Entropy of dissociation

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). 

From such crude data as are to be found in the literature we can calculate approximate values of the equilibrium constants, and hence of the free energies of dissociation for the various hexaarylethanes. From our quantum-mechanical treatment, on the other hand, we obtain only the heats of dissociation, for which, except in the single case of hexaphenylethane, we have no experimental data. Thus, in order that we may compare our results with those of experiment, we must make the plausible assumption that the entropies of dissociation vary only slightly from ethane to ethane. Then at a given temperature the heats of dissociation run parallel to the free energies and can be used instead of the latter in predicting the relative degrees of dissociation of the different molecules. 

As regards enthalpy of dissociation A//het(R-R ) and entropy of dissociation A5het(R-R )> only a few data are available (Table 4). Bordwell (1988 ... 

Table 4 Enthalpy and entropy of dissociation for some hydrocarbons in DMSO.

Equations (109) and (110) imply that Eq. (17) for the auxiliary condition on ASM is satisfied. The next independent constraint applied is then the requirement that the relative chemical potential of the species ac must equal the Gibbs energy of formation of AC(s) at the congruent melting point. This allows one to express the entropy of dissociation of ac as... 

In is species 2, Sb is species 3, and InSb is species 5 in the liquid. AH and AS" are the standard enthalpy and entropy of dissociation of the InSb species into the pure liquid elements. 

Denison, J. T., and J. B. Ramsey The free energy, enthalpy and entropy of dissociation of some perchlorates in ethylene chloride and ethylidene chloride. J. Am. Chem. Soc. 77, 2615 (1955). 

Measurement of Kd for any ion pair using the Fuoss formulation enables the degree of dissociation of that pair to be assessed, and this knowledge is essential in evaluating reactivity. The theory automatically provides a value for A0 for the salt and, if Kd is determined at a series of temperatures, then the corresponding enthalpy and entropy of dissociation can also be calculated. Use of the Denison and Ramsey equation then conveniently provides for an estimate of Kd in other... 

From our earlier discussion, you might expect that the dissociation of a proton from a carboxylic acid, which increases the number of independent particles, would lead to an increase in entropy. However, this effect is more than counterbalanced by solvation effects. The charged anion and proton both freeze out many of the surrounding molecules of water (fig. 2.4). Thus the ionization of a weak acid decreases the number of mobile molecules and so leads to a decrease in entropy. The entropy of ionization of a typical carboxylic acid in water is about —22 eu/mole. The entropy of dissociation of a proton from a quaternary ammonium... 

Even if the heat of decomposition of the fluoroborate is positive by as much as 10 kcal./mole, which seems unlikely, stability of the crystal would be limited to low temperatures by the high entropy of dissociation. The sum of the standard entropies of the likely products, NF3 + F2 + BF3, is... 

If values of pKi t for a dissociable group are measured at several temperatures, heats and entropies of dissociation can be determined from the... 

Detailed studies of the dissociation of macroion pairs, includii thermodynamic parameters of dissociation, are avaUable only for THF and oxepane Dissociation constants and the corresponding enthalpies and entropies of dissociation are given in Table 8 below. 

Tables 23 and 24 show some selected enthalpies and entropies of dissociation determined by Knudsen effusion mass spectrometry. In addition, partial pressures and equilibrium constants were evaluated. An account of partial pressures and equilibrium constants is given in a recent review by Hilpert [438].

Mass spectrometry of KF-Bep2 melt vapours. Enthalpy, entropy of dissociation and dimerization are... 

Knowledge of the dimerization and dissociation constants leads to the respective equilibrium constant of 6 x 10 15 M-1. Assuming a value of 15 eu for the entropy of dissociation, we calculate the heat of dissociation AH —25 kcal/mol. 

Although normal Arrhenius behavior was observed for k, anomalous increases of k with decreasing temperature were observed in polar solvents such as THF and dimethoxyethane (glyme) [97-99]. These results have been explained in terms of a temperature-dependent equilibrium between contact and solvent-separated ion pairs as shown in Scheme 7.13. This equilibrium shifts from the less reactive contact ion pairs k ) to the much more reactive solvent-separated ion pairs k ) as temperature is decreased because the contribution from the unfavorable (negative) entropy of dissociation (TAS /g) decreases and the enthalpy of dissociation (AH / ) is negative. The values of k and k are not very dependent upon solvent, but the equilibrium constants are very dependent on the polarity of the solvent (Table 7.2). It is noteworthy that the reactivity of the solvent-separated ion pairs approaches that of the free ions. These results also provide a rationalization for the effect of counterion on kj. Smaller cations such as lithium interact more strongly with solvent and form significant amounts of more reactive, solvent-separated ion pairs. 

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