Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 15Diffusionr n R 1Fig. 9. Apparatus for differential pressure measurement. From ref. 59.Pressure effects are observed on the dissociation of diatomic molecules andsmall polyatomic species such as CHO and HNO since the decomposition occursin a bimolecular process. In reaction (7)Br,+M + 2Br+M (7)M is Br, or any other gas that is present. By the principle of microscopic reversibility6',the reverse processes are also pressure-dependent.' A related pressureeffect occurs in unimolecular decompositions which are in their pressure-dependentregions (including unimolecular initiation processes in free radical reactions).According to the simple Lindemann theory6', the mechanism for the unimoleculardecomposition of a species A is given by the following scheme (for moredetailed theories see ref. 47b, p.283)A +A +A*+AA*+A +A +AA* + ProductsA* represents a molecule activated by collision. The rate of decomposition of Ais given byt For qualifying remarks see ref. 4, pp. 110 and 329.References pp. 104-111