1. Collision Theory :
According to collision theory, the reactant molecules collide with one another to give products. Out of millions of collision, between reactant molecules, only a small fraction is successful and gives product. For a collision to be successful, it should overcome two barriers,
a. Energy barrier
For a collision to be successful it should produce enough energy such that it can cross the maximum energy barrier called Threshold energy barrier. Out of many collisions, only those collisions will be successful which can produce enough energy to cross energy barrier.
b. Symmetry Barrier
Energy alone does not determine the rate of reaction as the reactant molecules can approach one another in different ways during collision out of many possible orientations, only one is symmetrically correct and can give correct product. So, for a collision to be successful, the colliding molecule should approach in correct symmetry.
Out of many possible collisions, only very few collisions can be symmetrically correct and can produce enough energy to cross energy barrier.
2. Activation Complex Theory
According to this theory, reactants approach one another and form a highly energetic transition state called Activation Complex which then changes to product. The minimum energy required by reactant to form activation complex is activation energy. The rate of reaction is determined by the fraction of reactant molecules that possess activation energy and can form activation complex.