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

Samples of cesium polymolybdates Cs₂Mo_nO_3n+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 Cs₂Mo_nO_3n+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, Δ_fH° (Cs₂MoO₄) = −1513.56 ± 1.11 kJ mol^–1, Δ_fH° (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: Δ_fH° (Cs₂Mo₃O₁₀) = −3077.17 ± 3.47 kJ mol^–1, Δ_fH° (Cs₂Mo₅O₁₆) = −4601.70 ± 5.96 kJ mol^–1, and Δ_fH° (Cs₂Mo₇O₂₂) = −6087.16 ± 8.79 kJ mol^–1, respectively. The convex shape of the curve constructed from the formation enthalpies of the Cs₂Mo_nO_3n+1 polymolybdates referred to the Cs₂MoO₄ and MoO₃ oxides is consistent with the Cs₂MoO₄–MoO₃ pseudobinary diagram.