Enthalpy dissociation

A/i, the enthalpy of atomisation of chlorine, which is also half the bond dissociation enthalpy. 

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... 

Figure 6.10 A plot of dissociation enthalpy versus substitution pattern for the gas-phase dissociation of alkyl chlorides to yield carbocations. More highly substituted alkyl halides dissociate more easily than less highly substituted ones.

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

Chloromethane. bond length of, 335 bond strength of, 335 dipole moment of, 335 dissociation enthalpy of, 195 electrostatic potential map of, 38, 143.335... 

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

Activation energy values for the recombination of the products of carbonate decompositions are generally low and so it is expected that values of E will be close to the dissociation enthalpy. Such correlations are not always readily discerned, however, since there is ambiguity in what is to be regarded as a mole of activated complex . If the reaction is shown experimentally to be readily reversible, the assumption may be made that Et = ntAH and the value of nt may be an indication of the number of reactant molecules participating in activated complex formation. Kinetic parameters for dissociation reactions of a number of carbonates have been shown to be consistent with the predictions of the Polanyi—Wigner equation [eqn. (19)]. 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

To ensure a proper initiation by light, the energy must correspond to the dissociation enthalpy of chlorine, which is 250 kJ/mol. Thus, initiation is possible by application of visible light of shorter wavelength such as 370-478 nm. Termination reactions are as follows ... 

Wc shall not distinguish between the average dissociation energy and the average dissociation enthalpy, a concept to be introduced in Section 6.20. The two quantities differ by only a few kilojoules per mole. 

Similar values have been obtained for AHffMesSi ) from two independent studies. The bond dissociation enthalpy DHfMeaSi-SiMea) = 332 +12 kJ moC was obtained from a kinetic study on the very low pressure pyrolysis of hexamethyldisilane and the enthalpy of formation of trimethylsilyl ion, AHf (MeaSi ) = 617.3 + 2.3kJmor, was determined using threshold photoelectron-photoion coincidence spectroscopy (TPEPICO). Both data are related to AHf°(Me3Si ). 

The stability of such adducts, usually expressed by their dissociation enthalpies AHoiss) has been intensely investigated in the past, both in solution and in the gas phase (Table 1). 

Table 1. Dissociation enthalpies AHdiss of adducts R3AI—ER3 [15]...

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

The observed dissociation enthalpies of f-Bu3Al—E(f-Pr)3 adducts (12.2 kcal/mol 9, 9.9 kcal/mol 10, 7.8 kcal/mol 11 and 6.9 kcal/mol 12) steadily decrease with increasing atomic number of the pnictine, as was expected (Fig. 3). Since steric interactions within analogously substituted adducts should become less effective with increasing atomic radius of the central group 15 element, the observed trend obviously results from the decreased Lewis basicity of the heavier pnictines. 

Fig. 3. Dissociation enthalpies of adducts f-Bu3Al-E( -Pr)3 as obtained from temperature-dependent NMR Studies...

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

The initial products in the trisulfane decomposition are H2S4 and approximately equal amounts of H2S and H2S2. This reaction is much faster than the tetrasulfane decomposition despite the higher dissociation enthalpy. Therefore, it was assumed that the radicals formed in the initial step are more reactive [64] ... 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

E. D. Sloan and F. Fleyfel. Hydrate dissociation enthalpy and guest size. Fluid Phase Equilibria, 76 123-140, 1991. 

K bond dissociation enthalpy, unless otherwise noted. 

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. 

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

---