Handbook of Solvents - George Wypych - ChemTech - Ventech!

562 Y. Y. Fialkov, V. L. Chumak
change is large enough. The analogous observations can be made regarding the character of
ΔS i and ΔS T values changes along with permittivity change.
If considerable endothermic effect of the ionic association for acid can be explained by
desolvation contribution of ionic pair formation (i.e., solvated molecule of acid), the endothermic
effect of ionic pair formation by such voluminous cations contradicts the physical
model of the process. Estimation of desolvation energy of ionic pair formation of salt ions
according to the equation 74 shows that this energy is two orders of magnitude lower than the
energy of heat movement of solvent molecules. For this reason, the process of ionic association
of salt ions is exothermal, as seen from ΔH T values. The exothermal character increases
with permittivity decreasing, i.e., with increase of ion-ion interaction energy.
Dependencies ΔH i on ΔS ifor salt and acid are not linear, i.e., the compensative effect is
not fulfilled. Consequently, the substantial change of the ionic association process is due to
change of solvent composition. Considering the same chemical characteristics of mixed
solvent components, this conclusion is wrong because the compensative effect if fulfilled
for van’t Hoff’s components is:
ΔH T = 0.448 ΔS T - 28.5 kJ/mol
ΔH T = 0.389 ΔS T - 17.3 kJ/mol
The smaller the magnitude of the characteristic temperature for salt (389K), in comparison
to acid, the smaller the barrier of the ionic pairs formation process. That is true because
the association process in the case of acid is accompanied by energy consumption for
desolvation.
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