Kinetic study of the reactions between lead metal and hydrogen bromide and hydrogen chloride

Abstract

The kinetics of the reaction between gaseous hydrogen bromide and hydrogen chloride and microparticulate lead metal have been investigated by gravimetric methods in the temperature ranges 427–514 K and 403–503 K, respectively. In both cases there is an initial rapid reaction to form a coherent, non-porous layer of lead(II) halide. The reaction of HBr with lead metal is diffusion controlled over the whole extent of reaction at temperatures of <448 K. At higher temperatures, although the reaction follows diffusion-controlled kinetics during the earlier parts of the reaction, phase-boundary kinetics become dominant later in the reaction when stresses in the PbBr₂ product layer produce a porous open structure. Parabolic rate constants vary from 0.232 × 10^–6 s^–1 at 403 K to 3.63 × 10^–6 s^–1 at 503 K, and phase-boundary rate constants from 0.67 × 10^–5 s^–1 at 403 K to 1.79 × 10^–5 s^–1 at 503 K. Activation energies have been evaluated to be 20.9 kJ mol^–1(initial reaction), 53.9 kJ mol^–1(diffusion-controlled regime) and 31.1 kJ mol^–1(phase boundary-controlled regime). Diffusion-controlled parabolic kinetics are observed over a wide range of HBr pressures (0.47–10.5 kN m^–2) at 473 K, and phase-boundary kinetics do not appear to operate even at higher extents of reaction at the higher pressures. Both the initial rates and the parabolic rate constants increase with increasing pressure of HBr. The reaction of HCI with lead metal was also found to proceed according to parabolic kinetics with rate constants varying from 0.112 × 10^–6 s^–1 at 427 K to 0.610 × 10^–6 s^–1 at 514 K with an activation energy of 30.2 kJ mol^–1. The rate-determining process in the regime of parabolic kinetics is considered to be anion diffusion.