Standard enthalpy of solution and formation of cesium chromate. Derived thermodynamic properties of the aqueous chromate, bichromate, and dichromate ions, alkali metal and alkaline earth chromates, and lead chromate

Abstract

Calorimetric measurements of the enthalpy of solution of cesium chromate gave ΔH soln = (7622 ± 24) cal th mol −1 for a dilution of Cs 2CrO 4·21128H 2O. This result, along with the enthalpy of dilution gave the standard enthalpy of solution, ΔH soln o = (7512 ± 31) cal th mol −1, whence the standard enthalpy of formation, ΔH f 0(Cs 2CrO 4, c, 298.15 K), was calculated to be −(341.78 ± 0.46) kcal th mol −1. Recomputed thermodynamic data for the formation of the other alkali metal chromates have been tabulated. From their solubilities and enthalpies of solution, the standard entropies, S 0(298 K), of BaCrO 4 and PbCrO 4 were estimated to be (38.9 ± 0.9) and (43.7 ± 1.2) cal th K −1 mol −1, respectively. There is evidence that ΔH f 0(SrCrO 4, c, 298.15 K) may be in error. Thermochemical, solubility, and equilibrium data, have been combined to update the thermodynamic properties of the aqueous chromate (CrO 4 2−), bichromate (HCrO 4 −), and dichromate (Cr 2O 7 2−) ions. The new values at 298.15 K are as follows: S 0/ cal th K −1 mol −1 ΔH f 0/kcal th mol −1 ΔG f 0/kcal th mol −1 CrO 4 2−(aq) (13.8 ± 0.5) −(210.93 ± 0.45) −(174.8 ± 0.5) HCrO 4 −(aq) (46.6 ± 1.8) −(210.0 ± 0.7) −(183.7 ± 0.5) Cr 2O 7 2−(aq) (67.4 ± 3.9) −(356.5 ± 1.5) −(312.8 ± 1.0) For CrO 4 −(aq), ΔH f 0 has been taken directly from the most recent critical compilation of thermochemical data.