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ELECTROCHEMISTRY PHYSICAL CHEMISTRY B.Sc FIRST YEAR SECOND SEMESTER

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Title: ELECTROCHEMISTRY PHYSICAL CHEMISTRY B.Sc FIRST YEAR SECOND SEMESTER


1
ELECTROCHEMISTRYPHYSICAL CHEMISTRY B.Sc FIRST
YEARSECOND SEMESTER
2
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.

3
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

4
RELAXATION EFFECTS OR ASYMMETRY EFFECTS

-
-
-
_
-
-

-

-

-
-
-
-
-
-
-
-
(a)
(b)
Symmetrical ionic atmosphere around a positive ion
Ionic atmosphere becoming asymmetrical when
central ion moves
FIG1
5
ELECTROPHORETIC EFFECT
_
_
_

_
_
_
_
FIG2
6
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.

?c ?0-82.4/(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
8
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)
FIG4 DEMONSTRATION OF THE MIGRATION OF IONS
9
HITTORFS 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.
10
WHEN 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
11
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.

12
CASE 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
13
CONCLUSION
  • Fall in conc. In the anodic compartment due to
    migration of Ag ions(x-y)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 ionx-y/y

14
TRANSPORT 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

15
DETERMINATION 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

16
APPARATUS FOR THE DETERMINATION OF TRANSPORT
NUMBER
MILLI-AMMETER
VARIABLE RESISTANCE
EXPERIMENTAL SOLUTION
VOLTAMETER OF COULOMETER
FIG7 HITTORFS TRANSPORT NO. APPARATUS FOR THE
DETERMINATION OF TRANSPORT NO
17
FACTORS ON WHICH TRANSPORT NUMBER DEPENDS
  • Nature of the ion
  • Nature of the other ion present
  • Hydration of the ions
  • Concentration
  • Temperature

18
CONDUCTOMETRIC 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
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