Covalent bond energy

There is a difference in the bond energies that are associated with the different types of bonds within the crosslinks and the main-chain carbon-carbon bonds that exist within the rubber molecules themselves. These bond energies are shown in Table 4.3 [8]. 

Bond type Bond energies (kj/mol) Location in crosslinked rubber 

From the above, it is apparent that carbon-sulfur and sulfur-sulfur bonds have lower dissociation energies, and so will break more easily than the main-chain carbon-carbon bonds in the rubber molecules when heat is applied to the crosslinked rubber. 

---

The total stabilization energy of a cluster rarely exceeds 25 kcal mol , i.e., a small fraction of a strong covalent bond energy (ca. 100 kcal mol ). Its partitioning into electrostatic, induction, and dispersion terms differs from cluster to cluster. In some cases, one particular energy term is dominant. More typically, many attractive terms contribute to the overall stabilization of non-covalent clusters, as it often happens to hydrogen-bonded complexes. Nevertheless, the electrostatic interaction plays a dominant role, and in the case of polar subsystems. 

One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (-3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, Dq (uncorrected for zero-point vibrational energy), between two atoms i and j, are the covalent bond energy, Xjj, and the ionic resonance energy, IRE. The evaluation of Ay takes the form ... 

The hydrogen bond energy between H and O is approximately 5 kcal/mol. Hydrogen bond is much weaker than the covalent bond since the typical covalent bond energy is about lOOkcaFmol. 

Values of the heat of adsorption of hydrogen on various metals calculated by Eley (110) using Pauling s covalent bond energy formula, i.e.. 

The second term is the covalent bond energy that arises from occupying the molecular eigenstates, n, with electrons, namely... 

Photochemical cleavage (see p. 236). The energy of light of 600 to 300 nm is 48 to 96 kcal/mol (200 to 400 kJ/mol), which is of the order of magnitude of covalent-bond energies. Typical examples are photochemical cleavage of chlorine and of ketones ... 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

Clearly, s-p mixing will have a stabilising effect upon the Li2 molecule, although the bonding can be qualitatively described in terms of s-s overlap alone. The dissociation energy of Li2 is only 105 kJ mol-1 compared with the covalent bond energies in Table 6.2, this is rather... 

R. S. Mulliken,/. Chem. Phys., 23, 1833, 1841 (1955). Electronic Population Analysis on LCAO-MO Molecular Wavefunctions. I. (no subtitle). II. Overlap Populations, Bond Orders, and Covalent Bond Energies. 

---