Mechanisms of Complex Reactions and the Association of H and O2

Abstract

The kinetics of the upper explosion limit and the nonexplosive thermal reaction of H₂ and O₂ uniquely demonstrates the formation of HO₂ in collisions H+O₂+M, its destruction on surfaces and its ability to propagate chains by gas phase reactions with H₂. Below 500° HO₂ gradually ceases to be a chain carrier in the gas phase; assuming its further reactions to be heterogeneous, involving self‐neutralization and reaction with H₂, the kinetics of the mercury‐photosensitized H₂–O₂ reaction are well interpreted. Bates' mechanism for this latter reaction, in which HO₂ is postulated to react in the gas phase, is shown to be inconsistent with the thermal reaction. Data on the oxygen‐inhibited photosynthesis of HCl and on the H concentration in the mercury‐photosensitized H₂–O₂ reaction indicate that a fraction of the associations of H and O₂ occurs in binary collisions. A quantum‐mechanical treatment by Rosen shows that this process is possible. The partially binary character of the association is consistent with data on the photo‐oxidation of HI. The reactions of HO₂ in a mixture of H₂, Cl₂, O₂ and HCl are discussed. Recent confirmation of the reaction HCO+O₂=HO₂+CO greatly strengthens the proposed mechanism of the oxidation of aldehydes and hydrocarbons. The rate coefficient of the reaction H+O₂+H₂=HO₂+H₂ is found to be 0.81×10¹³ cm⁶ mole^—2 sec.^—1 at room temperature. With O₂ as a third body the rate coefficient is about ⅓, and with N₂ about ½ of this value. The rate coefficient of the reaction H+O₂=HO₂ is found to be 0.74×10⁸ cm³ mole^—1 sec.^—1 at room temperature. Comparison of the above value of the rate coefficient of H+O₂+H₂ with that estimated from data on the upper explosion limit shows that this reaction probably has no positive temperature coefficient, while a negative temperature coefficient is not ruled out.