Chapter 10: Effect of Electrolytes on Chemical Equilibria CHE 321: Quantitative Chemical Analysis Dr. Jerome Williams, Ph.D. Saint Leo University

Overview Activity & Ionic Strength Activity Coefficients

Activity & Ionic Strength Equilibrium constants for chemical reactions should be written in terms of their activities of participating species. Activity is related to concentration by activity coefficient.

Activity &Ionic Strength Effect of added electrolyte is independent of the chemical nature of electrolyte but depends on ionic strength of solution. Ionic Strength (u) = 0.5 ([A]Z A 2 + [B]Z B 2 + [C]Z C 2 …)

Activity & Ionic Strength If one has a strong electrolyte consisting of monovalent charged ions, the ionic strength is identical to the total molar salt concentration. Ionic strength > molar-concentration if solution contains ions with multiple charges. (see Table 10.1)

Figure 10-2 p237

Table 10-1 p238

Activity Coefficients Activity is related to concentration by activity coefficient. a X = [X] γ X Activity of a species is a measure of its effective concentration as determined by colligative properties, electrical conductivity, and by mass action effect. Review Properties of Activity Coefficients (pp )

Figure 10-3 p240

Activity Coefficients Debye-Huckel Equation permits calculation of activity coefficients of ions from their charge and average size. -log γ A = Z 2 X (u) 1/2 / ( α X (u) 1/2 Ordinarily we neglect activity coefficients and simply use molar concentrations in applications. Works most of the time.

Table 10-2 p242

Suggested Problems HW Set 9: 10.1, 10.3, 10.7, 10.9, 10.13, 10.14