The development and the function of the interface participating in the nucleation and growth reaction between crystalline potassium bromide and chlorine gas

Abstract

A kinetic and microscopic study has been completed for the gas-solid reaction KBr + Cl2→ KCl + BrCl to elucidate the mechanism of this nucleation and growth process. The work was concerned with reaction in two temperature intervals, under 400 K and over 450 K: reactivity was negligible in the intervening range (400-450 K). The low temperature reaction (273-320 K in 25 kPa Cl2) was characterized by long induction periods followed by a strongly acceleratory nucleation process yielding ‘half-cube’ shaped nuclei which grew at constant rate. Nucleation was catalysed by SnCl4. Growth of nuclei ceased and required a second induction period for resumption of growth when reaction was interrupted by evacuation. The red-brown colouration of nuclei is attributed to liquid halogen (Br2) retention; this was confirmed by titration. Product KCl crystallites were oriented in the KBr (111) directions, shapes of nuclei varied with reaction conditions. From kinetic measurements, and microscopic examinations of interface textures, it is concluded that reaction occurs within a thin zone (of around 10 pm) at the nucleus periphery the chemical change proceeding between participants dissolved in liquid halogen. The probable reaction sequence is KBr dissolution in Br2—BrCl—Cl2,Cl2+Br−⇋[BrCl2]−⇋Cl−+BrCland KCl precipitation. Textural examinations of nuclei produced during the high temperature reaction (over 600 K) revealed the development of a subsurface pore system permeating unrecrystallized reactant. It is concluded that reaction in solid adjoining these pores yields product KCl that later recrystallizes, with (111) orientation relative to the reactant, in the form of irregularly shaped nuclei.