Tài liệu Chapter 8A: Activities doc

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Figure 2
Effect of Ionic Strength
¾ Ionic strength is a measure of the concentration and
charge of the ions in solution. It is defined by the
equation:
¾ where A, B, C, … are charged species in
the solution
¾ [A], [B], [C], … represent the species molar
concentrations of ions A, B. C
¾ Z
A,
Z
B,
Z
c, …
are their respective charges.
()
)C]Z[B]Z[A]Z([
2
1
μStrength Ionic
2
c
2
B
2
A
+++=
KNO
3
K
+
+ NO
3
-
1 mole
1 mole 1 mole
0.1 M
0.1 M 0.1 M
2 H
2
O <==> H
3
O
+
+ OH
-
10
-7
M
10
-7
M
()
)C]Z[B]Z[A]Z([
2
1
μStrength Ionic
2
c
2
B
2
A
+++=
M
Na
2
SO
4
2 Na
+
+ SO
4
2-
1 mole
2 mole 1 mole
0.1 M
0.2 M 0.1 M
()
)C]Z[B]Z[A]Z([
2
1
μStrength Ionic
2
c
2
B
2
A
+++=
M
M
KNO
3
K
+
+ NO
3
-
1 mole
1 mole 1 mole
0.05 M
0.05 M 0.05 M
()
35.0)21.012.0105.0105.0(
2
1
]ZSO[]ZNa[]ZNO[]ZK([
2
1
)C]Z[B]Z[A]Z([
2
1
μStrength Ionic
2222
2
SO
-2
4
2
Na
2
NO
-
3
2
K
2
c
2
B
2
A
-2
4
-
3
=+++=
+++=
+++=
++
++
xxxx
Na
2
SO
4
2 Na
+
+ SO
4
2-
1 mole
2 mole 1 mole
0.1 M
0.2 M 0.1 M
M
Effect of Ionic Strength
¾Dependent on
1) concentrations
2) charges
¾Independent on
The chemical properties of the ions
Therefore, the effect of ionic strength on a reaction (e.g.,
solubility or dissociation constant) is not
dependent on the
types
of spectator ions, but on the concentration and
charges
of spectator ions.
Any Questions?
Activity Coefficients
¾ Definition
A factor to describe quantitatively the effective
concentration of participants in an equilibrium at any
given ionic strength .
¾ Equation
a
x
= [X] γ
x
 a
x
= is the activity or observed concentration
 [X] = the expected or theoretical molar concentration
 γ
x
= the activity coefficient
pm
900
600
400-450
350
300
250
800
600
500
450
400
900
400
1100
500
Mean Activity coefficients
¾Equation
For example:
The mean activity coefficient of the electrolyte
A
a
B
b
is defined as
ba
1
b
B
a
A
)γ(γ t coefficienactivity mean γ
+
±
==
¾Feature 8-1