By Sunil Bhardwaj

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The collision theory is based upon two important assumptions.

(a) The reacting molecules are considered as rigid spheres.

(b) The molecules having sufficient energy of activation will only go for chemical reaction.

Detailed study on the nature and behaviour of molecules has shown that these assumptions are not correct. Thus, the collision theory suffers because of the following drawbacks.

1. The theory totally ignores the structure and properties of the reacting molecule. The bigger or complex molecules do not behave as rigid spheres.

2. A chemical reaction depends on many factors and not only on the molecules having sufficient activation energy.

3. On the basis of collision theory simple molecular reactions like the formation of hydrogen iodide from hydrogen and iodine in gaseous phase can be explained. But it is unable to explain complex molecular reactions.

4. The theory does not account for the higher rates of chain reactions.

5. The theory is only applicable to homogeneous gas phase bimolecular reactions which are generally of the second order.

6. Sometimes, different rates are determined for the same reaction. Collision theory fails to explain such different rates.

7. The theory fails to account for the first order kinetics of bimolecular gaseous reaction at high pressure.

8. There is no way to determine activation Energy from the theory itself.

9. In collision theory we consider that the reacting molecules must approach with suitable orientation and collide. But for polyatomic molecules with many degrees of freedom there is no particular way that the molecule can go for suitable orientation.

10. In many number of reactions, it is observed that the product of the collision frequency (Z) and the Boltzman factor \({ e }^{ \frac { -{ E }_{ a } }{ RT } }\) is considerably higher than the experimentally determined values from reaction rates. The probability factor P is required for such reaction.