Elements dissociation energy

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

FIGURE 2.19 The dissociation energies for bonds between hydrogen and the p-block elements. The bond strengths decrease down each group as the atoms increase in size. 

The effect of TOS on the product distribution during the pyrolysis of R22 over CU-AIF3 catalyst is shown in Fig. 3. The amoimt of halogen ion trapped in NaOH solution was determined by IC. The concentration of Cl formed during the pyrolysis of R22 was higher than the concentration of F at all TOS. This result is a consequence of the facile cleavage of the C-Cl bond in comparison to the C-F bond. Bond dissociation energy for the C-element of R22 is followed by the order C-C1[Pg.235]

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

The dissociation energy for C-H bond in methane (E = 436 kj/mol) is one of the highest among all organic compounds. Its electronic structure (i.e., the lack of n- and n-electrons), lack of polarity, and any functional group makes it extremely difficult to thermally decompose the methane molecule into its constituent elements. 

Element E Ionization energy E2 bond dissociation energy Spin-orbit 3P2 3Po... 

An important question concerns the kind of relationships that exist between the dissociation energy, Dq, and various other molecular properties. What are the main factors responsible for the chemical bonding How does Dq depend upon the charge on the heavy atom Does the HOMO-LUMO interaction of the fragments determine the magnitude of Dq And what is the role of d orbitals on higher elements ... 

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