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)/d t = k (I) 2 ( M ) is not obeyed. For each of the inert gases k , the experimentally determined termolecular rate constant, increases with the ratio (I 2 )/( M ), where (I 2 ) and ( M ) are the concentrations of iodine m olecules and inert gas molecules respectively. The dependence of k on (I 2 )/( 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 (I 2 )/( M ) increases and compensated for the real increase in k with (I 2 )/( M ). Except at low (I 2 )/( M ) values, k is a linear function of (I 2 )/( M ), the gradient being the same for all five inert gases. A rapid termolecular reaction I+I+I 2 =I 2 +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 10 32 k M (ml. 2 mol. -2 s -1 ) He Ne A Kr Xe 0.67 0.92 1.84 2.25 2.99