Kinetics of the reaction: H+HI?H2+I at 298 K and very low pressures

Abstract

The reaction \scriptfont4=\seveni \scriptscriptfont4=\fivei $\rm H+HI\mathop\rightarrow\limits ^{1}H_{2}+I$ was studied at 298 K and millitorr pressures employing the “Very Low Pressure Reactor” (VLPR) kinetic technique. H-atoms were generated by dissociating H2 molecules (of a H2/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 of I and HI were measured at both 20 and 40 eV ionizing potentials while those of H and H2 were measured at 40 eV due to the very weak signal of these species at lower ionization potentials. Three different exit flow orifices were utilized in the reported VLPR experiments of about 2, 3, and 5 mm inner diameter to vary the species concentration under steady-state reaction conditions. A rate constant of k1=(2.1±0.2)×10−11 cm3/molecule.s was determined for the forward reaction at 298 K, which lies between the two previously reported values directly measured at 298 K. Satisfactory mass balance relations were obtained for the iodine atoms (from the HI and I species) which were better than 90% for most of the experiments. The value of the reported rate constant (k1) is 14.3% higher than the value measured by Umemoto et al. [6], and 33.3% lower than the value measured by Lorenz et al. [4]. Based on this comparison, the activation energy E1 of the forward reaction probably lies between those two previously reported values of 580 and 720 cal/mol. Transition State Calculations of A1 and A2 for the reaction of \scriptfont4=\seveni \scriptscriptfont4=\fivei $\rm H+I_{2}\mathop\rightarrow\limits ^{2}HI+I$ are in good agreement with the data on both reactions and suggest an activation energy of about 500±100 cal/mol for E2.© 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 915–925, 1997.