Hydrogen Iodide Reaction. A Paradox for Chemical Kinetics

Abstract

Sullivan has measured the second-order rate constant for the thermal reaction of hydrogen with iodine and also the third-order rate constant for the reaction in a photostationary system obeying the kinetics for H2+I+I→2HI. If the photostationary rate constant is extrapolated to the temperature of the thermal measurements, all of the thermal rate can be accounted for as due to the iodine atoms known to be present in equilibrium with the iodine molecules. If iodine molecules and pairs of iodine atoms can both react with hydrogen molecules and make independent contributions to the over-all rate, the observations seem to require that the molecular reaction is of negligible importance. However, the transition-state structure of lowest free energy should be accessible either from an iodine molecule or from a pair of atoms; absolute reaction rate theory then requires that the rate in the photostationary system be less than that observed experimentally. The paradox can be resolved if the transition-state structure of the lowest free energy is not accessible from iodine molecules or if the rate in the thermal system is determined by a pseudothermodynamic iodine activity rather than by independent contributions from kinetically equivalent mechanisms, but reasons are given for rejecting both of these resolutions. It is proposed that the paradox be resolved by rejecting the customary description of the reaction by absolute reaction rate theory; such a description employs partition functions based on molecular states defined by energies in randomly phased normal modes of motion and assumes a transmission coefficient of unity. An alternative description is developed based on trajectories uniquely defined by the initial coordinates and momenta of the reactant atoms. Failure to observe the molecular reaction is then ascribed to the large mass ratio of iodine to hydrogen; because of this mass ratio, the H–H bond will not be split during a collision and the centers of mass of the H2 and I2 systems will follow trajectories corresponding to elastic collisions. The conclusions of this paper could be tested further by repeating the thermal and photostationary measurements in solution, by studying the reaction in crossed molecular beams, and by measuring the photostationary reaction of iodine with deuterium.