Calorimetric Determination of the Formation Enthalpies of Cs Polymolybdates at 298.15 K and 0.1 MPa

Abstract

Samples of cesium polymolybdates Cs2MonO3n+1 with n = 2, 3, 5, and 7 have been synthesized by melting and cooling stoichiometric mixtures of commercially available Cs2MoO4 and MoO3 powders. The single-phase character of the samples has been checked by X-ray diffraction. The standard enthalpies of formation of the Cs2MonO3n+1 polymolybdates with n = 1, 2, 3, 5, and 7 have been measured by solution calorimetry in dilute (CsOH·H2O) solvent at 298.15 K under 0.1 MPa. The values found for the mono and dimolybdates, ΔfH° (Cs2MoO4) =-1513.56 ± 1.11 kJ mol-1, ΔfH° (Cs2Mo2O7) =-2301.84 ± 2.37 kJ mol-1, are in good agreement with the already available calorimetric measurements. To our knowledge, this is the first time that the enthalpies of formation of cesium tri, penta, and heptamolybdates are measured: ΔfH° (Cs2Mo3O10) =-3077.17 ± 3.47 kJ mol-1, ΔfH° (Cs2Mo5O16) =-4601.70 ± 5.96 kJ mol-1, and ΔfH° (Cs2Mo7O22) =-6087.16 ± 8.79 kJ mol-1, respectively. The convex shape of the curve constructed from the formation enthalpies of the Cs2MonO3n+1 polymolybdates referred to the Cs2MoO4 and MoO3 oxides is consistent with the Cs2MoO4-MoO3 pseudobinary diagram.