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)

Abstract

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.

The Application of Periodic Density Functional Theory to the Study of Uranyl-Containing Materials: Thermodynamic Properties and Stability

“…It must be emphasized that while the experimental isobaric heat capacity function of gamma uranium trioxide at 1000 K is above the asymptotic Dulong-Petit limit, our computed function satisfies properly the requirement of being below this limit [100]. Finally, the theoretical results for metaschoepite mineral phase agree very well with the experimental thermodynamic properties reported by Barin [12] even at temperatures of the order of 800 K, the percent differences of the calculated specific heat, entropy, and Gibbs energy with respect to the corresponding experimental values being 5.4%, 3.2%, and 2.0% at 800 K. The present theoretical data have permitted to discriminate between the experimental thermodynamic functions of metaschoepite reported up to date because the experimental functions reported by Tasker et al [145] deviate from those of Barin [12] and from our theoretical results already at moderate temperatures [104].…”

Periodic DFT Study of the Thermodynamic Properties and Stability of Schoepite and Metaschoepite Mineral Phases

“…Computed heat capacity (A), entropy (B), and Gibbs free energy (C) functions of metaschoepite. The experimental values were reported by Tasker et al (in red) and Barin (in violet).…”

“…Although the range of thermal stability of metaschoepite appears to be from 0 to 425 K, it will be shown below that the thermal stability of metaschoepite in the presence of H 2 O 2 is much larger. For this reason, the values of the TPs of this material are provided in Tables S.5 to S.8 of the Supporting Information and shown in Figure for the extended range of temperatures from 0 to 1000 K. The calculated isobaric specific heat at the last temperature considered in the present work (1000 K), C p = 207.1 J·K –1 ·mol –1 is 17.1% below the Dulong–Petit asymptotic value ( C p = 249.9 J·K –1 ·mol –1 ).…”

Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates

“…The calorimetric data enable the calculation of the enthalpy of formation from the elements, AH?, at 298 K for metaschoepite and P-U02(0H)2, as in the following reaction ( (Hemingway 1982) and -1826.1 f 1.7 kJ mol-' (Guillaumont et al 2003). Santalova et al (1971) and Tasker et al (1988) independently measured the enthalpy of hydration of y-UO3 to U03(H20)2 based on differences in the enthalpy of solution of the two solids in dilute HF. The value given by Guillaumount et al (2003) was calculated using the average enthalpy difference between those of Santalova et al (1971) and Tasker et al (1988) along with AH?…”