Atomic heat of formation

ATOMIC HEAT OF FORMATION. Of a substance, tine difference between the enthalpy of one mole of that substance and the sum of the enthalpies of its constituent atoms at the same temperature the reference state for the atoms is chosen as the gaseous state, The atomic heat of formation at 0 K is equal to the sum of all the bond energies of the molecule, or to the sum of all the dissociation energies involved in any scheme of step-by-step complete dissociation of the molecule. 

It has been suggested thal AHjjw, the heal of formation or a molecule from its constituent atoms (see also Atomic Heat of Formation), could be computed from a tabic of average bond energies, and ihe assumption of additivity ... 

Group increments for silicon compounds based on the data reviewed here, and derived in accordance with the group additivity scheme29-31, are shown in Table 27. Radical and atomic heats of formation for non-silicon-containing species used in this chapter to derive bond dissociation energies are shown in Table 28. Miscellaneous inorganic silane heats of formation are included in Table 29. 

This is very close to our atomization heat of formation value above (—195.7 kJ mol x). 

For the atomic heats of formation, experimental data are taken, while the energy to remove all valence electrons also depends on parameters of the semiempirical model. In Equation 6.34, the electronic energies of the atoms, EelA, have to be calculated in their corresponding ground states. 

An important aspect of fluorine thermochemistry is the effect of increasing a-fluorine substitution on the C—F bond dissociation energy (BDE). This geminal stabilization is illustrated by the C—F BDE in the fluorinated methanes—the CHF series with associated C—F and C—H BDEs given below (for radical heats of formation used see Table 10, for halon atom heats of formation see Table 8) ... 

In view of the instability of the trioxide it is interesting to speculate on the stability of the oxyfluorides. Xenon oxide tetrafluoride should be an exothermic compound since the atomic heat of formation using average bond energies of 32 kcal. and 17 kcal. respectively for Xe-F and Xe-O is —160 kcal. mole-. The combined heat of atomization of four fluorine atoms (4 x 18.3 kcal.) and one oxygen atom (59.2 kcals.), 132.4 kcal., is less than this. Xenon dioxide difluoride, by such considerations, should be endothermic. It is of interest that xenon oxide tetrafluoride is easily made by controlled hydrolysis of xenon hexafluoride and is a thermally stable entity (m.p. —28) whereas xenon dioxide difluoride has been observed only mass spectrometrically < >. 

All quantum-chemical methods, even if they are considered as derived from first-principles, make use of at least the first group of parameters. Examples are the atomic masses and atomic heats of formation. Atomic orbital exponents used in all LCAO-based methods are an example of the second group. 

This is very close to our atomization heat of formation value above (-195.9 kJ mol" ), and a little more negative than the experimental value (-190.7 or -189.8kJmol" [128]). 

Thus the desired quantity, the heat of formation of the molecule, can be calculated from the experimental heats of formation of the atoms and the semiempirical energies of the atoms and the molecule. The calculation of Eq. (6.13) is automatically done by the program using stored values for atomic heats of formation and semiempirical atomic energies, and the freshly calculated calculated molecular energy, and one normally never sees These calculations are for the gas phase, and if one wants the... 

There is little doubt that direct experimental determination of the heats of atomization, heats of formation of compounds, free energies, and other thermodynamic properties of crystals can provide the basis for theoretical calculations of the bond energies, which are used to establish the nature of bonds. 

The heat of formation is calculated by subtracting atomic heats of formation from the binding energy ... 

The heat of formation is the energy released as heat when atoms situated at theoretically infinite distance approach, bind, and form the molecule of interest. The core includes, by definition, the atomic nucleus and the electrons that do not participate in chemical bonds, that is, the nonvalence electrons. The semiempirical method PM6 estimates the heat of formation as the sum of the total repulsion energy of the cores and the total heat of formation of the atoms. Each semiempirical quantum mechanics method calculates, in its own manner, the energy of repulsion of the cores and utilizes a different set of values for the atomic heat of formation. Consequently, the values of the heat of formation determined for the same molecule by different semiempirical... 

Heats of combustion have been used to calculate empirical resonance energies in another way, of theoretical interest. The method is illustrated by reference to a compound CaH Nc. Consideration of the equations set out below leads to the relationship shown between d//c, the heat of combustion, and AHfay the atomic heat of formation. 

With a resonance hybrid such as pyridine, the atomic heat of formation is given by... 

---