The isotope exchange reactions H++D2⇄HD+D+ and D++H2⇄HD+H+ in the temperature range 200–300 K

Abstract

Forward and reverse rate coefficients have been measured in the temperature range 200–300 K for the two reactions H++D2⇄HD+D+ and D++H2⇄HD+H+. Equilibrium constants derived therefrom agree with theoretical van’t Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is suggested that these reactions can be used as kinetic thermometers to measure independently the temperature of ion–molecule reaction cells. The system provides a particularly clear example of the role of statistical factors in chemical kinetics, 1 for the forward reactions and 1/2 for their reverse reactions; and the system illustrates further the relationship between statistical factors in kinetics, symmetry numbers in statistical mechanics, and the corresponding thermodynamic entropy changes. Constraints upon the temperature dependence of the rate coefficients are derived from consideration of thermodynamics and collision dynamics, and the data are seen to conform to these over a limited temperature range. A further trend is suggested by the data, supporting previous observations of isotope-exchange reactions—the rate coefficients of the exoergic reactions decrease with increasing temperature—and may be described in terms of the partitioning of the system according to the number of states available to the products and the original reactants. It is suggested that this should be a general result for reactions where the exoergicity is comparable to the temperature of measurement.