Critical Evaluation of the Standard Molar Entropies, Enthalpies of Formation, Gibbs Energies of Formation and Heat Capacities of the Aqueous Trivalent Rare Earth Ions, and the Corresponding Standard Molar Entropies, Enthalpies of Formation and Gibbs Energies of Formation of the Thermodynamically Stable RECl3·7H2O(cr) and RECl3·6H2O(cr)

Abstract

In an earlier evaluation (F.H. Spedding, J.A. Rard, A. Habenschuss, J. Phys. Chem. 81: 1069–1074, 1977) the standard molar entropies of the aqueous trivalent rare earth ions RE3+(aq) were calculated based on the standard molar entropies of the hydrated rare earth chlorides RECl3·7H2O(cr) (RE = La, Pr) and RECl3·6H2O(cr) (RE = Nd, Gd, Tb, Dy, Ho, Er, Lu) at T = 298.15 K and p = 0.1 MPa, along with their corresponding standard molar enthalpies and Gibbs energies of solution, which are known for nearly all of the RECl3·7H2O(cr) and RECl3·6H2O(cr) including YCl3·6H2O(cr). However, the entropies of these RECl3·7H2O(cr) and of several of the RECl3·6H2O(cr) have large uncertainties because the source heat capacities only extend up to T = 223–262 K and not to T = 298.15 K and the crystals used for these measurements contained some occluded solution (water excess). Simple methods are described to correct these entropies of the RECl3·7H2O(cr) and RECl3·6H2O(cr) and to estimate the standard molar entropies of the RECl3·6H2O(cr) from those of the corresponding anhydrous RECl3(cr). By using these entropies, standard molar enthalpies of solution and revised results for the standard molar Gibbs energy changes to form the saturated solutions, revised and CODATA-compatible values of the standard molar entropies of the RE3+(aq), $$ S_{\text{m}}^{\text{o}} ({\text{RE}}^{3 + } ,\,\,{\text{aq}},\,\,298.15\,\,{\text{K}}) $$ where RE = (La, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y), were evaluated or estimated (RE = Pm). By combining these results with critically-assessed standard molar enthalpies of formation of the RE3+(aq), the standard molar Gibbs energies of formation of these trivalent rare earth aquo ions were calculated, as were the corresponding standard molar Gibbs energies and enthalpies of formation of the thermodynamically stable RECl3·7H2O(cr) and RECl3·6H2O(cr) phases at T = 298.15 K and p = 0.1 MPa. In addition, standard molar heat capacities of most of the RE3+(aq) were evaluated from published results for dilute solutions from flow microcalorimetry.