Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 535
( ) ( [] [ ] )
[ ] [ ] [ ]
/
K = EF E − EF F − EF
us
0 0
[9.79]
Inserting [9.76] in [9.77] and [9.79] in [9.78], we come to the equation relating the equilibrium
constants of all considered processes:
( )( )
K = K 1+ E 1 + F
[9.80]
EF us A A
A similar form of equation [9.80] has been proposed elsewhere. 25
If it is impossible to neglect the solvent concentration (or the concentration of solvate
component of the mixed solvent) in comparison with [E] 0 and[F] 0, equation [9.80] is written
in the form:
A
S ( )( FA )
K = K 1+ K 1 + K
[9.81]
EF us EA
Let E be acidic (acceptor) reagent in reaction [9.68], F basic (donor) reagent. Then, if
A is acidic solvent, one can neglect specific solvation of the reagent E. Thus equations for
equilibrium constant of the process [9.68] can be written in the form
A
( ) ( )
K = K 1+ K K = K 1 + K
[9.82]
EF us FS EF us FS
If A is a basic component, equation [9.82] can be re-written to the form
A
( ) ( )
K = K 1+ K K = K 1 + K
[9.83]
EF us EA EF us EA
Equations [9.82] and [9.83] were developed 26-28 for the case of specific solvation.
Thus for K EF calculation one must obtain the equilibrium constant of processes: [9.71]
-K us, [9.69] - K EA and [9.70] - K FS from conductance measurements. The constant K EF is
identified in literature as “calculated by taking into account the specific solvation”. 29 The
value K EF characterizes only the universal solvation effect on the process of
heteromolecular associate formation. The approach cited above can be illustrated by equilibrium:
(CH 3) 2SO + o-CH 3C 6H 4OH ↔ (CH 3) 2SO•CH 3C 6H 4OH [9.84]
studied in binary mixed solvents:
A) CCl 4 - hepthylchloride 30 formed by two solvate inert components [9.84]
B) CCl 4 - nitromethane 31 formed by solvate inert component (CCl 4) and
acceptor (nitromethane) component
C) CCl 4 - ethyl acetate 32 formed by solvate inert component (CCl 4) and
donor (ester) component;
The isotherms lnK vs. 1/ε (298.15K) are presented in Figure 9.8. These dependencies
(right lines 1,2,4,5) are required for calculation of equilibrium constants of the
heteromolecular association process free from specific solvation effect. It can be seen from
Figure 9.8 that the values lnK, regardless of solvent nature, lie on the same line 3, which describes
the change of equilibrium constants of the process [9.84] in the universal solution
CCl 4-heptylchloride.