11chapter7part7(1).ppt pankajpandey spc govt. Ajmer

Pankaj's solutions

1. Debye-Huckel-Onsager Equation
Introduction
In this chapter, we introduce the basic concepts of
- Debye-Huckel-Onsager Equation.
- Ion Association .
- Conductivity at High Frequencies and Potentials.
- Independent Migration of Ions

2. Objectives
•When you complete this chapter you should
be able to:
• - Explain Debye-Huckel-Onsager Equation.
• - Explain Ion Association .
• - Explain Conductivity at High Frequencies
and Potentials.
•- Explain Independent Migration of Ions.

3. Debye-Huckel-Onsager
Equation
cQP )Λ(ΛΛ
oo

where P and Q can be expressed in terms of various constants and properties of the
system.
For the particular case of a symmetrical electrolyte [i.e., one for which the two ions
have
equal and opposite signs (z
+
=z
-
=z)], P and Q are given by

εε
2
εε24

εε
2
3
2/1
B0
22
B0
22
2/1
B0
22




















Tk
Lez
Tk
wez
Q
Tk
LezzeF
P
Based on the assumption of complete
dissociation
~For aqueous solution of uni-univalent
electrolytes,
the theoretical equation is found to be obeyed
very satisfactorily up to a concentration of
about
210
-3
mol dm
-3
; at higher concentrations,
deviation are found.
~The corresponding equations for other types
of
electrolytes in water are also obeyed
satisfactorily
at very low concentrations, but deviations are
found at lower concentration than with uni-
univalent electrolytes.
Figure
7.7
)Λ(slope
o
QP
o
Λerceptsint 
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4. Problem 7.10
The molar conductivity of KBr solutions as a function of concentration at 25
C is given in the following table. By a linear regression analysis of suitable
variables find
the value of 
o
for KBr.
c, 10
-3
M0.250.360.500.751.001.602.005.0010.00
, S cm
2
mol
-1
150.1
6
149.8
7
149.55149.1
2
148.78148.0
2
147.64145.4
7
143.15
Solution
c
c303.8314.151Λ 

o
=151.14 S cm
2
mol
-
1
c=0
0.00 0.03 0.06 0.09 0.12
130
140
150
160
170
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5. 7-5 Independent Migration of Ions
The plots of  against concentration can be
extrapolated
back to zero concentration to give the 
o
value.
~This extrapolation can only satisfactorily be made
with
strong electrolytes.
~With weak electrolytes there is a strong dependence of

on c at low concentrations and therefore the
extrapolations
do not lead to reliable 
o
values.
Kohlrausch’s law of independent migration of ions
Each ion is assumed to make its own contribution to the
molar conductivity, irrespective of the nature of the other
ion with
which it is associated. ooo
λλΛ

anion cation and vities of n conducti are the io λandλ
oo


12o
Na
o
K
12o
Cl
o
Na
o
12o
Cl
o
K
o
mol S cm 4.23λλ
mol S cm 5.126λλ(NaCl)Λ
mol S cm 9.149λλ(KCl)Λ












This will be the same whatever the
nature of the anion.
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6. Problem
7.5
The electrolytic conductivity of a saturated solution of silver chloride, AgCl, in pure
water at 25 C is 1.2610
-6

-1
cm
-1
higher than that for the water used. Calculate the
solubility of AgCl
in water if the molar ionic conductivities are Ag
+
, 61.9 
-1
cm
2
mol
-1
; Cl
-
, 76.4 
-1
cm
2

mol
-1
. Solutio
n 121
ClAgAgCl molcmΩ3.1384.769.61λλΛ

 
393
6
dmmol1011.9cmmol
3.138
1026.1
solubility





Problem
7.16
The molar conductivity at 25 C of a 0.01 M aqueous solution of ammonia is 9.6 
-1
cm
2

mol
-1
. For NH
4
Cl, 
o
=129.8 
-1
cm
2
mol
-1
and the molar ionic conductivities are OH
-
,
174.0 
-1
cm
2
mol
-1
; Cl
-
, 65.6 
-1
cm
2
mol
-1
, respectively. Calculate 
o
for NH
3 and the
degree of ionization
in 0.01 M solution.Solutio
n
121o
OH
o
Cl
o
ClNH
o
OH
o
NH
o
OHNH molcmΩ2.2380.1746.658.129λλΛλλΛ
4
4
4

 
0403.0
2.238
6.9
Λ
Λ
α
o

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