Jess Sproul
Chapter 9 Notes
Chapter 9
Electrolyte Effects: Activity or
Concentration
9A-1: How Do Ionic Charges Affect
Equilibria?
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The magnitude of the effect that the
electrolyte has is highly dependent on the
charge of the ions participating in the
equilibrium.
When only neutral species are involved in
the equilibrium, the electrolyte has no real
effect, no matter how concentrated.
9A-1: How Do Ionic Charges Affect
Equilibria? (cont.)
In the figure seen at the right,
the molar solubility of the doubly
charged ions in BaSO4 increases by
a factor of 2 in a solution of 0.02 M
KNO3 over a solution of pure water.
On the other hand, the molar
solubility of a solution of Ba(IO3)2 with
one singly and one doubly charged
ion only increases by a factor of 1.25
in a solution of 0.02 M KNO3 over a
solution of pure water. And finally,
the molar solubility of AgCl with two
singly charged ions only increases by
a factor of 1.2 in a solution of 0.02 M
KNO3 over a solution of pure water.
9A-2: What Is the Effect of Ionic
Strength on Equilibria? (cont.)

The ionic strength of
a solution of a
strong electrolyte
consisting solely of
singly charged ions
is identical with its
total molar salt
concentration. But
when the ions are
no longer singly
charged, the ionic
strength increases.
Type
Electrolyte
Example
Ionic Strength*
1:1
NaCl
c
1:2
Ba(NO3)2,
Na2SO4
3c
1:3
Al(NO3)3,
Na3PO4
6c
2:2
MgSO4
*c = molarity of the salt
4c
9A-3: The Salt Effect

The electrolyte effect results from the electrostatic
attractive and repulsive forces that exist between
the ions of an electrolyte and the ions involved in
an equilibrium. These forces cause each ion from
the dissociated reactant to be surrounded by a
sheath of solution that contains a slight excess of
electrolyte ions of opposite charge.
9A-3: The Salt Effect
The electrolyte effect results from the electrostatic attractive and repulsive forces
that exist between the ions of an electrolyte and the ions involved in an
equilibrium. These forces cause each ion from the dissociated reactant to be
surrounded by a sheath of solution that contains a slight excess of electrolyte
ions of opposite charge. These charged atmospheres lessen the charges on
the ions in equilibrium, the Ba2+ and SO42- in the figure, thus decreasing
their overall attraction to each other and increasing solubility.
9B: Activity Coefficients
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The term activity, a, is used to account for the
effects of electrolytes on chemical equilibria.
The activity, or effective concentration, of species X
depends on the ionic strength of the medium and is
defined as
ax = γx [X]
where ax is the activity of X, [X] is its molar
concentration, and γx is a dimensionless quantity
called the activity coefficient.
The activity coefficient and the activity of X vary
with ionic strength.
9B-1: Properties of Activity
Coefficients
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In dilute solutions, the activity coefficient for
a given species is independent of the nature
of the electrolyte and dependent only on
ionic strength.
For a given ionic strength, the activity
coefficient of an ion departs farther from
unity as the charge carried by the species
increases.
At any given ionic strength, the activity
coefficients of ions of the same charge are
approximately equal.
The activity coefficient of a given ion
describes its effective behavior in all
equilibria in which it participates.
9B-1: Properties of Activity
Coefficients (cont.)
9B-2) The Debye-Hückel Equation
•
The Debye-Hückel equation takes the ionic
atmosphere model and derives a theoretical
expression that permits the calculation of
activity coefficients of ions from their charge
and their average size.
9B-3: Equilibrium Calculations with
Activity Coefficients

Equilibrium
calculations with
activities yield
values that are
better in agreement
with experimental
results than those
obtained with molar
concentrations.
9B-4: Omitting Activity Coefficients
in Equilibrium Calculations
We shall ordinarily neglect activity coefficients
and simply use molar concentrations in
applications of the equilibrium law. This
simplifies calculations and decreases
necessary data. For most purposes, the error
introduced by this method is minimal.
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Significant discrepancies occur when the ionic
strength is larger than 0.01 or when the ions
involved have multiple charges.
With dilute solutions of nonelectrolytes or of simply
charged ions, the use of concentrations in a masslaw calculation often provides reasonably accurate
results.