A study of sensitized explosions - X. The kinetics of decomposition of chloropicrin and of the hydrogen-oxygen and hydrogen-chlorine reactions sensitized by chloropicrin

Abstract

A kinetic scheme is suggested for the decomposition of chloropicrin which is shown to be in agreement with the results of the work of Smith & Steacie (1938 a, b ), and which provides an explanation of the sensitizing action of chloropicrin on the hydrogen-oxygen and hydrogen-chlorine reactions described in parts VIII and IX (Ashmore & Norrish 1950 a, b ): CCl 3 NO 2 → Cl + ... k 1 CCl 3 NO 2 → COCl 2 + NOCl... k 2 Cl + NOCl → Cl 2 + NO... k 3 Cl + CCl 3 NO 2 → remove Cl... k 4 Estimates of the relative concentrations of chlorine atoms at various stages of the decomposition at different temperatures are made. The concentration of chlorine atoms passes through a maximum, and at temperatures above 340°C this maximum is probably reached within 1 msec. It occurs earlier, and is greater in magnitude, the higher the chloropicrin pressure at a given temperature or the higher the temperature for a given concentration of chloropicrin. These variations are shown to provide a direct explanation of the results for the range 340 to 400°C described in part IX, and to enable the results of part VIII to be explained by a modification of the scheme proposed by Dainton & Norrish (1941) for the nitrosyl chloride-hydrogen-oxygen reaction. This modification consists of an allowance for the variation in the initial number of chain-centres arriving in the reaction vessel, the variation being due to changes in the precise location within the entry tube of the maximum concentration of the chlorine atoms which initiate the chains. Kinetic schemes are suggested for the hydrogen-chlorine-chloropicrin reaction in the temperature region 100 to 200°C. A straight chain scheme, with chains initiated from reaction k 1 , is combined with a thermal condition for ignition to give a satisfactory account of the variation of the ignition limits with concentration of chloropicrin, with proportion of reactants, and with temperature, and also to account for the course of the slow reaction below the ignition limit. A branched chain mechanism is also discussed and is shown to account for the results only if the branching reaction is of the form H+Cl 2 +CCl 3 NO 2 →HCl+Cl+Cl+NO+COCl 2 . The experimental results do not allow a decision to be made between these possible schemes.