The dissociation of water vapour behind shock waves

Abstract

The dissociation of water vapour in dilute (< 7 %) mixtures with argon has been studied behind shock waves. The growth of OH concentration in the initial stages of dissociation was followed using a short-duration flash-absorption technique which recorded the OH (0, 0) band with high resolution. Profiles of OH concentration were constructed for various conditions of temperature and concentration and a computer analysis was used to match these profiles to a proposed reaction H2O+m —k1,M H + OH+M, sequence. The results indicate that basically the decomposition proceeds by the reaction and rate constants for this reaction were determined over a temperature range of 2570 to 3290 K with M = Ar and H 20. The overall rate equation can be expressed as k1,M[M] = A{[Ar] + n/[H2O]}T-n e-Do/RT with D0 being 494 k J mol-1 (118 kcal mol-1 *), and the calculated parameters being: A = (4.0 ±0.5) x 1023 cm3 mol-1 s-1, n = 20 + 7, N = 2.2 ±0.8 This result, together with the equilibrium constant of the reaction, provides an assessment of the rate of the reverse process, the recombination of H and OH, which in conjunction with previous assessments in flames and shock tubes presents a unified and most consistent set of rate data over a wide temperature range.