A flash photolytic study of the explosive reaction between hydrogen and oxygen sensitized by nitrogen dioxide, chlorine and bromine

Abstract

When hydrogen and oxygen were flashed in the presence of nitrogen dioxide, chlorine or bromine under suitable conditions explosion occurred. The results are compatible with the accepted mechanism for the hydrogen–oxygen reaction, with the addition of steps involving molecules and radicals produced from the sensitizers, when the unusual conditions pertaining to the flash system are considered. In addition to the chain branching mechanism there is a considerable thermal contribution to the explosion, since heat must be evolved at a sufficient rate to compensate for the heat lost to the surroundings. This was confirmed by direct observation of the temperature of the system by following the vibrational temperature of nitric oxide throughout the incubation period and explosion. With nitrogen dioxide as sensitizer, the nitric oxide formed during the flash exhibited a sensitizing effect attributed to the reaction proposed by Ashmore & Tyler (1962) HO 2 + NO → NO 2 + OH which interrupts the normal termination sequence. With halogen sensitizer ( X 2 ), H X formed during the flash exhibits an inhibiting effect, more pronounced with bromine, owing to the reaction: H X +H → H 2 + X . A peninsular explosion region was observed, for which the upper limit was analogous to the second limit in conventional thermal systems, but the lower limit did not involve heterogeneous termination steps. The onset of explosion at this limit was found to be chain-thermal in nature. During the explosion sensitized with chlorine a transitory continuous spectrum was observed, but not identified.