Collision frequency termolecular

Zabc termolecular collision frequency ev fraction of surface sites that are vacant... 

In this last expression, the preexponential factors are all similar in containing a product of two collision frequencies, a steric factor, and a mean lifetime. The latter may be approximated in a number of ways, each of which yields about 10 sec. Since bimolecular collision frequencies are about 10 liters/mole-sec, this would make Z V about 10 liters /mole -sec. The collision theory thus leads to a frequency of termolecular collisions of about 10 liters /mole -sec, which as we shall see from Table XII.9, is about the order of magnitude observed for the fastest reactions. 

The kinetic molecular theory of gases predicts that an increase in temperature increases molecular velocities and so increases the frequency of in-termolecular collisions. This agrees with the observation that reaction rates are greater at higher temperatures. Thus there is qualitative agreement between the collision model and experimental observations. However, it is found that the rate of reaction is much smaller than the calculated collision frequency in a given collection of gas particles. This must mean that only a small fraction of the collisions produces a reaction. Why ... 

The rate of a termolecular reaction is controlled by the three-body collision frequency Zabc- This frequency is defined as the number of two-body collisions Zx.YZ for each particle X (X = A, B, C) with an unstable complex YZ formed from other particles (YZ = BC, AC, AB) [443]. 

One may estimate the relative frequency of bimolecular and termolecular collisions using equations 4.3.1 and 4.3.11. 

A termolecular reaction rate is proportional to the frequency of the collision of three bodies. These reactions are proportional to [A]3, [A]2[B], or [A][B][C], depending on whether the molecules taking part in the reaction are of one, two, or three species. If these species are the starting materials, the reaction is third-order however, termolecular reactions are rare because of the low probability that three species will come together. 

The frequency of occurrence of three-body collisions is very much smaller than that of two-body collisions. Consequently, if a termolecular step is essential to the progress of the reaction, the reaction is very slow. 

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