Kinetics of the reaction of lead(II) oxide with hydrogen bromide in the temperature range 398–548 K

Abstract

The kinetics of the reaction of both the tetragonal and orthorhombic modifications of lead(II) oxide with hydrogen bromide at a pressure of 1.33 kN m^–2 have been studied in the temperature range 398–548 K. Additionally, the reaction with the orthorhombic modification has been examined at HBr pressures of 0.67, 3.33 and 6.65 kN m^–2. The kinetic behaviour of the two modifications is substantially different, an observation which is largely attributed to the difference in particle size. Reaction profiles for the orthorhombic modification (mean particle diameter ca. 15 µm) comprise a rapid initial reaction followed by a slow diffusion-controlled reaction giving conversions to lead(II) bromide of ca. 5% at 398 K rising to ca. 62% at 548 K after 300 min. In contrast, reaction with the tetragonal modification (mean particle diameter ca. 2 µm) is rapid at all temperatures giving much higher conversions of ca. 47% at 398 K rising to ca. 92% at 548 K after only 40 min. Analysis of the rapid reaction in terms of a phase-boundary model yields rate constants (k_PB) varying from 1.54 × 10^–4 s^–1 at 448 K to 2.55 × 10^–4 s^–1 at 548 K. The slow, limiting reaction observed for the orthorhombic modification is best described by the diffusion-controlled model of Zhuravlev, Lesokhin and Tempel'man, with derived rate constants (k_ZLT) varying from 1.65 × 10^–8 s^–1 at 398 K to 7.86 × 10^–6 s^–1 at 548 K, essentially independent of applied HBr pressure. Activation energies determined from Arrhenius analysis of the rate-constant data showed that for the phase-boundary-controlled reaction with the tetragonal modification is much lower [10.6(5) kJ mol^–1] than that for the diffusion-controlled reaction with the orthorhombic phase [71.8(17) kJ mol^–1].