Proton affinity of water and the mechanism and kinetics of production of H3O+ in flames of H2, O2 and N2

Abstract

From mass spectrometric measurements of the concentrations of H₃O⁺ in flames of H₂, O₂ and N₂ without additives, it is concluded that equilibrium levels of H₃O⁺ are gradually attained in the burnt gases. These are governed by a balance of H + H + OH ⇌ H₃O⁺+e^–(III) which in turn is affected by the species H and OH having concentrations above those for equilibrium especially in cooler flames. Observations made where process (III) is balanced demonstrate that its equilibrium constant is given by K₃= 2.0 × 10^–29 exp(–1.41 × 10⁴K/T) cm³ molecule^–1. This expression corresponds to a value for the proton affinity of water of 699 ± 17 kJ mol^–1.

H₃O⁺ is thus produced in the burnt gases of these flames and the reaction responsible appears to be the forward step of (III), operating with an overall rate coefficient of 8.2 × 10^–36 exp(–1.41 × 10⁴K/T) molecule^–2 cm⁶ s^–1. The process is, however, thought to be a two stage one, rather than a true termolecular reaction. Application of the principle of microscopic reversibility indicates that the products from dissociative recombination of H₃O⁺ and a free electron are two hydrogen atoms and a hydroxyl radical.