Chapter 7 Electrochemistry

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Chapter 7 Electrochemistry
7.4 Activity and activity coefficient
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Some facts of strong electrolytes
solution present species
0.52 moldm-3 KCl 95 K 5 KCl
0.25 moldm-3 Na2SO4 76 Na 24 NaSO4
0.1 moldm-3 CuSO4 44 CuSO4
Effective concentration is rather different from
the actual concentration Activity coefficient is
essential for quite dilute solutions
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1. Concepts
For ideal solution or dilute solution of
non-electrolytes
For nonideal solution of non-electrolytes
For electrolytic solution such as dilute HCl
solution
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Therefore
mean activity
Because solution only containing single ion
does not exist, the activity of individual ion is
unmeasurable, therefore, we use mean activity in
stead.
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For a salt with general formula MvXv-
Definition
Molality-scale mean ionic activity coefficient
Cf. Levine pp. 295-297
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mean ionic molality
mean ionic activity coefficient
mean ionic activity
Mean ionic molality can be expressed in term of
the molality of the solution, mean ionic activity
coefficient can be measured experimentally, and
then mean ionic activity can be determined.
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Exercises
1) Write the expression for the activity (a) of
Mg3(PO4)2 in terms of its molality and mean ionic
activity coefficient.
2) The mean ionic activity coefficient of an
0.005 molkg-1 K2SO4 aqueous solution is
measured to be 0.781. Calculate the mean ionic
activity of the solution.
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2. Methods for determination of mean ionic
activity coefficient
(1) (2) (3) (4) (5)
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3. Influential factors
1) Concentration-dependence
Discussion
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m ??
0.001 0.965
0.01 0.905
0.1 0.797
0.5 0.754
1 0.803
5 2.70
10 20.0
20 486
Activity coefficient of LiBr in water at 25 oC
and 1 atm
Cf. Levine p.299
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2) temperature
Table Dependence of ? on temperature for 11
type electrolytes
T/? 0 10 20 25
KCl 0.768 0.769 0.770 0.769
KOH 0.795 0.798 0.798 0.798
NaOH 0.767 0.768 0.766 0.766
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3) Valence types and concentration
type electrolyte 0.1 m 0.2 m 1.0 m
11 RbNO3 0.734 0.658 0.430
11 NH4ClO4 0.730 0.660 0.482
12 BaCl2 0.508 0.450 0.401
12 CaCl2 0.510 0.457 0.419
13 LaCl3 0.314 0.274 0.342
13 FeCl3 0.325 0.280 0.270
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4) ionic strength
Lewis, who noted that the nonideality observed in
electrolytic solutions primarily stems from the
total concentration of charges present rather
than from the chemical nature of the individual
ionic species, introduced ionic strength in 1921.
It is merely an hypothesis!
Valid when c lt 0.01 m
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Self reading Ira N. Levine, Physical
Chemistry, 5th Ed., McGraw-Hill, 2002. pp.
294-300 Section 10.6 solutions of
electrolytes Section 10.7 determination of
electrolyte activity coefficients