Why are reaction mechanisms a necessary part of the collision theory model?

ANS:- According to  collision
theory.

1)      Reactant particles must collide in order to
react.

2)      Not every collision results in a reaction .
There must be a certain minimum energy (Threshold energy) in the collision to bring
about the necessary reorganization of the bonds in the colliding 
particles.

3)      The colliding particles must be
correctly oriented with respect to each other in the collision otherwise new bonds may
not formed.

Consider a bimolecular
reaction

A  + B
  ==>  product

Consider one mole of A and one mole
of B present. Then at STP, there will be 1030 collisions/sec. If all collisions are
effective then the reaction will be very fast. But it is not so. When A and B molecules
are slow moving, the collision may be so gentle that there is no change in the identity
of the molecule during the collision. Electron clouds associated with A and B repel each
other. But if A or B or both A & B have much kinetic energy before collision,
they can easily use their kinetic energy to do work against the repulsive forces and
electron and atom rearrangement takes place. Hence the chemical reaction takes place.
The amount of energy required to convert the starting reagents (A and B) to the product
is called the ENERY OF ACTIVATION.