Kinetics of Hydrogen Halides in Shock Waves. II. A New Measurement of the Hydrogen Dissociation Rate

Abstract

Decomposition rates for H2, diluted in Ar, were studied behind incident shock waves over the temperature range 2900° to 4700°K. HCl and the infrared emission from this molecule were used in a manner to trace the course of decomposition of the H2. In terms of recombination rate constants, we found (cc, moles, sec units) k−3=1018T−1, k−4=2.5k−3, and k−5=20k−3, where the subscripts 3, 4, and 5 refer respectively to Ar, H2, and H as third bodies.

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“…Therefore, the TBR rate of Flower & Harris (2007) would also need to be reduced by this same factor of four. This would, upon use of the exact H 2 partition function, bring their TBR rate into excellent agreement with the rate of Jacobs et al (1967) and Palla et al (1983) at temperatures in the experimental range.…”

“…The TBR rate adopted by Palla et al (1983) was identical to the expression given by Jacobs et al (1967) apart from a change in units. Flower & Harris (2007) used the CID expression given by Jacobs et al (1967) together with their own determination of the equilibrium constant to derive a very different TBR rate constant. The discrepancy between these two TBR rate constants, therefore, is due to the adopted equilibrium constants that were used.…”

“…The experimental paper (Jacobs et al 1967) upon which these TBR rate constants are based cautions that their data lies in the middle of a range of other experimental data whose values vary over an order magnitude. Furthermore, substantial errors may be introduced by extrapolating collisional data, obtained over a small range of temperatures, to temperatures that lie outside that range.…”

“…The equilibrium constant used by Flower & Harris (2007) relies on the Saha equation and their determination of the H 2 partition function. When fitted to a simple temperature-dependent function, Flower & Harris (2007) found that their value is approximately 4.5 times greater than a similar fit obtained from the JANAF Thermochemical Tables at a temperature of 1000 K. Using this equilibrium constant together with the CID expression given in Jacobs et al (1967) yielded a TBR rate constant which was approximately six times larger at T = 1000 K than the TBR expression given in Jacobs et al (1967). It appears that there are two separate reasons for the factors of 4.5 and 6 discrepancies at 1000 K. The first reason is fitting error.…”

“…Two of these rate constants (Palla et al 1983;Flower & Harris 2007) were based on the shock tube measurements of Jacobs et al (1967) who gave analytic expressions for TBR and the inverse process of collision-induced dissociation (CID). The TBR rate adopted by Palla et al (1983) was identical to the expression given by Jacobs et al (1967) apart from a change in units.…”