P.A.G. O'Hare

Fluorine bomb calorimetry. Part 14.—Enthalpies of formation of the hexafluorides of sulphur, selenium, and tellurium and their thermodynamic properties to 1500°K

The energies of formation of the hexafluorides of sulphur, selenium, and tellurium were measured by direct combination of the elements in a bomb calorimeter. From these measurements the standard enthalpies of formation ΔH°f298·15(g), were calculated (in kcal mole–1) : SF6, –291.77±0.24; SeF6, –266.95±0.14; TeF6, –327.20±0.56. These values are substantially more negative then those reported by Yost and Claussen in 1933. The thermodynamic properties of these compounds have been computed and tabulated from 0 to 1500°K.

The standard enthalpy of formation of LaNi5 The enthalpies of hydriding of LaNi5−xAlx☆

Abstract The enthalpies of reaction in HCl(aq) of LaNi 5 (cr), La(cr), and Ni(cr) were measured. From these measurements and appropriate auxiliary quantities, the standard enthalpy of formation, Δ f H m o , of LaNi 5 (cr) was determined. The enthalpies of reaction between H 2 and LaNi 5 , as well as LaNi 4.5 Al 0.5 , and LaNi 4 Al, have been measured by hydrogen titration calorimetry. The enthalpy of solution of H 2 in the α-phase of (LaNi 5 +H 2 ) is −28 kJ · mol −1 , and in the two-phase plateau region the enthalpy of hydriding by H 2 is −(31.6±0.2) kJ · mol −1 . In the two-phase plateau region of (LaNi 5− x Al x +H 2 ) the enthalpy of hydriding by H 2 is −38 kJ · mol −1 for LaNi 4.5 Al 0.5 and −46 kJ · mol −1 for LaNi 4 Al. The relation between the enthalpy of hydriding of the alloy by H 2 and its aluminum content was found to be Δ hyd H m o (LaNi 5− x Al x , cr, 298.15 K) = −(31.6 + 14.4 x ) kJ · mol −1 . The following thermodynamic quantities at T = 298.15 K are reported: Δ f H m o /(kJ · mol −1 ) Δ f S m o /(J · K −1 · mol −1 ) Δ f G m o /(kJ · mol −1 ) LaNi 5 −159.1±8.3 3.5±0.8 −160.1±8.3 LaNi 5 H 6.2 −258.6±8.3 −339.5±0.9 −157.4±8.3

Thermochemistry of molybdates I. Standard enthalpy of formation of cesium molybdate (Cs2MoO4)

The standard enthalpy of formation of cesium molybdate, Δ°f(Cs2MoO4, c, 298.15K), is reported. The result obtained, −(362.00±0.11)kcalthmol−1, is based on calorimetric measurements of the enthalpy of reaction of molybdenum trioxide with aqueous cesium hydroxide and of the enthalpy of solution of cesium molybdate in aqueous cesium hydroxide. Thermodynamic data are given for the formation of Cs2MoO4 both from its elements and from the oxides Cs2O and MoO3 at 298.15 K.

Thermodynamic studies of zeolites: clinoptilolite

Calorimetric studies are described of a carefully characterized specimen of clinoptilolite (Malheur County, Oregon, U.S.A.) of composition: S r 0.036 M g 0.124 C a 0.761 M n 0.002 B a 0.062 K 0.543 N a 0.954 A l 3.450 F e 0.017 S i 14.533 O 36.000 ⋅ 10.922 H 2 O . Values are reported of the standard molar enthalpies of formation ΔfHom at 298.15 K and of the standard molar enthalpy increments {Hom(T) − Hom(298.15 K)} for both clinoptilolite and dehydrated clinoptilolite, as well as the low-temperature molar heat capacity Cop,m and, by derivation, the standard molar entropy increment {Som(T) − Som(0)} for clinoptilolite alone. The conventional thermodynamic properties have been calculated for both compounds.

Thermochemistry of uranium compounds XVII. Standard molar enthalpy of formation at 298.15 K of dehydrated schoepite UO3 · 0.9H2O. Thermodynamics of (schoepite + dehydrated schoepite + water)☆☆☆

Abstract Solution-calorimetric measurements in HF(aq) have yielded the standard molar enthalpy of formation Δ f H m o (UO 3 · 0.9H 2 O, 298.15 K) = −(1506.3 ± 2.1) kJ · mol −1 . On the basis of an estimated standard molar entropy, S m o (298.15 K) = (125 ± 5) J · K −1 · mol −1 , the standard molar Gibbs energy of formation Δ f G m o (298.15 K) was calculated to be −(1374.4 ± 2.6) kJ · mol −1 . The equilibrium dissociation pressures of the reaction: UO 3 · 2H 2 O(cr, schoepite) = UO 3 · 0.9H 2 O(cr) + 1.1H 2 O(g), are approximately 1.7 and 91 kPa at 298.15 and 373.15 K, respectively. The standard equilibrium constants at 298.15 K for the dissolution of schoepite and dehydrated schoepite in acid are 6.03 × 10 4 and 1.20 × 10 5 , respectively, and are given for several other temperatures.

