|MadSci Network: Chemistry|
Dear Mr. McLaughlin, The tissue of scientific knowledge, on closer inspection, is full of holes. In contrast to alkali halides, the determination of lattice enthalpies (or energies) of sulfates and carbonates is complicated by the following factors: these ions don't exist in the gaseous state, nor do the corresponding uncharged molecules. The ions are only coarsely approximated by point charges. The crystal structures of the salts are non-cubic and polymorphous. At room temperature, crystalline hydrates (with structures different from the anhydrous forms) are the equilibrium phases in contact with water. Of course, all this would not prevent a dedicated experimental scientist from getting the results, but the summed uncertainties would probably reduce the utility of the final value. Quite generally, hydration enthalpies of anions are lower than those of cations. In "Gmelins Handbuch der anorganischen Chemie", 21, Natrium, Ergaenzungsband , Lieferung 3, Verlag Chemie GmbH, Weinheim/Bergstr., 1966, p. 1092, for Na2SO4, values of the lattice energy U, calculated by Russian scientists (K.B. Yatsimirsky, J. Gen. Chem. USSR 26 (1956)) are given as 470 and 485 kcal/mol, depending on the Kapustinsky-equation used. The lattice entropy is 98cal/mol.degree. For Na2CO3, on page 1320, we find U=535 kcal/mol, and delta S= 97cal/mol.deg. I don't know the type of approximations involved in the Kapustinsky equation.. Best regards Werner Sieber
Try the links in the MadSci Library for more information on Chemistry.