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(OH)2 /center dot/ H2O (schoepite)

Three precise calorimetric methods, viz., low-temperature adiabatic, high-temperatuare 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 /center dot/ H2O. The following results have been obtained at the standard reference temperature T = 298.15 K:standard molar enthalpy of formation Δ/sub f/H/sub m/0(T) = /minus/1825.4 +- 2.1 kJ mol/sup /minus/1/; molar heat capacity C/sub p,m/0(T) = 172.07 +- 0.34 JK/sup /minus/1/; and the standard molar entropy S/sub m/0(T) = 188.54 +- 0.38 JK/sup /minus/1/ mol/sup /minus/1/. The molar enthalpy increments relative to 298.15 K and the molar heat capacity are given by the polynomials: /H/sub m//sup 0/(T) /minus/ H/sub m//sup 0/(298.15 K)//(J mol/sup /minus/1/) = /minus/38209.0 + 84.2375 (T/K) + 0.1472958 (T/K)2 and C/sub p,m/0(T)/(JK/sup /minus/1/ mol/sup /minus/1/) = 84.238 + 0.294592 (T/K), where 298.15 K < T < 400 K. The present result for Δ/sup f/H/sub m/0 at 298.15 K has been combined with three other closely-agreeing values from the literature to give a recommended weighted mean Δ/sub f/H/sub m/0 = /minus/1826.4 +- 1.7 kJ mol/sup /minus/1/, from which is calculated the standard Gibbs energy of formation Δ/sub f/G/sub m/0 = /minus/1637.0 +- 1.7 kJ mol/sup /minus/1/ at 298.15 K. Complete thermodynamic properties of schoepite are tabulated from 298.15 to 423.15 K. 19 refs., 6 tabs