Reaction probabilities for the reactions of hydrogen atoms at selected initial energies in hydrogen iodide–iodine mixtures

Abstract

The reactions of photochemically generated H atoms formed at selected initial energies in HI-I₂ mixtures have been investigated. A kinetic analysis of reaction rates in systems with and without added I₂ allowed the measurement of integrated reaction probabilities for the reaction H + HI → H₂+ I. At initial H atom energies of 49 and 106 kJ mol^–1 the respective reaction probabilities were 0.11 ± 0.02 and 0.35 ± 0.02. At 178 kJ mol^–1, after allowance for partial photolytic yield of electronically excited I atoms, a reaction probability close to unity is indicated. In each set of experiments the analysis also yielded the ratio of thermal rate constants for reactions of H atoms with I₂(k₆) and HI(k₃) as 11.6 ± 0.4. The reaction probabilities are discussed with particular reference to the very high values found in earlier work for other H atom + hydrogen halide reactions.