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. 272-273)
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 = 0.512 Z 2 X (u) 1/2 / (1+ 3.3 α X (u) 1/2 Ordinarily we neglect activity coefficients and simply use molar concentrations in applications. Works most of the time.