Hydrogen fluoride bond dissociation energy

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

Similarly, the original bond dissociation energy of hydrogen fluoride (147.6 kcal/mol (618 kJ) [/]) was later corrected to 136 kcal/mol (570 kJ) [31. 

It is reasonable to postulate that the main driving force for the reaction of (MeC5H4)3U(t-Bu) with hexafluorobenzene is thermodynamic. A weak uranium-carbon bond and a carbon-fluorine bond have to be broken. This is offset by the formation of a strong uranium-fluorine bond and either a carbon-carbon or a carbon-hydrogen bond. As illustrated in equation 8, the uranium-fluorine bond energy can be estimated to be on the order of 150 kcal/mol based on known thermochemical data for uranium fluorides (21), The carbon-fluorine bond dissociation energy for hexafluorobenzene is reported to be 154 kcal/mol (17), The... 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

Hydrogen fluoride vapors contain monomeric HF molecules, dimers (HF)2 and polymeric species, (HF) , with n varying from 3 to 8. The stmctures of the polymers remain unknown, but the stmcmre of the dimer has been determined by spectroscopic measurements and by high-level quantum chemical calculations. The structure indicated in Fig. 18.8 represents a recommended structure based on information from both experiments and calculations [12]. The complex consists of two monomeric units with essentially the same H-F bond distance as in an isolated monomer. The monomers are joined through a H- F bond which is twice as long as bond distance in the monomer, but nearly 100 pm shorter than the sum of the van der Waals radii of H and F (Table 8.4). Note the similarity to the structure of HFICI (Fig. 18.6). The dissociation energy of the dimer at zero kelvin is Do = 13 kJ mol [13]. 

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