Second Law values

Values of A°(T) were obtained by using simulant elements which have the same number of bonding electrons as each intermetallic phase. Second law values of AH (298) were also obtained by use of heat capacities of the same simulant and constituent... 

The appropriate second-law values for the sublimation of condensed EuCla are ... 

The standard entropy of EuC12(s), S298, computed from the experimental second-law values and S298 EuCl2(g) is 32.9 2.4eu by use of the third-law thermal functions. 

If all the experimental data were perfect, the second-law value of AH calcrrlated from Eq. (2) and the third-law value calculated from Eqs. (4), (5), and (7) would be identical. However, in this case, the third-law result should be the more reliable, since it is difficult to determine dpIdT with very high accuracy. 

The second law values of the eat of reaction for the reactions (1), (2), (3) and I (A) are AH = A.8 3.0 kcal/mol, q AH = 15.6 2.2 kcal/mol, AH° = 12.6 ll.A kcal/mol and AH = 2.1 1.5 kcal/mol respectively. These values agree well with the third law values within the limits of error. However, the third law values are much more reliable than those from the second law. Therefore only the third law values have been used for the evaluation of the atomization energy of Li2H2 ... 

Let us apply the simplest form of van t Hoff s equation to the data from various distributions of E given in Table 5.4. Notice that if Eq. 5.72 is valid, then the ratio of the equilibrium constants equals that of the desorption energy factors calculated from 5.71. Then the Second Law value of the effective jet would come from ... 

The third law evaluations made in this review reduce the scatter of the enthalpy values by more than 50%. The scatter of the values obtained from the second law is an effect of the small temperature ranges used in most of the studies. The second law value in [75HOA/REY] for the equilibrium with liquid selenium is not included in the average in Table V-12 because it stems from measurements which result in an enthalpy of fusion of selenium that is twice as large as the correct value. 

The difference between the average of the second law values and those calculated from molecular data is not large, but the second law values are widely scattered due to the small temperature ranges of the measurements. The entropy value in [84PUP/RUS] is selected since it is the result of the best documented molecular calculations,... 

Second law values of the enthalpy of formation calculated as discussed in Appendix A from the electrochemical measurements in [60FIN/WAG], [66SAD/SEM], and [73SHA/MIS] are tabulated in Table V-39 together with the results of direct synthesis calorimetry investigations made by Hajiev [70HAJ] and Boone and Kleppa [92BOO/KLE]. 

Second law entropies were calculated by the review as discussed in Appendix A from the evaporation studies summarised in Table V-40. The second laws values agree well with the value calculated from molecular data. 

Table V-62 Re-evaluated enthalpy of formation of a-Ag2Se at 298.15 K obtained from high temperature measurements using (a) the second and (b) the third law. The error limits of the second law values represent the uncertainties in the linear regressions.

Table V-66 The standard entropy of RuSe2(cr) determined in different investigations. The second law values were derived by the review in evaluations of experimental data given in the references.

The sublimation thermodynamics of EuSe was studied in the temperature range 1808 to 2131 K by Hariharan and Eick [74HAR/EIC] using mass spectrometry and Knudsen cells. The evaporation reaction is EuSe(cr) Eu(g) + Se(g). The second law value... 

Both second and third law values of the enthalpy of reaction were determined using two different detection techniques, electrometer analogue and ion counting for Reaction (V.57) whereas only the former was used for Reaction (V.58). For Reaction (V.57), the third law reaction data were significantly more consistent than the second law values and, as such, were selected by [78KLE/CUB]. The values selected for the enthalpy of reaction were (136.4 4.6) and (332.6 4.6) kJ-moP for Reactions (V.57) and (V.58), respectively. The enthalpy of formation calculated from the two reactions, utilising the enthalpy of formation of Zrl4(g) and the selected auxiliary enthalpy of... 

The authors reported an extensive set of torsion balance and Knudsen cell measurements for the determination of the enthalpy of sublimation of Nip2 between 950 and 1250 K. Third law and second law values are in much better agreement if the values of (G (7 )-//° (298.15 K))/r for the sublimation reaction for CoF2(cr) are substituted for those in the literature for NiF2(cr). The work appears to have been carefully carried out, and if the interpretation were correct, this would indicate that (a) 5 "(Ni, cr,... 

The selected values, which are based on the second-law values, are thus ... 

With these data, we have four semi-independent measurements to define the three enthalpies of formation of ThBr(g), ThBr2(g), ThBr3(g), as shown in Table VllI-24. We have used a least-square analysis to find the optimal solutions to the over-determined set of linear equations, with the results shown in Table Vlll-25. These second law values, in fact, form a veiy consistent set, and are the basis for the selected values ... 

The standard enthalpy of formation, AH ), and the dissociation energy, D, are the two thermodynamic properties which have been deduced from the experimental results. The total mass effusion rates were combined with the vapor phase compositions derived from the mass spectrometric measurements to obtain the change in the enthalpies of the reaction (1). The second law values corresponded to the mean temperatures of measurements. The third law values corresponded to the standard temperature of either 298.15 K or OK. 

The dissociation energy D (in kJ/mol) was extracted from Knudsen-effusion mass spec-trometric measurements at temperatures above 2456 K Dq = 358 + 15 (3.71 0.16 eV) and 298 = 361 + 15. Dg is an average of a Third-Law value (based on assumptions shown in the first column of the table above), Dg=354.8+8.6, and a value derived from AHg for PtY + Pt Pt2+Y and Dg(PtY), Dg=360.3 +11.1. A Second-Law value, Dg=357.9 + 59.0 was in fortuitously good agreement with the former data [35]. The Third-Law value should, however, be decreased in view of an electronic contribution to the partition function [40] see also [41] and a more recent electronic structure calculation [9]. 

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