The Heat Capacities and Entropies of Calcium and Barium Sulfates from 15 to 300°K. The Entropy and Free Energy of Sulfate Ion

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Abstract
The heat capacities of BaSO4 and small precipitated crystals of CaSO4–2H2O have been measured from 15 to 300°K. The heat capacity of large naturally occurring gypsum crystals was measured from 60 to 274°K. The Cp against log T plots of the molal heat capacities of the two samples of CaSO4–2H2O differing in crystal surface by a factor of roughly 106 show slight deviations from each other. We believe these variations to be outside the limits of experimental error. However, they do not produce a change in entropy at 298.1°K. S°298.1 of CaSO4–2H2O and BaSO4 were determined to be 46.36 and 31.51 E.U. respectively with an accuracy of 0.05 percent. The final value of S°298.1 of SO4—— from this series of measurements is 3.5 E.U. The free energy of formation of SO4—— has been calculated as — 175,300 cal.