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.