Abstract
Heat capacities and phase-transition enthalpies have been measured for three ethanoate hydrates by adiabatic-shield calorimetry between 270 and 400 K. Enthalpies and entropies have been derived for the exact stoichiometries and tabulated for selected temperatures. The molar enthalpy of transition of CH 3CO 2Na·3H 2O to (CH 3CO 2Na + aqueous solution) at 331.52 K is (37.86±0.25) kJ·mol −1. The molar enthalpy of transition of CH 3CO 2Na·3H 2O to an aqueous solution is (38.7±0.3) kJ·mol −1, when the enthalpy of dissolution of remaining anhydrous enthanoate up to 358 K is included. The enthalpy of transition of CH 3CO 2Li·2H 2O to (CH 3CO 2Li + aqueous solution) at 324.71 K is (24.25±0.20) kJ·mol −1. The enthalpy of transition of α-(CH 3CO 2) 2Mg·4H 2O to {aqueous solution + a less hydrated solid phase, presumably (CH 3CO 2) 2Mg·H 2O} at 336 K is (35.8±0.5) kJ·mol −1. Water-vapour saturation pressures above the samples have been measured at selected temperatures. Lattice constants determined from Guinier photographs at 300 K for CH 3CO 2Na·3H 2O are: a = 1234.5 pm, b = 1045.1 pm, c = 1040.8 pm, β = 111.70°; for CH 3CO 2Li·2H 2O: a = 682.1 pm, b = 1088.4 pm, c = 659.6 pm; for α-(CH 3CO 2) 2Mg·4H 2O: a = 480.5 pm, b = 1198.6 pm, c = 855.3 pm, β = 95.38°; and for (CH 3CO 2) 2Mg·H 2O: a = 1175.9 pm, b = 1753 pm, c = 666.3 pm. CH 3CO 2Na·3H 2O might be of interest as an energy-storage material because of its high molar enthalpy of fusion, if supercooling and segregation of anhydrous ethanoate can be prevented.
Published Version
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