Thermal reactions of lead(IV) chloride complexes in the solid state. Part IV. Thermolysis of alkali metal hexachloroplumbates

Abstract

Alkali metal hexachloroplumbates of general formula M 2(I)PbCl 6, with M(I) = K, Rb, Cs, undergo thermal decomposition on heating to 700 K. The thermal processes in the solid state have been studied by differential thermal analysis (DTA) and thermogravimetry (TG) with a derivatograph. The thermolysis occurs in one step M 2(I)PbCl 6→2M(I)Cl (s)+PbCl 2(s)+Cl 2(g) The experimental TG and DTG curves were used to estimate the enthalpy of thermal dissociation. Based on these curves the kinetic parameters of thermolysis have been evaluated. In the linearization procedure of the kinetic equation. ▪ the methods of Freeman and Carroll, Coats and Redfern, Horowitz and Metzger and Gorbachev were applied. Sixteen f(α) functions which describe the primary mechanism of thermal decomposition of solid substances—nucleation, growth of nuclei, phase boundary reactions and diffusion processes—were used in the least squares error analysis. The dehydration of calcium oxalate monohydrate was performed to confirm the validity of the analytical and computational procedures. The differences in values of the kinetic and thermodynamic parameters for hexachloroplumbates thermolyses have been discussed. It was found that K 2PbCl 6 showed atypical thermal properties. This is probably caused by the different crystal structure of K 2PbCl 6 as compared with those of Rb 2PbCl 6 and Cs 2PbCl 6, as well as the abnormal course of its thermolysis. The thermal properties of alkali metal hexachloroplumbates have been compared with those for analogous compounds of the type M 2(I)M(IV)X 6 (M(IV) = Pt, Te, Tc; X = Cl, Br).