Chapter 7 activity

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. BaSO
4
2-fold increase in solubility when KNO
3
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, K
a
’
= 1.75 x 10
-5
At 0.02 M NaCl, K
a
’
= 2.29 x 10
-5
At 1.01 M NaCl, K
a
’
= 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 (m)
memorize m = ½ S C
i
z
i
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
A
c
= [C] g
C
K = A
C
cA
D
d
/ A
B
bA
A
a
K = g
C
c
[C]
c
g
D
d
[D]
d
/ g
B
b
[B]
b
g
A
a
[A]
a

9. Activity and Activity Coefficients
Examples:
Sparingly soluble salt
BaSO
4(s)
= Ba
2+
+ SO
4
2-
K
sp
= A
Ba
A
SO4
= [Ba][SO
4
] g
Ba
g
SO4
pH
pH = - log A
H+
= -log [H
+
] g
H+

10. Activity and Activity Coefficients
Weak Acid
HA + H
2
O = A
-
+ H
3
0
+
K
a
= ([A
-
][H
3
O
+
] / [HA]) (g
A-
g
H3O
/ g
HA
)

11. To calculate the activity coefficient, use the
Debye-Huckel Equation,
(be aware of limitations)
Equation
305
1
51.0
log
2
m
a
m
g
+
=-
Z
Depends on:
Z: charge
a: diameter of hydrated ion (pm)
m: 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
m  0, g
x
 1, a
x
 [X], K
’
 K
• Dilute solutions, g
x
is independent of the nature of
electrolyte and dependent on m
• For a given m, g
x
departs farther from unity as
the charge increases

15. Activity coefficient gets
smaller
as ionic strength increases