Energy heat of atomization

Fig. 4. Trends in cohesive energies (heats of atomization) for alkali metals (column lA) and tetravalent nontransition metalloids (column IVB) of the periodic table. The abscissa is the mean inter-electronic spacing of the valence electrons expressed in units of the Bohr radius aQ.

T. W. Richards, The Significance of Changing Atomic Volume 111. The Relation of Changing Heat Capacity to Change of Free Energy. Heat of Reaction. Change of Volume, and Chemical Affinity". Z. Physik. Chem. 42. 129-154 (1902). 

The D values may be easy or difficult to measure, and they can be estimated by various techniques, but there is no question as to what they mean. With E values the matter is not so simple. For methane, the total energy of conversion from CH4 to C + 4 H (at 0 K) is 393 kcal mol (1644 kJ mol ). " Consequently, E for the C—H bond in methane is 98 kcal mol (411 kJ mol ) at OK. The more usual practice, though, is not to measure the heat of atomization (i.e., the energy necessary to convert a compound to its atoms) directly but to calculate it from the heat of combustion. Such a calculation is shown in Figure 1.11. 

Heats of combustion are very accurately known for hydrocarbons. For methane the value at 25°C is 212.8 kcal mol (890.4 kJ mol ), which leads to a heat of atomization of 398.0 kcal mol (1665 kJ mol ) or a value of for the C—H bond at 25°C of 99.5 kcal mol (416 kJ mol ). This method is fine for molecules like methane in which all the bonds are equivalent, but for more complicated molecules assumptions must be made. Thus for ethane, the heat of atomization at 25°C is 676.1 kcal mol or 2829 kJ mol (Fig. 1.11), and we must decide how much of this energy is due to the C—C bond and how much to the six C—H bonds. Any assumption must be artificial, since there is no way of actually obtaining this information, and indeed the question has no real meaning. If we make the... 

Energy of individual atoms Heat of atomization of real molecule... 

We define here Wlh and W2h theories, respectively, as the modifications of Wl theory for which AVnZ basis sets are only used on elements of groups V, VI, VII, and VIII, but regular VnZ basis sets on groups I, II, III, and IV. (The h stands for heteroatom , as we originally investigated this for organic molecules.) For the purpose of the present paper, we have repeated the validation calculations described in the previous section for Wlh and W2h theories. (For about half of the systems, Wl and Wlh are trivially equivalent.) Some representative results can be found in Table 2.1 for atomization energies/heats of formation, and in Table 2.2 for ionization potentials and electron affinities. 

The Dewar resonance energy (DRE) is found as the difference between the heats of atomization of a given conjugated molecule and the classical Kekule reference structure... 

Group IIB elements bond energies of, 11 316 heats of atomization, 11 313 ionization potentials, 11 310, 311 valence state promotion energies, 11 311, 312... 

Chain Reactions in]. Nevertheless, the energy evolved depends only on the initial and final states and not on intermediate ones. Once the reaction is completed, the net heat evolved is exactly the same as if the reactant molecules were first dissociated into their atoms, and then reacted directly to form the final products (Hess Law). If a compd be formed directly from the atoms, the heat of atomization (Qa.) which was required to generate them from the molecules... 

A second procedure makes use of heats of atomization. The heat of atomization of carbon is the energy of converting graphite to carbon atoms. With diatomic molecules such as hydrogen, nitrogen, oxygen, and fluorine, it is the energy required to convert them into atoms. The heats of atomization for most elements in their normal form are known from experimental data. ... 

The heat of atomization of a compound may also be derived from the calculated energy and the calculated heats of atomization of the elements. The difference between the heats of atomization of the compound and that of its constituent elements gives the heat of formation of the compound. 

We assume that we have a solid metal M which reacts with a diatomic, gaseous nonmetal X2 (e.g., CI2, F2, 02). Similar cycles can be written for solid elements such as sulfur as the nonmetal. In either case, before we can connect U with AHf we must form gaseous ions of M and X. We need not only the relevant ionization potentials (IP) and electron affinities (EA), but also the heats of atomization of solid M and gaseous X2. These atomization energies are traditionally referred to as heats of sublimation AHsuh of M(s) and of dissociation AHd ss of X2. For NaCl itself, we have... 

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