Collision theory:
According to collision theory, a reaction takes place because the molecules collide with each other. The number of collisions that take place per second per unit volume of the reaction mixture is called collision frequency.
At ordinary temperature and pressure, the value of collision frequency is so high that if all the collisions were effective i.e. result into chemical reaction, the reaction should be completed in the fraction of a second. However, in actual practice, this is not so. This is explained on the basis of the following two factors:
1. Energy factors:
For a collision to be effective, the colliding molecules must have energy more than a particular value. The minimum energy which the colliding molecules must have in order that the collision between them may be effective is called threshold energy.
Thus at ordinary temperature and pressures, most of the molecules may not possess energy to or greater than threshold value.
2. Orientation factor:
In some cases it is found that even if a large number of colliding molecules have energy more than threshold value, still the reaction is slow. This is improper orientation of the colliding molecules at the time of collision.
