ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

On the applicability and predictive power of additive equations in the evaluation of standard enthalpies of formation of crystallohydrate salts of rare-earths and actinoides

N.I. Yanakiev

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, B-1040 Sofia

Abstract: The validity of equations for the additive evaluation of the standard enthalpies of formation (∆H_f°) of crystallohydrates of rare-earth elements and actinoides is established on the basis of experimental data. Rozenfeld's equation, i.e.  ∆H_f°crystallohydrate) =  ∆H_f° (anhydrous salt) + b₁ N where N is the hydration value and b₁ is the increment for one water molecule, shows the best approximation to the experimental data and offers the best predictive possibilities. For N = b₁ is —307.5 kJ mol^-1 , for each from the second to the sixth water molecule bx is — 300.4 kJ mol^-1, while for any water molecule belonging to the second coordination sphere bx is — 292.9 kJ mol^-1 . The present considerations corroborate the previously published conclusion that the experimentally obtained AHf values for crys­talline phosphate dihydrates of 12 rare-earth elements should be recon­sidered. The use of a general increment of low value ( — 297.1 k J mol^-1 ) is shown to be one of the causes for considering in the literature some∆H_f° of crystallohydrates as subject to nonlinear changes and lack of additivity. Rozenfeld's equation is also shown to be useful in the rapid check of crystallohydrate ∆H_f° values in the cases where they do not agree with experimental data for other crystallohydrates of the same salt.

Full paper in Portable Document Format: 416a731.pdf

Chemical Papers 41 (6) 731–738 (1987)