Thermochemistry of group III and group IV halides.

Enthalpies of solution of rare earth trihalides have been determined in water, in aqueous hydrochloric acid, and in several non-aqueous solvents such as MeOH and DMSO. In addition further enthalpies of solution were obtained in a few mixed aqueous solvents, i.e. H20-Me0H, H20-Et0H, H2O-ButOH and H2O-DMSO. The determination of many thermodynamic solution properties of these trihalides, especially enthalpies of solution, requires the use of anhydrous materials. These are by no means readily available in high purity for this group of compounds. Therefore a section on the various preparative methods that have been published in the literature and also on the preparative methods investigated in this work precedes the thermochemistry. The solution thermochemistry (i.e. enthalpies of solution and solubilities in aqueous and non-aqueous solvents) recently reviewed by the author has been included as Appendix 7. The solubilities of rare earth iodates in H20-Me0H were investigated too, and these data along with the data on enthalpies of solution are discussed in terms of enthalpies and Gibbs free energies of transfer from water to mixed aqueous solvent. For comparison with the rare earth trihalide results, the enthalpies of solution of GaCl3, Gal3, InCl3, InI3, ThCl4, ThI4, UCl4, GeI4, and SnI4 were determined in similar aqueous and non-aqueous solvents. The number of solvents studied in the GeI4 and SnI4 work was extensive as both donor and non-donor solvents were investigated. As a continuation of the work on SnI4 and GeI4, and in view of possible relevance to the lamp manufacturing industry, their solubilities were determined in a range of organic liquids (i.e. halocarbons, hydrocarbons, and aromatic solvents). These results were compared with those for other solutes that formed 'regular solutions', primarily iodine. Since it was found that both SnI4 and Gel4 decom-posed in certain solvents (e.g. MeOH and EtOH) the kinetics of these reactions were investigated. The thermal decomposition of solvates of rare earth compounds was studied using TGA and DSC techniques as these too were of interest to the lamp industry. For similar reasons the process of dissolving molybdenum metal in sulphuric acid - nitric acid mixtures was investigated.