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

Self reading: Ira N. Levine, Physical Chemistry, 5th Ed., McGraw-Hill, 2002. pp. 294-300 Section 10.6 solutions of electrolytes Section 10.7 determination of electrolyte activity coefficients

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

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:

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

For a salt with general formula Mv+Xv- Definition: Molality-scale mean ionic activity coefficient Cf. Levine pp. 295-297

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.

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 0.005 mol·kg-1 K2SO4 aqueous solution is measured to be 0.781. Calculate the mean ionic activity of the solution.

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

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:

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

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?

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

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