Enthalpies of formation derivatives

Table 15 shows the new values of bond dissociation energies and radical enthalpies of formation derived using the relationship... 

Two examples illustrate this point. The enthalpy of formation of gaseous 1-heptene was determined from the enthalpy of hydrogenation to be —62.6 1.6 kJmol-1. The enthalpies of combustion and of vapourization (35.6 0.2 k.I mol 1) of the liquid were later measured, from which the enthalpy of formation of the gas was determined to be — 62.3 1.0 kJ mol-1. Yet Ped-ley did not incorporate the former measurement in the selected value although it is listed among the literature cited. This choice of experimental values is in contrast to a selected composite enthalpy of formation for gaseous 1-pentene which includes an enthalpy of formation derived from the enthalpies of combustion and vapourization of the liquid (—21.5 0.8 kJmol-1) and an enthalpy of formation derived from the equilibrium isomerization of 1-pentene and trails-2-pentene (—21.0 1.4 kJ mol-1). 

The enthalpy of formation derived from Johnston s measurements is in good agreement with the adopted value but is not averaged in because, as indicated by Kondakov et al. (2), the measurements are believed less accurate. 

Halstead and Moore (9) and of Tamaru and Shiomi (1 ) are in good agreement, but the pressures reported by Johnston (lA) are too low due to failure to reach equilibrium. The enthalpy of formation derived from 3rd law a H of Halstead and Moore (9) or Tamaru and Shiomi (1 ) is in good agreement with the value adopted. However, the decomposition of Ca(0H>2 may yield non-standard state CaO in the final product which was shown in a similar decomposition of Mg(0H) . See Mg(0H) table ( ) for details. 

Westrum and Pitzer (3) measured the dissociation vapor pressures of KHFj (cr, t) by a static method. Their pressure data have been analyzed by both 2nd law and 3rd law methods as given below. The enthalpy of formation derived from the 3rd law... 

Enthalpies of formation derived from the enthalpy changes of different chemical reactions are tabulated below, of sample characterization we can only assume from sample preparation that the following reactions Involve o-PbF (cr... 

We choose the enthalpy of formation derived from the result of Torgeson et al. (J ) which is considered to be better since it relates directly to the oxides. Also the AjG (MgSi02, t, 1873 K) thus calculated is in very good agreement with that derived from the activities of SiOg and MgO (in the Si02 - MgO system) measured by Rein et al. (6). The A H (MgSiO, cr, 298.15 K) derived from the work of Reesman et al. is less accurate and involves uncertainties in the auxiliary data used to calculate the value. 

Infrared and NMR data are included In Table 2. These data and the estimates of the enthalpy of formation derived therefore indicate relatively strong hydrogen bonding (AH = -6 to -8 kcal/mol). 

For the /V-clhyl isomer there are archival4 enthalpies of formation (derived from the calculated results presented in Reference 26). There is a much more ancient23, and very disparate, value of —29 k.l mol 1 for the liquid, which is nearly identical to that experimentally determined in Reference 20 and reported in Reference 19. A contemporary measurement24 reports somewhat more endothermic enthalpies of formation. Our predilection would be to accept the latter measurement. From a recent measurement28 of the enthalpy of formation of /V - e l h y I - 3 - rn c l h y I an i I i n c, we find values of —29.5 2.3 and 30.5 3.8 k.l mol for the liquid and gaseous species, respectively. We would expect reaction 7 to be roughly thermoneutral. 

There are surprisingly few alkoxyanilines for which thermochemical data seemingly exist. The simplest compounds are 2- and 4-methoxyaniline, known also as o- and p-anisidine. For the former compound, Reference 81 reports an enthalpy of formation of —46 kJmol-1 for the solid. For the latter, Reference 5 chronicles a value of —172 kJ mol-1, also for the solid. These two values are incomprehensively incompatible with each other. They are also incompatible with an enthalpy of formation derived from equation 2, ca —132 kJmol-1 (using—114.8 kJmol-1 for the enthalpy of formation of liquid anisole). 

The sole other arenediol we know of, phenanthrene-9,10-diol, has an enthalpy of formation derived from the ancient calorimetric resnlts in Reference 88. Acknowledging that there has been some dispute about the enthalpy of formation of the parent hydrocarbon, we adopt the value of 113.0 2.1 kJ moU for phenanthrene . The estimated enthalpy of formation of the phenanthrenediol, based on twice the OH/H increment exchange energy, is —293.8 kJ moU, ca 50 kJ moU more negative than the actual measurement. The apparent destabilization of the real diol is considerably greater than that for the related vicinally dihydroxylated naphthalenes or for catechol. 

The total vapour pressure of liquid selenium at equilibrium was measured by a static method in the temperature range 673 to 958 K and given as log o (/ /bar) = 5.217 - 4991 T. A third law evaluation of the enthalpy of formation of Se2(g) at 298.15 was made by this review using the vapour pressure at 940 K and the selected data for the heat capacities and entropies of Se(l) and Se2(g). A mole fraction of Sc2(g), - sej(g) 0.63 was estimated from the selected selenium data. The enthalpy of formation derived... 

New density and total pressure measurements of selenium vapour are presented. A previous investigation by the same author, [67RAU], is re-evaluated and combined with the new data in an evaluation of the thermodynamic properties of the molecules Se2(g)-Seg(g). The enthalpies of formation and entropies were varied in a procedure to obtain a good fit to the observed densities. The procedure contains many parameters that can be varied and different sets may fit the experimental data almost equally well. The thermodynamic properties derived for the species Se2(g)-Seg(g) are too dependent on the choices made by the author in order to be considered by the review. However, the new measurements of the total vapour pressure in the temperature range 1073 to 1373 K were evaluated by the review for the thermodynamic properties of Sc2(g) using the third law and mole fractions of Se2(g) estimated from the selected selenium data. The enthalpy of formation derived was Af//°(Se2, g, 298.15 K) = (138.9 1.9) kJmoP. ... 

Table A-107 Atomisation energies and enthalpies of formation derived from measurements in [78SMO/PAT].

Table A-109 Enthalpies of formation derived by the review from the measurements in [79SUL/PRU].

As individuals who have used enthalpies of formation derived from the analysis of appearance energy measurements, we do not particularly appreciate this prejudice nor can we give one key reference that either legitimizes nor invalidates the use of thermochemical data from this source. 

Therefore this review selects the following standard enthalpy of formation, derived from the data in [1953EYR/WES] ... 

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