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

Collision theory explains reaction rates as arising from the frequency of effective molecular collisions, governed by energy and orientation requirements.

Collision theory of bimolecular gaseous reactions, developed by Max Trautz and William Lewis, treats reactant molecules as hard spheres and assumes that a chemical reaction occurs as a result of collisions between the reacting molecules.

Key postulates:

1. For a reaction $A + B \rightarrow \text{Products}$, molecules A and B must collide with each other for the reaction to occur.
2. The rate of reaction is proportional to the number of collisions taking place per unit time per unit volume, called the collision frequency, $Z_{AB}$.
3. Not every collision leads to a reaction (most collisions are simply 'elastic', molecules bouncing off unchanged). Only a small fraction of collisions are effective in forming products — these are called effective collisions, and require two conditions to be met simultaneously:

• (i) Threshold energy: the colliding molecules must possess a certain minimum kinetic energy, called the threshold energy, so that after collision the molecules have enough energy to overcome the energy barrier and form an unstable 'activated complex'. The additional energy the reactants (at their average energy) need to reach the threshold energy is the activation energy, $E_a$. The fraction of molecules possessing energy $\geq$ threshold energy is given by the Boltzmann/Maxwell distribution factor $e^{-E_a/RT}$.
• (ii) Proper orientation: the colliding molecules must also collide with the correct spatial orientation, so that the old bonds can break and new bonds can form correctly at the reacting site. Even energetically sufficient collisions fail to react if molecules are not oriented correctly.

Rate expression: …