Uni-molecular reactions in gases have an exceptional theoretical interest, because they involve, the spontaneous change of isolated molecules. Initially these reactions were hampered much having co-existence between theoretical and experimental differences.Later with more studies it was put forward that the reaction rate is studied with reference to type of reactions, elementary and multi-step reactions. The concepts of radiation and collision were also related to these reactions by some. So in order to understand uni-molecular reactions in vapor phase many theories were put forward. Some of those are widely accepted even with their limitations.
Generalization:- If we assume that in such a reaction (A -> P), the molecule A acquires the necessary activation energy by colliding with another molecule, then the reaction should obey II order kinetics, not the I order which is actually observed in some uni-molecular reactions. Let’s discuss about these theories to understand the mechanism of uni-molecular reactions in vapour phase:
1) LINDEMANN THEORY:
According to this theory a uni-molecular reaction A-> P proceeds via following mechanism
Where,
A=energized A molecule that has acquired sufficient energy to enable it to isomerizes or decomposed i.e., the vibration energy of A exceeds the threshold energy for the overall reaction. According to the theory in first step, A molecule is produced by collision of a molecule A with other molecules.Then the kinetic energy of second molecule is transferred into the vibrational energy of the first. In the Lindemann mechanism,a time lag exists between the energization of A to A* and the decomposition of A* to product/de-enegization back to A. According to SSA (steady the state approx.) when a reactive (short lived) species is produced in the reaction as an intermediate its rate of formation is equal to its rate of decomposition. Here A* is reactive therefore,
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