The thermochemistry of some complex inorganic fluorides.

Thermodynamic data have been obtained on some ternary fluorides of the type AMF6 or A2MF6 where the central ion M is in the quadrivalent or quinquevalent oxidation state. Enthalpies of reaction and of solution of these fluorides were measured mainly by isoperibol calorimetry and from these measurements enthalpies of formation, enthalpies of anion solvation and related thermodynamic parameters were calculated. The effects of different solvent media, particularly aqueous hydrofluoric acid and the common non-aqueous organic solvents, on enthalpies of solution are discussed. Thermodynamic parameters which are related to the gaseous isolated anion, such as fluoride ion affinity and anion solvation enthalpy, require a knowledge of the lattice enthalpy of the appropriate salt which in turn requires a knowledge of the charge distribution within that salt. The author has developed a method whereby an approximate charge distribution can be estimated from the enthalpies of solution and of formation of a family of salts of the same anion provided that the lattice parameters are known and that there are at least two different crystal structure types within that family. This method was applied to the hexafluorosilicates and hexafluoromanganates. Factors which influence the rates of reaction of various hexafluoro elementate ions in aqueous solution have also been investigated. These are of relevance to any calorimetric determination of the enthalpy of hydrolysis since complete reaction must be ensured within the time limit set by the calorimeter design.