Collision hypothesis

The most natural source of energy for the activation of molecules is molecular collision, but since the rate is independent of the number of collisions it would appear at first sight that the simple collision mechanism can not be responsible for the activation. Three hypotheses have been proposed to account for the activation process the radiation hypothesis, the elaborated collision hypothesis, and the hypothesis of chain reactions. 

We have seen that the radiation hypothesis is not supported by experiment and that it can not be used to explain the fact that unimolecular reaction rates are uninfluenced by collisions. When investigators found this avenue of explanation closed they resumed consideration of the collision hypothesis. As early as 1922 Lindemann suggested that since a time interval exists between activation, of a molecule and its dissociation the apparent connection between the two phenomena would ordinarily be lost. This view was received with increasing favor as the radiation hypothesis became more and more discredited. Rodebush7 in 1923 howed that the known facts could be explained on the basis of collisions... 

We have seen that neither the requirements for activation energy nor the fact that the rates of unimolecular reactions are independent of collision frequency can be explained on the basis of the simple collision hypothesis or the radiation hypothesis. The elaborated collision hypothesis is able to explain them on the assumption of a time-lag in complex molecules between activation and decomposition. In this way a single molecule can collect energy from many successive collisions and store up a sufficient amount for activation. Just because a given hypothesis accounts for the facts, is no reason to consider that the hypothesis has been proved. There may be other hypotheses which will account equally well for the facts. The hypothesis of chain reaction offers a competing hypothesis which up to the present time has been increasing in favor. 

The first theoretical approach was proposed by Libby (1947) from his billiard ball collision hypothesis. His expression for the observed... 

An assumption made in all of the theories discussed is that activation and deactivation occur as single steps as opposed to processes in which energy is gained or lost in a series of steps. The assumption that large amounts of energy are transferred in molecular collisions is called the strong collision hypothesis. In essence, it means that consideration of the detailed collision dynamics is not necessary. The energy transfer process can be studied by... 

Suggest two ways that would enable you to test the asteroid collision hypothesis. 

Until now, it is virtually impossible to evaluate the function k(E, E ) for polyatomic molecules. For this reason, the theory of collisional activation and deactivation is to a considerable extent based on hypotheses concerning the general properties of the function k(E, E ). The two alternative hypotheses substantially simplifying the microscopic kinetic equations are the strong-collision hypothesis and that of stepladder activation and deactivation [336, 339, 486]. 

This time, the collision hypothesis is conserved in the energetic sense and involves two reactant molecules forming the activated complex, but the latter only contains one molecule A, and the second molecule is used to transfer energy. The crossing of the pass will therefore lead to the reaction product provided a transmission coefficient exists. We will later return to this. Thus, the elementary reaction will be represented by ... 

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