Activation energy and catalysis
“A substance which, though itself not
a catalyst, promotes the activity of a
catalyst is called a promoter.”
Activity of catalyst can often be increased by addition
of small quantity of a second catalyst
that is :
either not a catalyst itself
Or may be a feeble catalyst
Mo promotes the activity of Fe
Mixture of catalysts are used(ZnO, Cr2O3)
Change of lattice spacing
Increase of peaks and cracks
Note: The phenomenon of promotion is a common
feature of heterogeneous catalyst.
A substance which destroys the activity
of the catalyst to accelerate a reaction, is
called a poison and this process is called
Note: Presence of small amount of impurities(Heterogenous
SO2+O2 → 2SO3 (Pt, As2O3)
N2+3H2→2NH3 (Fe, H2S )
Synthesis of Water
2H2+O2→2H2O (Pt, CO)
The poison is adsorbed on the catalyst
surface in preference to the reactants
The catalyst may combine chemically with
When one of the products of reaction acts
as a catalyst for that reaction, this
phenomenon is called autocatalysis.
Rate of reaction raises, as the catalytic
product is formed, instead of decreasing
Hydrolysis of an ester(Acid act as catalyst)
Decomposition of arsine(As act as catalyst)
Oxidation of oxalic acid
When a catalyst reduces the rate of a
reaction, it is called as negative catalyst
Oxidation of Chloroform(2%Methanol)
Decomposition of hydrogen peroxide(Dil.acid)
Combustion of fuel (Tetraethyl lead)
According to collision theory , a reaction occurs by
the collisions b/w the reactant molecules (or ions).
At ordinary temperature the molecules do not possess
enough energy and here collisions are not effective
however when temperature of system is raised the
kinetic energy increases.
But the molecules do not react unless they attain a
minimum amount of energy.
Catalyst works by alternate route.
The minimum amount of energy required to cause a
chemical reaction is known as the activation energy.
The activated molecules on collision first form a
As a result of breaking and forming of new bonds the,
the activated complex dissociated to yield product
So large number of collisions occur in the presence of
catalyst, than would occur at same temperature without
presence of catalyst.
1. Intermediate compound formation
2.The adsorption theory