Enthalpy dissociation energies

The bond dissociation energy (enthalpy change) for a bond A—B which is broken through the reaction... 

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. 

In a parallel study Goursot and Wadso (322) determined calorimetri-cally the free energies, enthalpies, and entropies of dissociation of the conjugate acids of thiazoles in aqueous media (Table 1-51). 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

The endothermic radical lO has also been studied in the gas phase the interatomic distance is 186.7 pm and the bond dissociation energy 175 20kJmol . It thus appears that, although the higher oxides of iodine are much more stable than any oxide of Cl or Br, nevertheless, lO is much less stable than CIO (p. 849) or BrO (p. 851). Its enthalpy of formation and other thermodynamic properties are A//f(298K) 175.1 kJmol", AGf(298 K) 149.8 kJmol-, 5°(298 K) 245.5 J K- mor . 

However, the fact that the derived S—O bond dissociation energy in sulfuric acid is identical to that found in the acid derivatives, strongly supports the estimated enthalpy of formation for gas-phase sulfurous acid given by Benson18. 

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. 

A bond dissociation energy (Section 2.14) is strictly the energy required to break the bond at T = 0 a bond enthalpy is the change in enthalpy at the temperature of dissociation (typically 298 K). The two quantities differ by a few kilojoules per mole. 

The S-S bond dissociation energies of H2S2, H2S3 and H2S4 have been studied by Steudel et al. at the GCSD(T)//6-311 G(2df,p) level [42]. The calculated enthalpies AH° for the dissociation at the central bonds at 298 K are 247, 201 and 159 kJ mol respectively. The lower stability of the tri- and tetrasulfanes towards homolytic S-S cleavage is attributed to the stability of the generated HSS radical which is stabilized by the formation of a three-electron n bond. 

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