Lithium nitride (Li3N): standard enthalpy of formation by solution calorimetry

Abstract The enthalpies of reaction of a carefully prepared and characterized specimen of lithium nitride, Li 3 N, with H 2 O and with HCl(aq) were found to be −(581.62±1.42) kJ mol −1 [−(139.01±0.34) kcal th mol −1 ] and −(803.50±1.26) kJ mol −1 [−(192.04±0.30) kcal th mol −1 ], respectively. Enthalpies of solution of LiCl and NH 4 Cl were also measured. Combination of the measured results with auxiliary thermochemical data from the literature yielded values of −(165.14±1.55) kJ mol −1 [−(39.47±0.37) kcal th mol −1 ] and −(164.77±1.51) kJ mol −1 [−(39.38±0.36) kcal th mol −1 ], respectively, for the standard enthalpy of formation, ΔH f o (Li 3 N, c, 298.15 K). A weighted mean value, −(164.93±1.09) kJ mol −1 [−(39.42±0.26) kcal th mol −1 ], is recommended for ΔH f o (Li 3 N, c, 298.15 K). This result differs by about 8 kcal th mol −1 from previous determinations.

Thermochemistry of molybdates IV. Standard enthalpy of formation of lithium molybdate, thermodynamic properties of the aqueous molybdate ion, and thermodynamic stabilities of the alkali-metal molybdates

Abstract The enthalpy changes of the processes: Moo 3 +2 LiOH(aq , 0.2 mol dm −3 = Li 2 MoO 4 ( AQ )+ H 2 O(L) ; δH=−(18.54±0.04) kcal th mol −1 and Li 2 MoO 4 ( c )= Li 2 MoO 4 ( aq ); δH=−(7.61±0.01) kcal th mol −1 , have been determined in a solution calorimeter. These results, when combined with auxiliary thermochemical values, yielded the standard enthalpy of formation, ΔH f o (Li 2 MoO 4 , c, 298.15 K) = −(363.26 ± 0.12) kcal th mol −1 . Based on thermodynamic and solubility data from the literature for sparingly soluble metal molybdates, ΔG f o (MoO 4 2− , aq, 298.15 K) was deduced to be −(200.0 ± 0.3) kcal th mol −1 . This paper also includes estimates of ΔH f o for solid Rb 2 WO 4 and Cs 2 WO 4 , and of the enthalpy of sublimation for Cs 2 WO 4 . It is also pointed out that ΔH f o (PbMoO 4 ) may be in error, and a value of −245 kcal th mol −1 is suggested.

Thermochemistry of uranium compounds VI. Standard enthalpy of formation of cesium diuranate (Cs2U2O7). Thermodynamics of formation of cesium and rubidium uranates at elevated temperatures

Abstract Cesium diuranate, Cs 2 U 2 O 7 , was reacted with 1 mol dm −3 HCl in a solution calorimeter. The enthalpy of reaction, −(46087 ± 102) cal th mol −1 , leads to a standard enthalpy of formation, ΔH f o (Cs 2 U 2 O 7 , c, 298.15 K), of −(754.3 ± 1.6) kcal th mol −1 . Thermodynamics of the Cs + U + O and Rb + U + O systems are discussed with particular emphasis on the formation of cesium and rubidium uranates and diuranates in nuclear reactor fuel. Formation of the monouranates is expected to predominate, although diuranates are also predicted to form, but in rather low concentrations.

Thermodynamic investigation of trisodium uranium(V) oxide (Na3UO4) I. Preparation and enthalpy of formation

Abstract Trisodium uranium(V) oxide was prepared by the reaction between UO 2 and Na 2 O in liquid sodium. The sample was thoroughly characterized by means of chemical, X-ray, and metallographic analyses. Based on a solution calorimetric study, the enthalpy of formation, ΔH f o (Na 3 UO 4 ,c,298.15K), was found to be −(477.7 ± 0.9) kcal th mol −1 .

Standard enthalpy of solution and formation of cesium chromate. Derived thermodynamic properties of the aqueous chromate, bichromate, and dichromate ions, alkali metal and alkaline earth chromates, and lead chromate

Abstract Calorimetric measurements of the enthalpy of solution of cesium chromate gave ΔHsoln = (7622 ± 24) calth mol−1 for a dilution of Cs2CrO4·21128H2O. This result, along with the enthalpy of dilution gave the standard enthalpy of solution, ΔHsolno = (7512 ± 31) calth mol−1, whence the standard enthalpy of formation, ΔHf0(Cs2CrO4, c, 298.15 K), was calculated to be −(341.78 ± 0.46) kcalth mol−1. Recomputed thermodynamic data for the formation of the other alkali metal chromates have been tabulated. From their solubilities and enthalpies of solution, the standard entropies, S0(298 K), of BaCrO4 and PbCrO4 were estimated to be (38.9 ± 0.9) and (43.7 ± 1.2) calth K−1 mol−1, respectively. There is evidence that ΔHf0(SrCrO4, c, 298.15 K) may be in error. Thermochemical, solubility, and equilibrium data, have been combined to update the thermodynamic properties of the aqueous chromate (CrO42−), bichromate (HCrO4−), and dichromate (Cr2O72−) ions. The new values at 298.15 K are as follows: S 0 / cal th K −1 mol −1 ΔH f 0 /kcal th mol −1 ΔG f 0 /kcal th mol −1 CrO42−(aq) (13.8 ± 0.5) −(210.93 ± 0.45) −(174.8 ± 0.5) HCrO4−(aq) (46.6 ± 1.8) −(210.0 ± 0.7) −(183.7 ± 0.5) Cr2O72−(aq) (67.4 ± 3.9) −(356.5 ± 1.5) −(312.8 ± 1.0) For CrO4−(aq), ΔHf0 has been taken directly from the most recent critical compilation of thermochemical data.