1. §7.4 Activity and activity coefficient
Chapter 7 Electrochemistry
§7.4 Activity and activity coefficient

2. Self reading:
Ira N. Levine, Physical Chemistry, 5th Ed., McGraw-Hill, 2002. pp
Section 10.6 solutions of electrolytes
Section 10.7 determination of electrolyte activity coefficients

3. Activity coefficient is essential for quite dilute solutions
Some facts about strong electrolytes
solution
present species
0.52 mol·dm-3 KCl
95% K+ + 5% KCl
0.25 mol·dm-3 Na2SO4
76 % Na+ + 24% NaSO4¯
0.1 mol·dm-3 CuSO4
44% CuSO4
Activity coefficient is essential for quite dilute solutions

4. 1. Concepts For ideal solution or dilute solution of non-electrolytes
For nonideal solution of non-electrolytes
For electrolytic solution such as dilute HCl solution:

5. Therefore:
mean activity
Because solution only containing single ion does not exist, the activity of individual ion is unmeasurable.

6. For a salt with general formula Mv+Xv-
Definition:
Molality-scale mean ionic activity coefficient
Cf. Levine pp

7. mean ionic molality
mean ionic activity coefficient
mean ionic activity
Mean ionic molality can be expressed in term of the molality of the solution, mean ionic activity coefficient can be measured experimentally, and then mean ionic activity can be determined.

8. Exercises:
1) Write expression for the activity (a) of Mg3(PO4)2 in terms of its molality and mean ionic activity coefficient.
2) The mean ionic activity coefficient of an mol·kg-1 K2SO4 aqueous solution is measured to be Calculate the mean ionic activity of the solution.

9. 2. Methods for determination of mean ionic activity coefficient
(1)
(2)
(3)
(4)
(5)

10. 3. Influential factors 1) Concentration-dependence Discussion:  HCl
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.8
1.0
HCl
NaCl
Mg(NO3) 2 
m / mol·kg-1
ZnSO4
Discussion:

11. m 
0.001
0.965
0.01
0.905
0.1
0.797
0.5
0.754 1
0.803 5
2.70 10
20.0 20
486
Activity coefficient of LiBr in water at 25 oC and 1 atm
Cf. Levine p.299

12. 2) temperature independent on temperature? T/℃ 10 20 25 KCl 0.768
Table Dependence of ± on temperature for 1:1 type electrolytes
T/℃ 10 20 25
KCl
0.768
0.769
0.770
KOH
0.795
0.798
NaOH
0.767
0.766
independent on temperature?

13. 3) Valence types and concentration
electrolyte
0.1 m
0.2 m
1.0 m
1:1
RbNO3
0.734
0.658
0.430
NH4ClO4
0.730
0.660
0.482
1:2
BaCl2
0.508
0.450
0.401
CaCl2
0.510
0.457
0.419
1:3
LaCl3
0.314
0.274
0.342
FeCl3
0.325
0.280
0.270

14. 4) ionic strength
Lewis, who noted that nonideality observed in electrolytic solutions primarily stems from the total concentration of charges present rather than from the chemical nature of the individual ionic species, introduced ionic strength in 1921.
Valid when c < 0.01 m