Hinshelwood Theory

Hinshelwood Theory

Hinshelwood was the first scientist who correlated between the internal structure of a molecule with its kinetic properties. According to this theory, all atoms in a polyatomic molecule are in a state of vibrational motion, so a molecule may be taken as a complex of a certain number of independent vibrators capable of transferring energy from one to another.
Let us consider the following reactions-
Where A* is energiged molecule and A** is an activated complex.
An energized molecule has sufficient energy to get transformed into an activated complex by undergoing some vibrational changes. During such vibrational changes, the energy is localized in a particular bond or bonds that is/are going to be broken. The molecule may become energized much more rapidly than expected by collision theory. There is long time gap. however before an energized molecule can become an activated complex.
If the total energy (E) is distributed among s vibrators of a polyatomic molecule with no restriction as to the amount in individual degree of freedom, then fractions of molecules with energy between E & E + dE is given as-

Molecules with this energy make a contribution dk₁ to the k₁ and the total value of k₁ is the sum of such contributions for all possible energies.
A* are very efficiently removed by molecular collisions, the rate for this process being k₂.

Objection to Hinshelwood Theory

1.The number of the degree of freedom required to give aggrement with experiment on the basis of the Hinshelwood theory is about one-half of the total number of vibrational modes. There is no satisfactory explanation for this.
2. According to Hinshelwood theory, we can expect a strong tremperature dependence of the pre-exponential factor, especially for large values of s But no experimental evidences exist for this.
3. The Hinshelwood treatment can not account for the lack of linearity found experimently for the plot of 1/k¹ vs 1/[A].

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