Collision Theory
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| Collision Theory |
It is one of
the two main theories of reaction rates. It is useful to explain bimolecular
reactions in the gas phase. This theory was proposed to explain how chemical
reactions takes place.
Postulates of collision theory
This theory
has three major postulates.
1. 1. The collision
frequency “z” factor
for a
reaction to take place, collision must occur between reactants or reactants
molecules must collide for a reaction to take place. Rate constant of reaction
will be proportional to the number of collisions. Rate constant will be high if
collisions are high and vice versa. The factor will be “z” which is known as
collision frequency. Collision frequency is defined as “’number of collisions
per unit volume”.
Rate = K = z
2. 2. The active
collision “q” factor
Product
formation takes place in active collisions only which is the “q” factor. Active or effective collision is defined as
the collision in which colliding molecule has energy equal to or greater than
activation energy. Suppose there are total 10 molecules and total 5 collisions
out of which active collisions are 2 then
rate ∝ no of active collisions
 |
Figure 8.4: Active collisions |
Here n*= number of
molecules having energy
n= total number of
molecules
q= fraction of
activated molecules.
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1. 3. The stearicfactor “p”
For a reaction to take place
colliding molecule must be properly oriented.
The orientation of HCl in (1) in above reaction is the best to form transition state.
Limitations
of collision theory
Collision
theory was unable to explain unimolecular reactions. For collision two
molecules are required which indicates that every reaction must be of second
order. But there are many reactions that are of first order. So,
- rate
- change in pressure causes change in order of reaction.
e.g low pressure= second order, high pressure=
first order
to explain such
reactions Lindeman
proposed theory of unimolecular reactions which is explained next.
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