Low-temperature heat capacity measurements

The obtained value AHf(PUF3,c) = - 1585.7 + 2.9 kJ.mol-1 cannot be considered as entirely satisfactory as the reliability of the adopted value for the enthalpy of dehydration is not demonstrated. The only experimentally known enthalpies of formation for the actinide trifluorides are AHf(PuF3,c) and AHf(UF3,c) accuracy is therefore essential if these two data are used to estimate the enthalpies of formation of the other trifluorides. The low temperature heat capacity measurements of Osborne et al. (22) using 242PuF3(c) yield S°(PuF3,c 126.11+0.38 J.K"l.mol"i. 

EuRu4Sbn is metallic and becomes ferromagnetic for temperatures below 3.3 K (Takeda and Ishikawa, 2000b). The low temperature saturation moment is about 6.2/xb, 89% of the Eu+2 value. Low temperature heat capacity measurements indicate that the magnetic entropy removed due to magnetic order is also only about 90% of its expected value (Rln8). It is likely that the lanthanide site is not completely filled in this compound although mixed valence behavior can not be ruled out with the available data. 

The standard molar entropies at 298.15 K derived from the low-temperature heat capacity measurements are summarized in table 3. Similar to the heat capacity, the entropy can be described as the sum of the lattice and excess components (Konings, 2001, 2002) ... 

Fig. 14. The reduced enthalpy increment of LaF3 (in J-K 1-mol 1) o, Henderson (1970) Lyon et al. (1978) , value at 298.15 K derived from the low-temperature heat capacity measurements of Lyon et al. (1978).

The Nemst calorimeter is used for low-temperature heat capacity measurements. The sample is contained in a small metal case equipped with a heater and thermometer and is placed in an isoperibol (isothermal) jacket of large heat capacity, which in turn is surrounded by an evacuated chamber surrounded by, for example, a liquid N2 or H2 chamber (Fig. 11.77). A variant is to use an adiabatic jacket. Of course, what is measured is not Cp, but a hopefully reasonable approximation to it ... 

It will be of interest to consider the thermal measurements since this area has been one of major activity in recent years. It is well known that the measurement of heat capacities at low temperatures provides a powerful tool for studying many solid-state phenomena, including the energy separation degeneracy of low lying energy levels in crystalline substances. The advantage of low temperature heat capacity measure-... 

Low temperature heat capacity measurements have been reported by Roberts (1.5-20 K, ), Krier, Craig, and Wallace (12-320 K, 2), Dauphinee, Martin, and Preston-Thomas (20-330 K, 3), Simon and Zeidler (15-300 K, ), and Eastman and Rodebush, (70-290 K, ). A smooth curve was drawn through a large scale plot of the data, giving the most weight to the first three references. Above 300 K, the heat capacity data were adjusted to join smoothly with the enthalpy measurements of Douglas et al. ( ). 

Also considered were the low temperature heat capacities measured by Eastman and Rodebush (j 6, 64-283 K) and Kelley s equations (r ) which are based on the work of Bornemann and Hengstenberg (, 273-873 K). The Eastman and Rodebush data appears to be high above 200 K. At 900 K the enthalpy calculated from Kelley s equation is 4% higher than the adopted value. 

Low temperature heat capacity measurements by Anderson 10) y Bronson and MacHattie 42) y Keesom and van den Ende 176) y and Armstrong and Grayson-Smith 16) were used to calculate an entropy and enthalpy at 298 K. of 13.58 e. u. and 1536 cal./gram atom, respectively. From many sources, Kelley 186) derives an equation for the solid heat capacity above 298 K. Kubaschewski and coworkers 206) select 544.5 K. as the melting point and 2600 50 cal./gram atom for the heat of melting. Data on... 

Farr (109)y of the Tennessee Valley Authority, has compiled a resume of the physical and thermodynamic properties of the allotropic forms of phosphorus. Based on entropy calculations from low temperature heat capacity measurements, Stephenson (318) believes that red crystalline triclinic phosphorus (T.V.A. designation V) is the most stable form at room temperature. This point of view is buttressed by the x-ray work of Roth, DeWitt, and Smith (376). Consequently we have selected red phosphorus V as the reference state up to its sublimation point at 704° K. 

No low temperature heat capacity measurements of H2Se03(cr) have been found. An estimated value of the standard entropy 5°(H2Se03, cr, 298.15 K) = (116.0 14.0) J K -moP is derived below from measurements of Reaction (V.17). 

