It was stated in section 6.4 that the products are formed only when reaction molecular come close and collide together at one and the same time. During the collision, molecular rearrangement takes place which leads to the formation of products. For example, a second –order reaction of the types May by traced to a bimolecular collision process involving A and B, and A, respectively. However, the first-order reactions in the gas phase cannot be accounted for by the above collision theory. The first successful explanation of first-order reactions was provided by Lindemann. The mechanism is. (i) A + A ⟺ A* + A (rapd equillibrium) (ii) A* ⟶ Products Step (i) involves a rapid equilibrium reaction where in the forward reaction, a molecular A is activated by colliding with another A molecular with a rate constant k1 and in the backward reaction, the excited molecule A is deactivated by colliding with another A molecule with rate constant k-1. Step (ii) repre