Thermodynamics and Chemistry

CHAPTER 10 ELECTROLYTE SOLUTIONS

10.3 ELECTROLYTES INGENERAL 291

0 10 ^4

0

10 ^4

m^2 B=mol^2 kg^2

am

;B

(a)

b

0

0:5

1:0

0 0:5 1:0

m^2 B=mol^2 kg^2

am

;B

(b)

0

0:5

1:0

0 0:5 1:0

mB=mol kg^1



(c)

Figure 10.2 Aqueous HCl at 25 C and 1 bar.a

(a) HCl activity on a molality basis as a function of molality squared. The dashed line

is the extrapolation of the ideal-dilute behavior.

(b) Same as (a) at a greatly reduced scale; the filled circle indicates the solute reference

state.

(c) Mean ionic activity coefficient of HCl as a function of molality.

aCurves based on experimental parameter values in Ref. [ 74 ], Table 11-5-1.

10.3 Electrolytes in General

The formula unit of anonsymmetrical electrolyte solute has more than two ions. General
formulas for the solute as a whole are more complicated than those for the symmetrical case
treated in the preceding section, but are derived by the same reasoning.
Again we assume the solute dissociates completely into its constituent ions. We define
the following symbols:
CDthe number of cations per solute formula unit
Dthe number of anions per solute formula unit
Dthe sumCC
For example, if the solute formula is Al 2 (SO 4 ) 3 , the values areCD 2 ,D 3 , andD 5.

10.3.1 Solution of a single electrolyte

In a solution of a single electrolyte solute that is not necessarily symmetrical, the ion mo-
lalities are related to the overall solute molality by

mCDCmB mDmB (10.3.1)

From the additivity rule for the Gibbs energy, we have

GDnAACnBB

DnAACCnBCCnB (10.3.2)

giving the relation
BDCCC (10.3.3)