Chain-initiating Process in the Reaction between Hydrogen and Oxygen between the Second and Third Explosion Limits

Abstract

THERE has been much discussion concerning the reaction responsible for the initiation of chains, in the thermal reaction between hydrogen and oxygen, in the non-explosive region between the second and third explosion limits¹. In their early work², Lewis and von Elbe favoured the thermal dissociation of hydrogen, but in a later paper³ they reject this view because their value of the overall activation energy is "of the order of only 100 K-cals" which they regard as the sum of the activation energies of the chain-initiating and chain-propagating steps, and hence conclude that the energy of activation of the initiating reaction is less than 100 kcal. This value excludes the dissociation of hydrogen (and, of course, oxygen, but the thermal dissociation of this molecule is not considered for valid kinetical reasons) as the chain-initiating reaction, and to meet these conditions, these authors postulate the dissociation of hydrogen peroxide according to H₂O₂ ↠H₂O + O or 2OH, without being very precise about the origin of the hydrogen peroxide, other than assuming that the reaction H₂+ O₂ ↠H₂O₂ may play a part initially. Recently, Willbourn and Hinshelwood⁴ have made a fresh experimental study of the third limit and the slow reaction at lower pressures and, in contrast to Lewis and von Elbe, conclude that the initiation reaction is probably H₂ + M ↠2H + M.