Thermal Decomposition of HgCl₂ Vapour

Abstract

IN a recent communication, Gaydon¹ has shown that an absorption spectrum obtained from mercury chloride (HgCl₂) in a heated steel tube by Rao², and attributed by the latter to diatomic HgCl (not to be confused with calomel (Hg₂Cl₂) which does not exist as a gas), is really due to CuCl (present as an impurity). It is the purpose of this communication to show by thermodynamic calculation that there is no chance at all of obtaining the absorption spectrum of HgCl from thermally decomposed mercuric chloride vapour. The same is true for HgBr and HgL. This may be surprising, because numerous diatomic radicals are known; for example, OH, NH, CN, CuH, CuCl, CaH, CaF, CaCl, BeF, MgF, MgCl, ZnF, CdF, AlF, AlH, SnF, PbF, PbCl, MnF, FeCl, the presence of which in thermal equilibrium at high temperatures has been ascertained by their absorption spectra. The rather exceptional position of the diatomic halides of mercury is primarily due to their small energy of dissociation, the value of which is 24,000 cal./mol. in the case of HgCl³ and still lower for HgBr and HgI.