A third law value of the entropy at 298.15 K., S °(ZnSe, ot, 298.15 K) = (71.94 1.00) J-K -mol, was derived by the review from the low temperature heat capacity measurements in [80S1R/PET] as discussed in Appendix A. The second law entropies are widely scattered, but when obviously poor measurements are neglected, the weighted average given in Table V-50 agrees well with the third law value. The latter value is selected ... 

Table V-61 Determinations of the entropy of a-Ag2Se at 298.15 K. All values were evaluated using the second law except that in [62WAL/ART] which was obtained from low temperature heat capacity measurements. The error limits represent the uncertainties in the linear regressions where applicable.

Summary We have seen how data from experimental studies of equilibria are converted into a readily usable form, that is, into AH, AS and AG values. These data are combined with others, determined, for example, by reaction calorimetry (AH) or by Third Law (as opposed to Second Law ) determinations of entropy, using low-temperature heat capacity measurements. Before we can enter the first division of predictive thermodynamics, and confidently plan processes, we must learn to accept information from two further sources. First, there is a great reservoir of electrochemical expertise which we have not yet tapped. Secondly, we must learn to handle the refinements in free energy formulations, which make allowance for the slight variations of AH and AS with temperature. 

The standard entropy of millerite was determined at 298.15 K by application of low temperature heat capacity measurements [64WEL/KEL],... 

NiS04 7H20) was calculated, and compared to the difference in the values calculated from the low-temperature heat capacity measurements. 

The adiabatic calorimetry heat capacity data were available as supplementary material from the British Library Lending Division (SUP 21075), and were used in the preparation of the current review. The authors integrated the low-temperature heat capacity measurements and reported 138.7 J K -mol for 5 °(Nil2, cr). No contribution was added for the magnetic phase transition at 75 K to 76 K reported by Billerey et al. [77BIL/TER] and Kuindersma et al. [81KU1/SAN], and this may have led to an underestimation of between 0.3 J K mol and 0.5 J K mol in the value of 5 above 80 K. Indeed, the two most relevant values from the authors Run I (for 77.69 K and... 

Robie, R.A., and Hemingway, B.S., 1972, Calorimeters for Heat of Solution and Low-Temperature Heat Capacity Measurements U.S. Geological Survey Prof. Paper 755, 32 pp. 

There are no low-temperature heat capacity measurements, but Venkata Krishnan et al. [2001VEN/NAG] have measured the heat capacity by a DSC technique from ca. 320 to 820 K. They fitted their results to the expression ... 

The data of Blegen [136] were corrected later by Hendry (1977) [139]. Janaf [140] gives a standard entropy value of 112.967 J/mol K whereas the low temperature heat capacity measurements recommended by Koshchenko [94, 124] give 64.2 J/mol K. This value was accepted by Hillert et al. [37]. 

The entropies of most of the lanthanide sesquioxides have been determined by low-temperature heat capacity measurements, principally by Justice and Westrum Jr. [7,33-38]. Recently new measurements for Y2O3 [39] and Ce203 [24] and the first measurements of Pt203 [40] have been reported. The measurements for the lanthanide sesquioxides reveal the presence of Schottky and Kramer components related to electronic stmcture of the lanthanide ions. The trend in the lanthanide sesquioxide series, shown in Figure 7-3, is t3qjical for many lanthanide compounds, as discussed by Konings [41]. The heat capacity, as well as the entropy, can be described by a lattice and an excess component, the latter representing the Schottky... 

The standard entropy for this compound was derived from the low-temperature heat capacity measurements by Teany and Moruzzi [50]. The high-temperature heat content of EuO was determined shortly thereafter by McMasters et al. [51]. The two measurements are in good agreement, and the derived values are included in Tables 7-1 and 7-2. 

There is a large nuclear contribution to flie low-temperature heat capacity. Measurements of the heat capacity were carried out by Dreyfus et al. (1961a) (1.4- K), Lounasmaa (1962b) (0.38-4.2 K), Van Kempen et al. (1964) (0.06-0.71 K), and Krusius et al. (1969) (0.03-0.5 K) neutron transmission measurements by Postma et al. (1962) (0.95 K), Sailor et al. (1962) (0.07-4.225 K), and Brunhart et al. (1965) (0.05—4 K) and NMR measurements by Mackenzie et al. (1974) and McCausland (1976). As summarized in Part 15.10, these measurements were used to determine the magnetic interaction parameter, a, and the quadrupole coupling constant, P, and following the procedure as given in Section 2.4 were then used to determine the variation of the nuclear heat capacity with temperature. 

Low-Temperature Heat Capacity Measurements. Washington DC Government Printing Office, US Geological Survey Prof. Paper 755. 

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