Title: ELECTROCHEMISTRY PHYSICAL CHEMISTRY B.Sc FIRST YEAR SECOND SEMESTER
1ELECTROCHEMISTRYPHYSICAL CHEMISTRY B.Sc FIRST
YEARSECOND SEMESTER
2DEBYEHUCKEL 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.
3INTERIONIC 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
4RELAXATION EFFECTS OR ASYMMETRY EFFECTS



_












(a)
(b)
Symmetrical ionic atmosphere around a positive ion
Ionic atmosphere becoming asymmetrical when
central ion moves
FIG1
5ELECTROPHORETIC EFFECT
_
_
_
_
_
_
_
FIG2
6DEBYEHUCKELONSAGER EQUATION
 Debye and huckel (1923)derived a mathematical
expression for the variation of equivalent
conductance with concentration. This equation was
further improved by Onsager(19261927) and is
known as DebyeHuckelOnsager equation.
?c ?082.4/(DT)1/2 ?
8.20X105/(DT)3/2 ?0vC
Where ?c Equivalent conductance at concentration
c. ?0 Equivalent conductance at
infinite dilution. D Diectric
constant of the medium. ?
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 ? are constant
for a particular solvent.Therefore,at constant
temperature, the above equation can be written in
the form ?c ?0(AB?0)vc where A and
B are constants for a particular solvent
7 VERIFICATION OF THE ONSAGER EQUATION
 Two tests can be readily performed to verify the
onsager equation.These are  The plot of ?c vs vc should be linear.
 The slope of the line should be equal to AB ?0,
calculated by substituting the value of various
constants directly.
HCI ACID
KCl
AgNO3
Equivalent conductance
NaCl
vconcentration c
FIG3 TESTS OF ONSAGER EQUATION
8MIGRATION 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)
FIG4 DEMONSTRATION OF THE MIGRATION OF IONS
9HITTORFS THEORETICAL DEVICE
According to faradays 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.
10WHEN ELECTRODES ARE NOT ATTACKED The following
different cases may be considered Case 1When
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
C
CATHODIC COMPARTMENT
A
b
ANODIC COMPARTMENT
CENTRAL COMPARTMENT
a
_

 _ _ _ _ _
_ _ _ _ _ _ _ _ 

 _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ 
 _ _ _ _ _ _ _ _
_ _ _ _ _ 
2 
 _ _ _ _ _ _
_ _ _ _ _ _ _
I
II
2




2
III
2
IV
1
FIG 5 MIGRATION VELOCITY OF IONS AND CHANGE IN
CONCENTRATION WHEN ELECTRODES ARE NOT ATTACKED
11CONCLUSION
 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.
12CASE IV WHEN ELECTRODES ARE ATTACKABLE
C
CATHODIC COMPARTMENT
A
b
ANODIC COMPARTMENT
CENTRAL COMPARTMENT
a
_

 _ _ _ _ _
_ _ _ _ _ _ _ _ 

 _ _ _ _ _ _ _
_ _ _ _ _ 
 _ _ _ _ _ _ _ _
_ _ _ _ _ 
2 
 _ _ _ _ _ _
_ _ _ _ _ _ _
I
II
2




2
III
2
IV
1
FIG 6 MIGRATION VELOCITY OF IONS AND CHANGE IN
CONCENTRATION WHEN ELECTRODES ARE ATTACKED
13CONCLUSION
 Fall in conc. In the anodic compartment due to
migration of Ag ions(xy)gram equivalents  Fall in conc. around cathodeIncrease in conc.
Around anodey gram equivalents  Thus, the speed ratio will be given by
 Speed of Ag ions/Speed of Nitrate ionxy/y
14TRANSPORT NUMBER
 The fraction of the total current carried by
an ion is called its transport number or
Hittorfs number.  Transport number of anion na ua

uauc  Transport number of cation nC uC

uauc
15DETERMINATION OF TRANSPORT NUMBERS BY HITTORFS
METHOD
 Hittorfs 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.  ncNumber of gram equivalent lost from the
anodic compartment  Number of gram equivalent deposited in
the voltameter
16APPARATUS FOR THE DETERMINATION OF TRANSPORT
NUMBER
MILLIAMMETER
VARIABLE RESISTANCE
EXPERIMENTAL SOLUTION
VOLTAMETER OF COULOMETER
FIG7 HITTORFS TRANSPORT NO. APPARATUS FOR THE
DETERMINATION OF TRANSPORT NO
17FACTORS ON WHICH TRANSPORT NUMBER DEPENDS
 Nature of the ion
 Nature of the other ion present
 Hydration of the ions
 Concentration
 Temperature
18CONDUCTOMETRIC TITRATIONS
(i) Titration of a Strong Acid with a Strong
Base When a strong alkali like sodium hydroxide
is titrated against a strong acid like
hydrochloric acid, the following reaction
occurs. (H Cl) (Na
OH) Na Cl H2O
A
C
CONDUCTANCE
B
EQUIVALENCE POINT
VOLUME OF NaOH ADDED (ml)
19(II)Titration of a weak acid with a strong
base When a weak acid like acetic acid is
titrated against a strong base like sodium
hydroxide, the following reaction occurs.
CH3COOHNaOH CH3COONaH2O
B
CONDUCTANCE
EQUIVALENCE POINT
VOLUME OF NaOH ADDED(ml)
20(iii)Titration of a mixture of a weak and strong
acids with a strong baseWhen a mixture of a weak
and a strong acid like acetic acid and
hydrochloric acid is titrated against a strong
base like sodium hydroxide.
D
C
A
CONDUCTANCE
B
VOLUME OF NaOH ADDED
21(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
NH4Cl H2O
A
C
B
EQUIVALENCE POINT