Kinetics of Isotope-Exchange Reactions of Fluorine with Some Halogen Fluorides

Abstract

Quantitative kinetic data on the isotope-exchange reaction of fluorine with chlorine trifluoride, bromine pentafluoride, and iodine heptafluoride have been obtained using F18 (t½=112 min) tracer. The materials were handled in a metal vacuum line, and the exchanges were carried out in a 2-liter nickel reaction vessel at three temperatures in the range 181—257°C. While the experimental data do not uniquely determine the mechanism, they strongly suggest that exchange takes place either through a heterogeneous catalysis by the metal fluoride coating on the walls of the reaction vessel, or by a combination of the heterogeneous mechanism plus a homogeneous gas-phase exchange in those cases where a measurable amount of dissociation of the halogen fluoride occurs at the temperature of exchange. It has been shown to be unlikely that the observed exchanges take place through either an atomic mechanism or a purely homogeneous mechanism. It has also been shown that all fluorine atoms in the halogen fluoride molecules are exchangeable.