Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 533
Table 9.2. Equilibrium constants of adduct (nHAc•TBP) formation in the mixed solvent
n-hexane - nitrobenzene (298.15K) and coefficients of equation [9.65]
Solvent
HAc•TBP 2HAc•TBP 3HAc•TBP 4HAc•TBP
Hexane (ε=2.23) 327 8002 6.19×10 5
1.46×10 -7
H+NB (ε=9.0) 28.1 87.5 275 1935
H+NB (ε=20.4) 19.6 45.0 88.1 518
Nitrobenzene
(ε=34.8)
17.4 36.2
Coefficients of equation [9.65]:
60.8 337
a 2.69 3.28 3.58 5.21
b 5.82 10.73 18.34 21.22
When stoichiometric coefficients are equal to m and n (equation [9.64]), K add expression
is an equation of higher degree relative to c M
o
n
o ( M)
( MF M)
Kadd
K = c c −nc c −mc
/ , , [9.67]
EmFn M M E
Let us consider an example of the interaction between acetic acid and tributyl phosphate
(the change of permittivity of universal solvent permits us to change essentially the
output of the reaction product). When initial concentration of the components equals to
o
c M=0.1
mol/l, the output (in %) of complexes of different composition is listed in table below:
1:1 2:1 3:1 4:1
In hexane 84 46 32 22
In nitrobenzene 43 13 4.5 2
Relative concentration changes the more essentially, the larger the value of electro-
el
static component of the process free energy, ΔG ε =0 .
Just as in the previous cases, the solvent use in this case is an effective means of process
adjustment. To reduce the output of adduct HAc•TBF in nitrobenzene to the same
value as in hexane solution, nitrobenzene solution must be cooled down to -78 o C (taking
into account that enthalpy of adduct formation equals 15 kJ/mol). 24 Naturally, the process is
not possible because the nitrobenzene melting point is +5.8 o C.
Let us consider the effect of specific solvation on equilibrium constant of the
heteromolecular association process as an example of associate formation with a simplest
stoichiometry:
E + F ↔ EF
[9.68]
m