Rate of Reaction of Hydrogen with Iodine at Equilibrium

Rosenbaum and Hogness [E. J. Rosenbaum and T. R. Hogness, J. Chem. Phys. 2, 267 (1934)] measured the rate of para to ortho hydrogen conversion in equilibrium mixtures of hydrogen, iodine, and hydrogen iodide and found that the conversion rates were about twice as fast as would be predicted by the reactions $${\mathrm{H}}_2+{\mathrm{I}}_2\leftrightarrow {\displaystyle {lim}_{\text{(2)}}^{\text{(1)}}}2\mathrm{HI}.$$ In this paper the additional conversion in their experiments is shown to be explained by the reaction sequence $$\begin{array}{c}\text{(3)}\mathrm{I}+{\mathrm{H}}_2\to \mathrm{HI}+\mathrm{H}\\ \text{(4)}\mathrm{H}+\mathrm{HI}\to {\mathrm{H}}_2+\mathrm{I}\\ \text{(5)}\mathrm{H}+{\mathrm{I}}_2\to \mathrm{HI}+\mathrm{I}\\ \text{(6)}\mathrm{I}+\mathrm{HI}\to {\mathrm{I}}_2+\mathrm{H}.\end{array}$$ The rate constants for the rate determining steps (1) and (3) as determined from the equilibrium data are shown to be equal to those determined in systems far from equilibrium. Upper limits can be set on the rates of two para to ortho hydrogen conversion reactions which have previously been suggested to explain the results of Rosenbaum and Hogness: an upper limit of 2.5×10^—12 is determined for the collision efficiency of reaction (a) and a lower limit of 47 kcal is determined for the activation energy of (b) $$\begin{array}{c}(\mathrm{a}){\mathrm{H}}_2^p+\mathrm{I}\to {\mathrm{H}}_2^o+\mathrm{I}\\ (\mathrm{b}){\mathrm{H}}_2^p+\mathrm{HI}\to {\mathrm{H}}_2^o+\mathrm{HI}.\end{array}$$ The reactions of hydrogen, both with iodine atoms and with iodine molecules, are shown to be homogeneous in Pyrex vessels at 693°K.