Apparent enthalpy of formation

For the calculation of the enthalpies of formation of the tribromides and triiodides, the same reaction cycles were used as for the trichlorides. However, as the halide ion in the compounds are different from those in the solution (e.g., Lnh in HCl(aq)) the calculation of ArH2 becomes a bit more complex as we have to deal with the ternary system H2O-HCI-HX. In that case, it is assumed that the apparent enthalpy of formation of HI and HBr in HC1 solutions are the same as in HBr and HI solutions of the same molality. 

Finally, a series of values for the apparent enthalpy of formation of g-phase Pd hydride, along with the bulk Pd hydride ratios, is listed in Table II. The bulk ratios were obtained by dividing the hydrogen uptake at 75 torr from the second isotherm by the number of bulk Pd atoms, i.e., the total number of Pd atoms minus the surface Pd atoms, as determined by chemisorbed hydrogen. 

To calculate the apparent enthalpy of formation at elevated temperatures, we need the a, b, c values only for the compound of interest and not for its elements. Thus... 

Calculate the apparent enthalpy of formation from the elements of halite at 1000 K. 

In Figure 8.1, we show that H2O at 693.5 bars, 600°C (point D) has the same apparent enthalpy of formation (-232426 J mol ) as at 2000 bars, 700°C (point A), so these two points are joined by an isenthalp. The work done in this case is... 

Another example illustrating the apparent problems with the enthalpy-of-formation data for some tin compounds concerns the term (Sn—Sn). In order to avoid the complication related to the term (Sn—Cb), the bond enthalpy (Sn—Sn) in Table 4 was calculated from A/ °[(SnMe3)2, g]. As a confirmation, one can then derive the value for the same term from A ff °[(SnEt3)2, g]—they should be similar. Yet, this calculation yields (Sn—Sn) = 223.18 kJmol-1, a value which looks unreasonably high and questions the experimental value of A °[(SnEt3)2, g] in Table 3. 

To apply this equation to calculate the difference between the standard enthalpies of formation of AB and AB+, we need to know the values of Af//°(e ,g), A, and A. The heat capacities of AB and AB+ are easily evaluated lfom statistical mechanics calculations, provided that their structures and vibrational frequencies are available. Usually, A 0. However, with regard to AfH°(e, g) and A, the apparently simple task of assigning their values has been a source of controversy involving two scientific communities the calorimetrists and the mass spectrometrists. 

For the tertiary amines, the desired exchange values are available from experiment only for R = R1 = R2 = Me and R = R1 = R2 = Et. The gaseous enthalpy of formation for the hydrocarbon corresponding to tri-n-propyl amine has not been measured, but it may be reliably estimated17 as —251.0 kJmol-1. A derived 8(,(tert/n-Pr, n-Pr, n-Pr) is ca —90 kJmol-1. Because Ss(tert/Et, Et, Et) = —97 kJmol-1, it is apparent that the exchange quantities for tertiary amines are not constant, as was surmised from the slopes reported in Table 1. Most of the derivations involving tertiary amines in later sections are based on an ethyl or propyl substructure and so an intermediate value of —93 kJmol-1 is recommended. 

The success of earlier empirical schemes in predicting the sign and magnitude of the enthalpy of formation can in part be attributed to making predictions for AB compounds. This can hide asymmetrical effects which are more apparent when other stoichiometries are considered. The empirical and semi-empirical schemes do... 

We close this section with a statement of surprise and disappointment. For no apparent reason there are no published enthalpy of formation measurements for 1-hydroxy-2,2,6,6-tetramethylpiperidine, the parent species for the above 1-hydroxylated piperidines and the hydroxylamine counterpart to probably the most famous nitroxide radical, TEMPO . 

The enthalpy of formation of solid peroxybenzoic acid was determined by a combination of combustion and reaction calorimetry. The enthalpy of reaction 14 is calculated to be —18.2 kJmol. This result does not seem credible. There is no reason to doubt the enthalpy of formation of benzoic acid, and there is no apparent reason why the enthalpy of reaction should be so different from the aliphatic peracids. 

An additional problem with the enthalpies of formation of the two tetroxane species is irreconcilable. The —288 kJmoU difference between the enthalpies of formation of the parent and diphenylated species is much too large. After all, the difference between the enthalpies of formation of 1,3-dioxolane and its 2-phenyl derivative is —93 kJmoU and doubling this value for two substituents is but —186 kJmor. No explanation for this discrepancy is apparent. 

The aphorism about scientific data there s more than you think and less than you need provides both an excuse and a challenge for the would be archivist of data. Personal experience has shown it perilously easy to miss relevant thermochemical data. Measurements of enthalpies of formation apparently lie fallow in conference abstracts and in patents and are scatterred throughout papers officially on synthesis, structure, mechanisms and reactivity. Even computational theoretical papers have been known to contain new thermochemical data ... 

We consider this plausible assumption somewhat suspect. Admitting ignorance as to the difference in solvation energies of the two cyclic olefins and the methylcycloalkane, we note that summing the enthalpies of hydrogenation and formation of liquid methylenecyclobutane results in a value of the enthalpy of formation of liquid methylcyclobutane of ca —29 kJ mol-1, in marked contrast to the archival value of —44.5 1.4 kJ mol-1. The source of the discrepancy is not apparent. 

As an example, let us pose the question why does BF3 adopt a molecular structure, while A1F3 is apparently ionic As shown in Table 5.2, the ionic model (using the Kapustinskii equation) gives a fair approximation to the thermochemistry of formation of A1F3. Let us estimate the enthalpy of formation of a hypothetical ionic substance BF3(s), having a structure similar to that of A1F3. The lattice energy can be estimated by means of the Kapustinskii equation. We require the... 

Consider now the enthalpies of formation of our diverse indane heterocycles and corresponding one-ring species formed by de-benzoannelation (Table 1). De-benzoannelation resulting in aromatic one-ring species is generally more favorable than processes involving non- or antiaromatic species. The enthalpy of formation difference for aromatic species is typically 50-60 kJ mol-1, for nonaromatic species typically ca. 30 kJ mol-1, and for antiaromatic species ca. 24 kJ mol-1. The difference of the enthalpies of formation between the benzoannelated and one-ring dithiol-2-thiones is —11 kJ mol-1. No explanation for this discrepancy is apparent. 

---