Enthalpy predictions

Figure 2.7. The relation, suggested by de Boer etal. (1988), between the average number of stable intermediate phases in a binary system and the formation enthalpy predicted for the equiatomic composition.

The potential existence of these interrelationships has considerable significance both from a practical and theoretical standpoint. With them, it is possible to measure the O—H frequency shifts of any new hydroxyl compound with the three donors in Fig. 8 and locate these frequencies on the three straight constant base lines. The points can be connected to produce the constant-acid line whose slope and intercept can then be determined. The enthalpy of interaction of this acid with all the donors in the phenol correlation can then be predicted from the measured frequency shift for these donors and the constant acid line. By use of the E and C correlation, the internal consistency of the enthalpies predicted from the frequency shifts can be checked because the three enthalpies from the constant base line give three heats for the determination of the Ea and Ca parameters of the new add. 

Summing the experimentally determined solid-phase enthalpies of formation of the olefin and the sublimation enthalpies predicted by equation 23 results in the desired gas-phase quantities of 358 and 473 kJmol-1 for the enthalpies of formation of tri- and tetraphenylethylene. Combined with the above enthalpies of formation of ethylene, styrene and ( )-stilbene, our new numbers allow us to generate a reasonably straight line as a function of the number of phenyl groups ... 

Tel. 203-432-6278, fax 203-432-6144, e-mail bill doctor.chem.yale.edu Computer-Assisted Mechanistic Evaluation of Organic reactions and prediction of products. pK and reaction enthalpy predictions. VAX. 

Based on changes in entropy and enthalpy, predict whether this reaction is spontaneous or nonspontaneous ... 

Another obvious contributor to entropy is the steepness of the conformational energy minimum. Vibrational energy levels are sparser in a steep-sided energy well than in a broad flat-bottomed one. Calculations certainly suggest that potential minima do have different shapes, so the entropy predictions of the calculations should be given attention comparable to the enthalpy predictions. 

Equation (8.37) is a practical working relationship for the infinite dilution aqueous molecular component partial molar enthalpy as a function of temperature. There is, however, an underlying assumption made. The relationship derived above assumes equilibrium between liquid and vapor, or, in other words a saturated liquid-vapor situation. In a subcooled liquid the enthalpy predicted by equation (8.37) should be corrected for the enthalpy difference between the pure component at the prevailing pressure and the saturation pressure. 

Phase equilibria calculations from both an Orye-type BWR and the Soave Redlich-Kwong equation of state were compared with data from an LNG plant. Both correlations showed good comparisons with the plant data and experimental data measured by P-V-T, Inc. The enthalpies predicted using the Soave Redlich-Kwong equation of state were poorer than those predicted using the Lee-Kesler correlation. 

It can be seen that the results of SVR computation are rather satisfactory. Moreover, it has been found by SVR computation that the inclusion of radius ratio of metallic atoms significantly improves the accuracy of enthalpy prediction. A reasonable explanation of this fact is that the rare earth-transition metal compounds listed here do not have the... 

A measure of the quality of the results can be obtained by comparing the AHj values in O Tables 13-8 and O 13-9. Even for the aug-cc-pV(Q+d)Z basis set, there is a 2.9 kcal mol" difference, which is not within chemical accuracy, of course. However, the corresponding extrapolated values in the two tables differ by less than 1 kcal mol , which is within chemical accuracy. Our best results for AH°, then, are 1-2 kcal mol above Bensons mean value and in the range of values obtained by Wang and Zhang by isodesmic reactions, rather than their estimates based on dissociation reactions. Our reaction enthalpies predict that decomposition of S(0H)2 to SO2 + H2 is slightly exothermic, while decomposition to H2S + O2 is strongly endothermic. 

Before leaving this subject, we want to also calculate the transition enthalpy predicted by our model. We estimate the pressure corresponding to the circle in Fig.5.15 viaEq.(5.148) setting z = 1, i.e. 

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