Bond energies dissociation enthalpies

A/i the dissociation or bond energy of hydrogen (it is also, by definition, twice the enthalpy of atomisation two gram atoms being produced). 

The very low bond dissociation enthalpy of fluorine is an important factor contributing to the greater reactivity of fluorine. (This low energy may be due to repulsion between non-bonding electrons on the two adjacent fluorine atoms.) The higher hydration and lattice enthalpies of the fluoride ion are due to the smaller size of this ion. 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

A//298 is called the bond dissociation enthalpy or simply the bond enthalpy. Bond enthalpies are often also called bond energies because the small difference between the two values (ca. 2.5 kJ mol-1) can be ignored in many cases, particularly for polyatomic molecules. 

In this section we deal with the first of the physical effects which impinge on reactivity — the influences which heats of reaction and bond dissociation energies have on the course of chemical reactions. Both heats of reaction and bond dissociation energies are enthalpy values that are experimentally determined by thermochemical methods, in the first case usually by direct calorimetric methods, in the second by more indirect techniques 22). 

On the basis of published data for enthalpies of formation, sublimation, and vaporization, the dissociation enthalpies of terminal N—O bonds, DH°(N—O), in various organic compounds including nitrile oxides, were calculated and critically evaluated (18). The derived DH°(N—O) values can be used to estimate enthalpies of formation of other molecules, in particular nitrile oxides. N—O Bond energy in alkyl nitrile oxides was evaluated using known and new data concerning kinetics of recyclization of dimethylfurazan and dimethylfuroxan (19). 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

Some of the few complete sets of enthalpies for binary fluorides are collected in Tables XVIII and XIX. The inversion in dissociation heats for copper and beryllium fluorides can be associated with the closed-shell configurations of Be2+ and Cu+. The alternations in bond energies... 

Adiabatic detachment energy [7]. Abbreviations used rcoy (Em) = covalent radius of element E in a trivalent compound BE(E-E) = bond enthalpy of a single E-E bond D°298(E2) = dissociation enthalpy of the E2 molecule at standard conditions IE = ionization enthalpy EA = electron affinity AHf°(E2 g) = standard enthalpy of formation of the gaseous E2 molecule. 

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

Figure 5.5 Thermochemical cycles relating O-H bond enthalpy contributions ( s) with bond dissociation enthalpies (DH°) in phenol and ethanol. ER are reorganization energies (see text).

The value obtained for the reorganization energy of PhO, -29.3 kJ mol together with the PhO-H bond dissociation enthalpy, lead to... 

In summary, the previous example shows that bond dissociation enthalpies should not be correlated with bond lengths unless the relaxation energies of the fragments are comparable. On the other hand, when two bonds between the same pairs of atoms have identical bond lengths, it is sensible to assume that they have similar bond enthalpy contributions. Hence, in this case, a bond enthalpy contribution can be transferred from one molecule to another. 

We are now left with the evaluation of E s (Cr—CO), the Cr-CO bond enthalpy contribution in Cr(CO)6. The third thermochemical cycle in figure 5.6 shows how this bond enthalpy contribution can be evaluated from the Cr-CO mean bond dissociation enthalpy (107.0 0.8 kJ mol-1 see section 5.2) and the reorganization energy ER(CO ). 

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