The photosensitised decomposition of nitrogen trichloride. Part II.— The effects of surface and inert gases, and the mechanism of reaction

Abstract

In Part I it has been established that nitrogen trichloride vapour is decomposed by the photosensitive action of chlorine.The reaction is strictly of zero order, and the high values to which the quantum efficiency rises at low total pressures indicate the existence of a chain mechanism. Further evidence is now presented which locates the decomposition in the homogeneous gas phase, and rules out the possibility of accounting for the zero order on the basis of a surface reaction. The kinetics of all chain reactions so far studied are in accord with the view that they are propagated in a homogeneous phase; this does not preclude a reaction involving a chain mechanism from occurring exclusively on a surface, but such a possibility is susceptible of a simple test by a variation of the total available surface. This criterion has been applied in the present instance, and neither alteration of the total illuminated surface nor of the total available surface has any other than a secondary effect upon the reaction rate. It is therefore concluded that the chain mechanism in the photosensitised decomposition of nitrogen trichloride is primarily confined to the gas phase, and other evidence will be referred to later which supports this conclusion.