Second Limit of Hydrogen-I-Oxygen Mixtures: the Reaction H+HO2

Abstract

Measurements of the second limit of H2+ O2 mixtures at very low O2 concentrations have shown that the results at 500°C cannot be interpreted on the assumption that the only reaction between H atoms and HO2 radicals is the quadratic branching reaction H + HO2→2OH (8) whereas a precise interpretation is possible by introducing the additional reaction H + HO2→H2+ O2, (8a) with the ratio k8a/k8= 0.17. An alternative explanation in terms of destruction of H atoms at the surface has been eliminated by measurement of the first limit. Comparison of the above estimate at 500°C with the mean of k8a/(k8+k8b)≈1, obtained at 20°C suggests that E8–E8a≈1725 cal mol–1; if E8a= 0, (k8+k8b)= 3.1 × 1011 exp(–1725 cal mol–1/RT) dm3 mol–1 s–1. The present work does not distinguish between (8) and the kinetically equivalent reaction H + HO2→H2O + O (8b) but other workers have suggested that k8b/k8≈0.1. A higher activation energy and higher preexponential factor for the equivalent reaction, involving O—O bond fission rather than H abstraction, has also been found in the reaction H + H2O2.