
• Catalyst is a substance which speeds up and
speeds down a chemical reaction without
itself being used up.”
or
Ostwald (1895) redefined a catalyst as, “A
substance which changes the reaction rate
without affecting the overall energetics of the
reaction is termed as a catalyst and the
phenomenon is known as catalysis

3. Types of catalysis
• (1) Homogeneous catalysis : When the reactants and
the catalyst are in the same phase (i.e. solid, liquid or
gas). The catalysis is said to be homogeneous. The
following are some of the examples of homogeneous
catalysis.examples
NO( g )
2SO 2 (g) O 2 (g) 2SO 3 (g)
HCl(l)
CH 3 COOCH 3 (l) H 2 O(l) CH 3 COOH (l) CH 3 OH (l)

4. • (2) Heterogeneous catalysis : The catalytic process
in which the reactants and the catalyst are in different
phases is known as heterogeneous catalysis. Some of
the examples of heterogeneous catalysis are given
below.
Pt (s)
2SO 2 (g) O2 (g) 2SO 3 (g)
BrV2 O5
Fe (s)
N 2 (g) 3 H 2 (g) 2 NH 3 (g)
• (3) Positive catalysis : When the rate of the reaction is
accelerated by the foreign substance, it is said to be a positive
catalyst and phenomenon as positive catalysis. Some examples
of positive catalysis are given below
MnO2 (s)
2 KClO3 (s) 2 KCl(s) 3O2 (g)
o
270 C

5. • (4) Negative catalysis : There are certain,
substance which, when added to the reaction
mixture, retard the reaction rate instead of
increasing it. These are called negative catalyst or
inhibitors and the phenomenon is known as
negative catalysis. Some examples are as follows.
Diphenyl
2C6 H5 CHO (l) O2 (g) 2C6 H5 COOH (l)
amine(l)
• (5) Auto-catalysis : In certain reactions, one of the product
acts as a catalyst. In the initial stages the reaction is slow but
as soon as the products come into existences the reaction rate
increases. This type of phenomenon is known as auto-
catalysis. Some examples are as follows

6. • The rate of oxidation of oxalic acid by acidified
potassium permanganate increases as the reaction
progresses. This acceleration is due to the presence
of ions which are formed during reaction. Thus ions
act as auto-catalyst.
5 H2C2O4 2KMnO 4 3 H2SO 4 2MnSO 4 K2SO 4
10 CO2 8 H 2O

7. Characteristics of catalysis
• (1) A catalyst remains unchanged in mass and
chemical composition at the end of the reaction.
• (2) A small quantity of the catalyst is generally
sufficient to catalyses almost unlimited
reactions
• (3) The catalyst can not initiate the reaction: The
function of the catalyst is to alter the speed of the
reaction rather than to start it.
(4) The catalyst can not change the position of
equilibrium : The catalyst catalyse both forward and
backward reactions to the same extent in a reversible
reaction and thus have no effect on the equilibrium
constant.

8. • (5) The catalyst is generally specific in nature:
A substance, which acts as a catalyst for a particular reaction ,
fails to catalyse the other reaction , different catalysts for the
same reactant may for different products.
Al 2O3
C2 H 4 (g) H 2 O(g)
(Dehydration)
C 2 H 5 OH (l)
Cu
CH 3 CHO (g) H 2 (g)
(Dehydroge
nation )
(6) Catalytic promoters : Substances which themselves are not
catalysts, but when mixed in small quantities with the
catalysts increase their efficiency are called as promoters or
activators.
For example, in Haber’s process for the synthesis of ammonia,
traces of molybdenum increases the activity of finely divided
iron which acts as a catalyst.

9. • (7) Catalytic poisons : Substances which destroy the
activity of the catalyst by their presence are known as
catalytic poisons. For example, the presence of traces of
arsenious oxide in the reacting gases reduces the activity of
platinized asbestos which is used as catalyst in contact process
for the manufacture of sulphuric acid.
• (8) A positive catalyst lowers the activation energy

10. Uncatalysed
complex
Che
mica Energy
l barrier
pote Catalysed
ntial complex
ener Ea
gy
Ea
ER
Reactants G of
Initial
(A+B) reaction
state
EP
Final state Products (C + D)
Reaction sequence

11. Enzyme catalysis
• (1) Enzymes are complex nitrogenous substances these
are actually protein molecules of higher molecular
mass.Numerous reactions occur in the bodies of
animals and plants to maintain the life process. These
reactions are catalysed by enzymes. The enzymes are
thus, termed as bio-chemical catalysts and the
phenomenon is known as bio-chemical
catalysis.examples
Invertase
C12 H 22O11(l) H 2O(l) C6 H12O6 (l) C6 H12O6 (l)
Glucose Fructose
Zymase
C6 H12O6 (l) 2C2 H 5 OH (l) 2CO2 (l)
Diastase
starch Maltose (l)

12. Activity and Selectivity
• Activity : Activity is the ability of catalysts to accelerate
chemical reaction, the degree of acceleration can be as
high as times in certain reactions
• Selectivity : Is the ability of catalysts to direct reaction
to yield particular products (excluding other).
• Zeolite (Shape selective catalysis)
Zeolite are alumino–silicates of the general formula,
M x / n [ AlO 2 ]x .(SiO 2 )y .mH 2 O
where, M may be simple cation , n is the charge on the simple
cation, m is the number of molecules of water of crystallization.
The zeolite catalyst ZSM-5 converts alcohols to gasoline
(petrol) by dehydrating the alcohol