Thermochemistry of uranium compounds. XVI. Calorimetric determination of the standard molar enthalpy of formation at 298.15 K, low-temperature heat capacity, and high-temperature enthalpy increments of UO2(OH2)•H2O (schoepite)

Three precise calorimetric methods, viz., low-temperature adiabatic, high-temperature drop, and solution-reaction, have been used to determine as a function of temperature the key chemical thermodynamic properties of a pure sample of schoepite, UO2(OH)2•H2O. The following results have been obtained at the standard reference temperature T = 298.15 K: standard molar enthalpy of formation [Formula: see text] molar heat capacity [Formula: see text] and the standard molar entropy [Formula: see text] The molar enthalpy increments relative to 298.15 K and the molar heat capacity are given by the polynomials: [Formula: see text] and [Formula: see text], where 298.15 K < T < 400 K. The present result for [Formula: see text] at 298.15 K has been combined with three other closely-agreeing values from the literature to give a recommended weighted mean [Formula: see text] from which is calculated the standard Gibbs energy of formation [Formula: see text] at 298.15 K. Complete thermodynamic properties of schoepite are tabulated from 298.15 to 423.15 K.