Question

Give a detailed account of the collision theory of reaction rates of bimolecular gaseous reactions.

Answer 1

The collision theory of reaction rates of bimolecular gaseous reaction was proposed by Max Trautz and William Lewis to explain the rates of gaseous reactions. It is based on kinetic theory of gases.

Complete step by step answer:
According to collision theory of reaction rates of bimolecular gaseous reactions, the reactant – molecules are assumed to be hard spheres. These hard spheres of reactant hit on each other only a certain amount of collision result in a perceptible or notable change; these successful changes are called as successful collision and reaction occurs. The successful collisions must have enough energy, also known as activation energy, at the moment of impact to break the pre – existing bonds and form all new bonds. This results in the products of the reaction.
Increasing the concentration of the reactant brings about more collisions and hence more successful collisions. Increase in the temperature increases the average kinetic energy of the molecules in a solution and simultaneously increases the collision that has enough energy. The collisions are possible if $2$ or more molecules are present. The minimum energy must be possessed by reactant molecules to give products is called threshold energy $\left( {{E_T}} \right)$ and molecules having other than threshold energy arc are called normal molecules. Collisions between two normal molecules are called normal collisions which do not lead to the chemical reaction hence products are not formed. In addition to the normal molecules the molecules must acquire some extra amount of energy called activation energy $\left( {{E_a}} \right)$
${E_a} = {E_T} - {E_R}$
In detail account of the collision theory
(1) The reaction molecules are assumed to be hard spheres.
(2) When the molecules collide with each other, the reaction will occur.
(3) Collision frequency (z) is the number of collisions per seconds per unit volume of the reaction mixture.
(4) For a bimolecular elementary reaction
$A + B\xrightarrow{{}}$ Products
$\therefore$ Rate $= Z{e^{ - Ea/RT}}$
(5) All the collisions do not result in the reaction
(6) The probability factory (P) also called steric factor accounts for effective collisions rate $= pZ.{e^{ - Ea/RT}}$
(7) The proper orientation of reactant molecules results in bond formation number product is formed with improper orientation.

Note:
When a catalyst is involved in the collision between the reactant molecules less energy is required for the chemical change to take place and hence more collisions have sufficient energy for reaction to occur.