ELECTROCHEMISTRY
PHYSICAL CHEMISTRY
B.Sc FIRST YEAR
SECOND SEMESTER
DEBYE-HUCKEL THEORY
• The first successful attempts to explain the
variation of equivalent conductance of strong
electrolytes with dilution was made by Debye
and Huckel(1923).
• The fundamental idea underlying their work is
that because of electrical attraction among
the oppositely charged ions.
INTERIONIC EFFECTS
• The electrical attractions among the
oppositely charged ions which affect the
speed of an ion in the electric field are called
“interionic effects”.
There are two such effects :Relaxation effect or Asymmetry effect
Electrophoretic effect
RELAXATION EFFECTS OR ASYMMETRY
EFFECTS
+
_
-
-
+
-
-
-
-
-
+
-
-
-
-
(b)
(a)
Symmetrical ionic atmosphere
around a positive ion
Ionic atmosphere becoming
asymmetrical when central ion moves
FIG:1
ELECTROPHORETIC EFFECT
_
_
_
+
_
_
_
_
FIG:2
DEBYE-HUCKEL-ONSAGER EQUATION
Debye and huckel (1923)derived a mathematical
expression for the variation of equivalent
conductance with concentration. This equation was
further improved by Onsager(1926-1927) and is
known as Debye-Huckel-Onsager equation.
Λc = Λ0-[82.4/(DT)1/2 ή +8.20X105/(DT)3/2 λ0]√C
Where Λc =Equivalent conductance at concentration c.
Λ0 =Equivalent conductance at infinite dilution.
D = Diectric constant of the medium.
ή =Coefficient of viscosity of the medium.
T =Temperature of the solution in degree absolute.
c = Concentration of the solution in moles/litre.
As D and ή are constant for a particular solvent.Therefore,at constant
temperature, the above equation can be written in the form:
Λc= Λ0-(A+BΛ0)√c where A and B are constants for a particular solvent
VERIFICATION OF THE ONSAGER EQUATION
 The plot of Λc vs √c
should be linear.
The slope of the line
should be equal to A+B Λ0,
calculated by substituting
the value of various
constants directly.
HCI ACID
KCl
Equivalent conductance
Two tests can be readily
performed to verify the
onsager equation.These
are:-
AgNO3
NaCl
√concentration c
FIG:3 TESTS OF ONSAGER EQUATION
MIGRATION OF IONS AND TRANSPORT NO
The movement of ions towards the oppositely charged
electrode is called migration of ions.
KNO3
SOLUTION
KNO3
SOLUTION
IN JELLY
CHARCOAL
POWDER
CuCr2O7
SOLUTION
IN JELLY
(GREEN)
Cu2+ (Blue)
Cr2O72- (YELLOW)
FIG:4 DEMONSTRATION OF THE MIGRATION OF IONS
HITTORF’S THEORETICAL DEVICE
According to faraday’s second law of electrolysis, when the same
quantity of electricity is passed through solution of different
electrolytes, the ions are always liberated in equivalent amounts.
To explain this ,consider a cell containing the solution and
provided with the anode A and the cathode C.Let the solution
lying between the electrodes A and C be divided into three
compartment. Before electrolysis suppose there are 13 pairs of
ions.
WHEN ELECTRODES ARE NOT ATTACKED:- The following different cases may be considered
Case 1:When only anion moves.
Case 2: When cations and anions move at the same rate.
Case 3: when cations move at double the speed of the anions
I
+ + + +
_ _ _ _
II
+ + + +
_ _ _ _ _ _
+
III
+ +
_ _
_ _ _ _
IV
_
_
_
+
_
2
2
+
_
a CENTRAL
COMPARTMENT
+ + + + +
_ _ _ _ _
+ + + +
_ _ _ _
+
_
+
_
+ + + +
_ _ _ _
+
_
+
_
+
_
+ +
_ _
C
CATHODIC
COMPARTMENT
b
----------------------------------------------------------
ANODIC
COMPARTMENT
----------------------------------------------------------
A
_
+ + + +
_ _ _ _
+ + + +
_ _ _ _
2
+ + +
_ _
2
+ +
+
+
_
+
+
+ +
_ _
+
1
FIG: 5 MIGRATION VELOCITY OF IONS AND CHANGE IN
CONCENTRATION WHEN ELECTRODES ARE NOT ATTACKED
CONCLUSION
Fall in concentration around any electrode is directly
proportional to the speed of the ions moving away from it.
It means:
Fall in con. around anode =Speed of cation

No. of ions liberated on both the electrodes is equal.
CASE IV:- WHEN ELECTRODES ARE ATTACKABLE
I
+ + + +
_ _ _ _
+
II + + + + +
_ _
III
IV
_ _ _
+ + + + +
_ _ _ _ _
2
+
_
2
+ + + + +
_ _ _ _ _
a CENTRAL
COMPARTMENT
+ + + + +
_ _ _ _ _
+ + + + +
_ _ _ _ _
+ + + + +
_ _ _ _ _
+ + + + +
_ _ _ _ _
b
----------------------------------------------------------
ANODIC
COMPARTMENT
----------------------------------------------------------
A
C
CATHODIC
COMPARTMENT
+ + + +
_ _ _ _
+ + + +
_ _
2
+ +
_ _
2
+
_
+ +
+ +
+ + + + +
_ _
1
FIG: 6 MIGRATION VELOCITY OF IONS AND CHANGE IN
CONCENTRATION WHEN ELECTRODES ARE ATTACKED
_
CONCLUSION
Fall in conc. In the anodic compartment due to
migration of Ag+ ions=(x-y)gram equivalents
Fall in conc. around cathode=Increase in conc. Around
anode=y gram equivalents
Thus, the speed ratio will be given by:
Speed of Ag+ ions/Speed of Nitrate ion=x-y/y
TRANSPORT NUMBER
The fraction of the total current carried by an
ion is called its transport number or Hittorf’s
number.
Transport number of anion na= ua
ua+uc
Transport number of cation nC= uC
ua+uc
DETERMINATION OF TRANSPORT
NUMBERS BY HITTORF’S METHOD
Hittorf’s method:Principle:- The method is based upon the
principle that the fall in concentration around
an electrode is proportional to the speed of the
ion moving away from it.
nc=Number of gram equivalent lost from the anodic compartment
Number of gram equivalent deposited in the voltameter
APPARATUS FOR THE DETERMINATION OF TRANSPORT NUMBER
+
VARIABLE
RESISTENCE
+
EXPERIMENTAL
SOLUTION
_
MILLI-AMMETER
+
_
--- ---
VOLTAMETER OF
COULOMETER
FIG:7
(i) Strong Acid with a Strong Base
When a strong alkali, e.g., sodium hydroxide is
added to a solution of a strong acid, e.g.,
hydrochloric acid, the following reaction
occurs:
(H+ + Cl-) + (Na+ + OH-) = Na+ + Cl- + H2O
•
•
•
•
•
(ii) Strong Acid with a Weak Base
The titration of a strong acid with a weak base may be illustrated by the
neutralization of dilute
HCl by dilute NH4OH.
H+Cl- + NH4OH NH4
+ + Cl- + H2O