Effect of
Catalyst: A catalyst is a substance
which alters the rate
of a reaction without itself undergoing any permanent chemical change.
The action of the catalyst can be explained by intermediate complex theory.
According to this theory, a catalyst participates in a chemical reaction by
forming an intermediate complex. This is unstable and decomposes to yield
products and the catalyst.
A catalyst
increases the rate of a chemical reaction by providing an alternate pathway or
reaction mechanism by reducing the activation energy between reactants and
products. The important characteristics of a catalyst are:
1. A small
amount of the catalyst can
catalyse a large amount of reactants.
2. A catalyst does not alter Gibbs energy, ΔG of
a reaction. It catalyses the spontaneous reactions but does not catalyse non-spontaneous reactions.
3. A catalyst
does not change the equilibrium constant of a reaction, but it helps to attain
the equilibrium faster by increasing
the rate of both forward as well as the backward reactions.
Collision
Theory: This theory was developed by Max Trautz and William Lewis.
It is based on kinetic theory of gases. According to this theory, the
reactant molecules are assumed to be hard spheres and reaction is occurred when
molecules collide with each other. The number of collisions per second
per unit volume of the reaction mixture is known as collision frequency (Z).
Another factor
which affects the rate of chemical reactions is activation energy.
For a bimolecular
elementary reaction
A + B →
Products
Rate of reaction can be expressed as
Rate (r)= ZAB e - Ea /RT
Where ZAB
represents the collision frequency of reactants, A and B and e– Ea/RT
represents the fraction of molecules with energies equal to or greater than Ea.
Comparing with Arrhenius equation, we can see that A is related to collision
frequency.
A third factor
which affects the rate of a chemical reaction is the proper orientation. To account for this, a factor P called the Probability or Steric factor is introduced. So the above equation becomes:
Rate (r)= PZAB e - Ea /RT
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