KINETICS OF THE REACTION - H-]H-2+I AT 298 K AND VERY-LOW PRESSURES(HI)

The reaction H + HI -->(1) H-2 + I was studied at 298 K and millitorr pressures employing the ''Very Low Pressure Reactor'' (VLPR) kinetic t echnique. H-atoms were generated by dissociating H-2 molecules (of a H -2/Ar mixture) in a microwave discharge cavity that preceded the very low pressure well-mixed reaction vessel. Quadrupole mass spectrometry was used to analyze molecules and atoms. The mass signal intensities o f I and HI were measured at both 20 and 40 eV ionizing potentials whil e those of H and H-2 were measured at 40 eV due to the very weak signa l of these species at lower ionization potentials. Three different exi t flow orifices were utilized in the reported VLPR experiments of abou t 2, 3, and 5 mm inner diameter to vary the species concentration unde r steady-state reaction conditions. A rate constant of k(1) = (2.1 +/- 0.2) x 10(-11) cm(3)/molecule.s was determined for the forward reacti on at 298 K, which lies between the two previously reported values dir ectly measured at 298 K. Satisfactory mass balance relations were obta ined for the iodine atoms (from the HI and I species) which were bette r than 90% for most of the experiments. The value of the reported rate constant (k,) is 14.3% higher than the value measured by Umemoto et a l. [6], and 33.3% lower than the value measured by Lorenz et al. [4]. Based on this comparison, the activation energy E-1 of the forward rea ction probably lies between those two previously reported values of 58 0 and 720 cal/mol. Transition State Calculations of A(1) and A(2) for the reaction of H + I-2 -->(2) HI + I are in good agreement with the d ata on both reactions and suggest an activation energy of about 500 +/ - 100 cal/mol for E-2. (C) 1997 John Wiley & Sons, Inc.