This document discusses how activity coefficients can explain the effect of inert salts on solubility and acid dissociation constants. It provides examples showing that a precipitate is more soluble and a weak acid dissociates more when the ionic strength is increased by adding an inert salt. This is because the activity coefficients of the ions are less than 1 and decrease with increasing ionic strength, making the activities higher than concentrations. The Debye-Huckel equation can be used to calculate activity coefficients based on ionic charge and strength.

1. Activity and Activity Coefficients
Experimental Observation:
A precipitate is more soluble in a solution containing an
inert salt than it is in water
Ex. BaSO4
2-fold increase in solubility when KNO3 concentration
is increased from 0 to 0.02M

2. Activity and Activity Coefficients
Experimental Observation:
Degree of dissociation increases with the addition of an
inert salt
Ex. Acetic Acid
At 0.00 M NaCl, Ka
’
= 1.75 x 10-5
At 0.02 M NaCl, Ka
’
= 2.29 x 10-5
At 1.01 M NaCl, Ka
’
= 3.16 x 10-5

4. More soluble
as conc. of
inert salt increases

5. Activity and Activity Coefficients
• Effect due to electrostatic attractions between ions
• Effect is essentially independent of the kind of
electrolyte but depends on the ionic strength (µ)
memorizeµ = ½ Σ Ci zi
2
C = concentration of the ion “i”
z = charge on that ion

6. Activity and Activity Coefficients
aA + bB = cC + dD
K = [C]c
[D]d
/ [A]a
[B]b
Does not predict any effect on the ionic strength

7. Why?

8. Activity and Activity Coefficients
activity Activity coefficient
Ac = [C] γC
K = AC
c
AD
d
/ AB
b
AA
a
K = γC
c
[C]c
γD
d
[D]d
/ γB
b
[B]b
γA
a
[A]a

9. Activity and Activity Coefficients
Examples:
Sparingly soluble salt
BaSO4(s) = Ba2+
+ SO4
2-
Ksp = ABaASO4 = [Ba][SO4] γBaγSO4
pH
pH = - log AH+ = -log [H+
] γH+

10. Activity and Activity Coefficients
Weak Acid
HA + H2O = A- + H30+
Ka = ([A-][H3O+] / [HA]) (γA-γH3O / γHA)

11. To calculate the activity coefficient, use the
Debye-Huckel Equation,
(be aware of limitations)
Equation
305
1
51.0
log
2
µ
α
µ
γ
+
=−
Z
Depends on:
Z: charge
α: diameter of hydrated ion (pm)
µ: ionic strength

12. Ion size (will be given on a test)
Use D-H equation to calculate:
Do not use these values.

13. Why?

14. Activity and Activity Coefficients
Properties of Activity Coefficients:
• Measure of the “effectiveness” with which that
species influences an equilibrium
µ  0, γx  1, ax  [X], K’
 K
• Dilute solutions, γx is independent of the nature of
electrolyte and dependent on µ
• For a given µ, γx departs farther from unity as
the charge increases

15. Activity coefficient gets
smaller
as ionic strength increases