The recombination of atoms II. Causes of variation in the observed rate constant for iodine atoms

Abstract

A re-investigation of the recombination of iodine atoms in presence of the inert gases over a wider range of experimental conditions has shown that the simple termolecular rate law — d(I)/dt = k(I)² (M) is not obeyed. For each of the inert gases k, the experimentally determined termolecular rate constant, increases with the ratio (I₂)/(M), where (I₂) and (M) are the concentrations of iodine m olecules and inert gas molecules respectively. The dependence of k on (I₂)/(M) was obscured in previous work by the fact that a thermal effect, which results in a lowering of the apparent value of k as recombination proceeds, increases as (I₂)/(M) increases and compensated for the real increase in k with (I₂)/(M). Except at low (I₂)/(M) values, k is a linear function of (I₂)/(M), the gradient being the same for all five inert gases. A rapid termolecular reaction I+I+I₂=I₂+I'₃ with a rate constant k = 470 x 10^-32 ml.² mol^.-2 s^-1 is postulated to explain the linear relationships. B y extrapolation the values of k_M the third-order rate constants for the five inert gases are M 10³²k_M (ml.² mol.^-2 s^-1) He Ne A Kr Xe 0.67 0.92 1.84 2.25 2.99