The role of liquid halogen in the reaction of crystalline KBr with gaseous Cl₂

Abstract

Problems in formulating a mechanism^1,2 for the reaction of crystalline KBr with gaseous C1₂ (KBr(s)+Cl₂(g)→KCl+BrCl) (a nucleation and growth process) and the role of dislocation proliferation³ in promoting interface advance have been widely discussed. The evidence that local textural changes, in the vicinity of an active reaction interface, could be observed microscopically prompted us to examine the system using scanning electron microscopy. From these textural observations together with kinetic and other measurements, we conclude that liquid halogen (C1₂, BrCl and Br₂) and dissolved intermediates, contained within the pore system permeating the solid product assemblage (the nucleus), are essential participants in the chemical change. We believe that reactant KBr dissolved in condensed halogen retained in pores at the reaction interface then interacts with chlorine to form polyhalide intermediates (probably including [BrCl₂]⁻) subsequently depositing KCl product. Microscopic observations show that the nucleus is composed of KCl plates, penetrated by narrow (∼0.2 µm) channels. This reaction model could be applicable to other systems, as previous discussions of solid state nucleation and growth processes have only rarely explicitly considered the possible participation of an intranuclear liquid and dissolved intermediates^4,5.