Inhibition of the second limit of the hydrogen + oxygen reaction by hydrogen bromide

Abstract

The inhibition of the thermal reaction between hydrogen and oxygen by hydrogen bromide has been studied over a wide range of mixture composition and vessel diameter at 500°C using vessels coated with boric acid. Under these conditions, surface reactions are unimportant and the inhibition arises through the sequence of reactions : H + HBr = H₂+ Br (16), Br + H₂= HBr + H (17), Br + HO₂= HBr + O₂(18), Br + H₂O₂= HBr + HO₂(19), O + HBr = OH + Br, (22), where reaction (22) is much less important than reaction (16) as a primary inhibition step. In the kinetic analysis of this mechanism it has been necessary to simulate the course of the reaction by the direct numerical integration of the kinetic equations using the Atlas computer, since these equations were too complex for a more conventional approach to be used. This approach simulates closely the experimental situation and the computed limits are in good agreement with the experimental values over the whole range of mixture composition. This agreement has been obtained by a trial and error optimization using only two unknown parameters, k₈/k₁₈ and k₁₉, where reaction (8) is H + HO₂= 20H (8). The values of these quantities are 8.0 ± 1.6 and (1.0 ± 0.5)× 10¹¹ cm³ mol^–1 s^–1 respectively at 500°C.