Energy of dissociation

D is the chemical energy of dissociation which cair be obtained from thermodynamic data, aird is the reduced mass of the diatomic molecule... 

The species H2 and H3+ are important as model systems for chemical bonding theory. The hydrogen molecule ion H2+ comprises 2 protons and 1 electron and is extremely unstable even in a low-pressure gas discharge system the energy of dissociation and the intemuclear distance (with the corresponding values for H2 in parentheses) are ... 

The interaction of two alkali metal atoms is to be expected to be similar to that of two hydrogen atoms, for the completed shells of the ions will produce forces similar to the van der Waals forces of a rare gas. The two valence electrons, combined symmetrically, will then be shared between the two ions, the resonance phenomenon producing a molecule-forming attractive force. This is, in fact, observed in band spectra. The normal state of the Na2 molecule, for example, has an energy of dissociation of 1 v.e. (44). The first two excited states are similar, as is to be expected they have dissociation energies of 1.25 and 0.6 v.e. respectively. 

In an atom of the second column of the periodic system, such as mercury, the two valence electrons are in the normal state s-electroiis, and form a completed sub-group. Two such atoms would hence interact in a way similar to two helium atoms the attractive forces would be at most very small. This is the case for Hg2, which in the normal state has an energy of dissociation of only 0.05 v.e. But if one or both of the atoms is excited strong attractive forces can arise and indeed the excited states of Hg2 are found to have energies of dissociation of about 1 v.e. 

From such crude data as are to be found in the literature we can calculate approximate values of the equilibrium constants, and hence of the free energies of dissociation for the various hexaarylethanes. From our quantum-mechanical treatment, on the other hand, we obtain only the heats of dissociation, for which, except in the single case of hexaphenylethane, we have no experimental data. Thus, in order that we may compare our results with those of experiment, we must make the plausible assumption that the entropies of dissociation vary only slightly from ethane to ethane. Then at a given temperature the heats of dissociation run parallel to the free energies and can be used instead of the latter in predicting the relative degrees of dissociation of the different molecules. 

The experiments with a beam of silver particles were conducted at room temperature. The energy of dissociation of diatomic molecules of silver is 1.78 eV, the heat of evaporation of silver molecules is 95 kcal/mol [46], and the heat of evaporation of an uniatomic silver is 64 kcal/mol. Mass-spectrometric studies [46] of silver vapour above a metallic silver showed that the ratio of number densities of ions Ag /Ag2 is equal to two. In other studies [47], a considerably larger value of this ratio was found. At 1037 - 1147 C molecular mass of silver particles in vapours was found to be 278 90 [46], i.e., an average number of atoms in a molecule of silver is 2.56. 

How do matters stand with radical addition A comparison of Ee0 and re with the energy of dissociation of the resulting bond De showed that this influence certainly does exist [41-46]. The parameters Ee0 and re are juxtaposed with the dissociation energy of the bond formed Z>e(X—C) in Table 6.21. For reactions of one class, namely, X + CH2=CHY, the linear correlation Ee0 = const x [De(X—C)]2 holds const = 5.95 x 10 4mol kJ 1. The following linear correlation was found to be fulfilled for the reactions of all the 13 classes considered (see Figure 6.6) ... 

Figure 4.103 Incremental CO binding energies, Eq. (4.129), for M(CO) complexes of group 6 metals Cr (circles), Mo (squares), and W (triangles). For n = 3, the quantity plotted is the average energy of dissociating all three carbonyls from M(CO)3.

Theoretical considerations show that the free energy of dissociation of an acid in water, and hence the dissociation constant, is governed by the algebraic sum of the free energies for the solution of the undissociated acid in water, for vaporisation of the acid, for the formation of a free proton and an anion from the molecule of acid in the gas phase, and for hydration of the proton and anion. Thus the true acidity, given by the third of these... 

It is seen from the above that the energy of the molecule in the excited state must be enough to provide for the energy of dissociation, the kinetic energy of the atoms relative to each other and for the energy which is radiated as continuous spectrum. That a similar process occurs in absorption has been shown by Dieke and Hopfield. The work of Witmer on the analysis of bands in the Lyman region gives as the heat of dissociation a value of 4.34 volts and that found by Dieke and Hopfield was 4.38 volts. 

Spectroscopic energy of dissociation of a diatomic molecule in the Morse equation... 

The results of elctron-impact studies of phosphine by Halmann et al. are given in Table 3a. The authors used the appearance potentials, in conjunction with thermochemical data, to choose the probable reaction processes. In many simple cases the observed appearance potential A (Z) for an ion fragment Z from a molecule RZ is related to its ionisation potential 7(Z) and to the energy of dissociation 7)(R—Z) of the bond by the expression A (Z) = /(Z) + D (R—Z). This assumes that the dissociation products are formed with little, if any, excitation energy, and that /(Z) < /(R). The most abundant ion species in the usual mass spectrum of phosphine is PH, which is probably formed according to the following mechanism... 

The average energy of dissociation of the P-H-bond is known from thermochemical measurements, (P—H) = 3.35 eV. The dissociation energy of the hydrogen molecule is/)(H—H) = 4.48 eV. The appearance potential for PH formed according to the mechanism... 

Atomization Energy. The energy of dissociation of an atom or molecule into separated nuclei and electrons. Atomization energy is the quantity which is calculated in G2 and G3 Models. 

---