1. § 7.4 Activity and activity coefficient Out-class extensive reading: Ira N. Levine, pp. 294-300 Section 10.6 solutions of electrolytes Section 10.7 determination of electrolyte activity coefficients

2. solution present species 0.52 mol·dm -3 KCl95% K + + 5% KCl 0.25 mol·dm -3 Na 2 SO 4 76 % Na + + 24% NaSO 4 ¯ 0.1 mol·dm -3 CuSO 4 44% CuSO 4 Some facts of strong electrolytes Effective concentration is rather different from the actual concentration Activity coefficient is essential for quite dilute solutions

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

4. Therefore: Because solution only containing single ion does not exist, the activity of individual ion is unmeasurable, therefore, we use mean activity in stead. mean activity

5. For a salt with general formula M v+ X v- Definition: Cf. Levine pp. 295-297 Molality-scale mean ionic activity coefficient

6. 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.

7. Exercises: 2) The mean ionic activity coefficient of an 0.005 mol·kg -1 K 2 SO 4 aqueous solution is measured to be 0.781. Calculate the mean ionic activity of the solution. 1) Write the expression for the activity (a) of Mg 3 (PO 4 ) 2 in terms of its molality and mean ionic activity coefficient.

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

9. 3. Influential factors 1) Concentration-dependence Discussion: 0.0 0.10.20.30.4 0.50.6 0.0 0.2 0.4 0.6 0.8 1.0 HCl NaCl Mg(NO 3 ) 2  m / mol·kg -1 ZnSO 4

10. Activity coefficient of LiBr in water at 25 o C and 1 atm m  0.0010.965 0.010.905 0.10.797 0.50.754 10.803 52.70 1020.0 20486 Cf. Levine p.299

11. 2) temperature T/℃T/℃ 0102025 KCl0.7680.7690.7700.769 KOH0.7950.798 NaOH0.7670.7680.766 Table Dependence of  ± on temperature for 1:1 type electrolytes

12. typeelectrolyte0.1 m0.2 m1.0 m 1:1 RbNO 3 0.7340.6580.430 NH 4 ClO 4 0.7300.6600.482 1:2 BaCl 2 0.5080.4500.401 CaCl 2 0.5100.4570.419 1:3 LaCl 3 0.3140.2740.342 FeCl 3 0.3250.2800.270 3) Valence types and concentration

13. 4) ionic strength Lewis, who noted that the 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. It is merely an hypothesis! Valid when c < 0.01 m

14. 10-minute exam 1. Write the equilibria exist in concentrated NaCl solution and HAc (acetic acid). Pay attention to their difference. 2. For strong electrolyte, there is a linear relationship between and at infinite dilution. In the following figure, there are tow lines standing for NaCl and MgSO 4 dilute solution, respectively. Please indicate which line (the solid one or the dash one) is for NaCl solution.