Enthalpy increments

The heat capacity of thiazole was determined by adiabatic calorimetry from 5 to 340 K by Goursot and Westrum (295,296). A glass-type transition occurs between 145 and 175°K. Melting occurs at 239.53°K (-33-62°C) with an enthalpy increment of 2292 cal mole and an entropy increment of 9-57 cal mole °K . Table 1-44 summarizes the variations as a function of temperature of the most important thermodynamic properties of thiazole molar heat capacity Cp, standard entropy S°, and Gibbs function - G°-H" )IT. 

Enthalpy of a System The enthalpy increment of a system over the interval of temperature from Tj to T2, under the constraint of constant pressure, is given by the expression ... 

Heat capacities at high temperatures, T > 1000 K, are most accurately determined by drop calorimetry [23, 24], Here a sample is heated to a known temperature and is then dropped into a receiving calorimeter, which is usually operated around room temperature. The calorimeter measures the heat evolved in cooling the sample to the calorimeter temperature. The main sources of error relate to temperature measurement and the attainment of equilibrium in the furnace, to evaluation of heat losses during drop, to the measurements of the heat release in the calorimeter, and to the reproducibility of the initial and final states of the sample. This type of calorimeter is in principle unsurpassed for enthalpy increment determinations of substances with negligible intrinsic or extrinsic defect concentrations... 

Solution An enthalpy increment of 5 kcal mol-1, although modest by chemical standards, is highly significant compared with the ambient RT value of about 0.6 kcal mol-1 near T 300K. If we denote the new binding constant with a prime, and consider only the net enthalpy change A A H° = A Hof - A H° -5 kcal mol-1, we can see from (8.29) that... 

Following the procedure in Liebman, Campbell and Slayden , we derived the new S sec/prim, tert) enthalpy increment, numerically equal to 65 kJ mol-1. 

Enthalpy increment measurements to derive the high-temperature heat capacity have been reported for all lanthanide trifluorides except PmF3, as summarised in table 7. The results of the various authors are compared using the reduced enthalpy increment function ... 

Figure 14 shows a plot of this function for LaF3. It can be seen that the data for the enthalpy increment by Spedding and Henderson (1971) and Lyon et al. (1978) are in good... 

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).

Summary of the enthalpy increment measurements for the lanthanide trifluorides... 

Fig. 16. The reduced enthalpy increment of CidC 13 (in J K fmor1) , Sommers and Westrum Jr. (1977) o,...

Fig. 17. The reduced enthalpy increment of LaCl3 (in J-K 1 -mol 1 ) , Sommers and Westrum Jr. (1976) o, Dworkin and Bredig (1963a) broken line (1), Gaune-Escard et al. (1996) broken line (2), Reuter and Seifert (1994).

The enthalpy increment of the lanthanide trifluorides in the liquid phase has been measured for most compounds (see table 7). The heat capacity has been derived from these results by fitting them to a 2-term equation H°(T) — 7/°(298.15 K) = a(T/K) + b. In... 

The enthalpies of fusion that have been derived from the enthalpy increment equations for the solid and liquid phase are listed in table 13 and the derived entropies of fusion are plotted in fig. 29. It can be seen that the enthalpies and entropies of fusion for ErF3 to LuF3 are significantly lower than those of the other lanthanide trifluorides. Because these four compounds... 

For the liquid lanthanide trihalides data from enthalpy-increment measurements and heat capacity (DSC) measurements are available, as summarised in table C. 1 of Appendix C, the recommended values are given in table 14. The majority of the results have been reported by two... 

Reactions of type (a) for Na, K, Rb, and Cs can attain equilibrium, and those constants were calculated. Calculations are in good agreement with experimentally determined values. No calculations could be carried out for type (b) reactions. Only for Mg(I03)2 have enthalpy increments above 298 K been measured, but in this case S°(298 K) is not known. There are no experimental studies. For reactions of type (c) only the calculation for Ca could be done since no S°(298 K) values for the periodates of Li, Ba, and Sr are available. 

The gef for the iodate was calculated from the 298.15 K values in Wagman et al.,1 and the enthalpy increments determined calorimetrically by David et al.12 Unfortunately, equilibrium constants for the above reaction cannot be calculated, because no S°29815 is available for barium paraperiodate. The density at 293 K is 5.23 gem 3.13... 

This reaction is reversible.16 The temperature at which decomposition occurs depends therefore on the gas phase for example, under vacuum, decomposition becomes noticeable at as low as 500°C. The lack of S° values for this salt prevents any equilibrium calculations for the above reaction, as well as the gef calculation for the paraperiodate. The enthalpy and entropy increments are based on the calorimetric enthalpy increments of David, Mathurin, and Thevenot.12 Thermodynamic data for these salts are given in Tables 9.2 and 9.3. 

Enthalpy increments of LiI03 and Li5IOe have been measured49 up to 623 and 867 K, respectively. The experimental values were used to generate enthalpy and entropy increments for the two salts. Since entropy data are lacking, it was not possible to calculate equilibrium constants for the decomposition reactions. 

Very little information about this salt is available. According to Unezawa et al.,52 the anhydrous salt, prepared by dehydrating the 4-hydrate is stable up to 575°C. Above this temperature it decomposes to the oxide. No thermodynamic data, not even at 298.15 K, are listed in the standard compilations. Enthalpy increments were measured calorimetrically by Sanyai and Nag.14... 

The enthalpy increment for this process is known as the hydrogen atom affinity (HA) and can be obtained from the thermodynamic cycle in equation 10 provided that PA and the ionization potential for the base B are known, by using equation 11,... 

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