1. ELECTROCHEMISTRY PHYSICAL CHEMISTRY B.Sc FIRST YEAR SECOND 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
FIG:1

5. ELECTROPHORETIC EFFECT_ _ _ + _ _ _ _
FIG:2

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( ) 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

7. VERIFICATION OF THE ONSAGER EQUATION
Two tests can be readily performed to verify the onsager equation.These are:-
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
AgNO3
Equivalent conductance
NaCl
√concentration c
FIG:3 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)
FIG:4 DEMONSTRATION OF THE MIGRATION OF IONS

9. 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.

10. _ + Case 1:When only anion moves.
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 C
CATHODIC COMPARTMENT A
ANODIC COMPARTMENT
CENTRAL COMPARTMENT b 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 conc. around anode = Speed of cation
Fall in conc. Around cathode speed of anion
No. of ions liberated on both the electrodes is equal.

12. CASE IV:- WHEN ELECTRODES ARE ATTACKABLE
CATHODIC COMPARTMENT A
ANODIC COMPARTMENT
CENTRAL COMPARTMENT b 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 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

14. 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

15. DETERMINATION OF TRANSPORT NUMBERS BY 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

16. FACTORS ON WHICH TRANSPORT NUMBER DEPENDS
Nature of the ion
Nature of the other ion present
nCl- IN HCL= uCl-
uH+ + uCl_
nCl_ IN NaCL= uCl-
uNa+ + uCl_
Hydration of the ions
Temperature

17. Concentration CdI2 Cd2+(aq)+2I- (aq) Cd2+ (aq) + 4I-(aq) CdI 2-4 (aq)_ + _ +
Cd2+
IMAGINARY PARTITION
FIG:7 ELECTROLYSIS OF CONCENTRATED CdI2 SOLUTION

18. APPARATUS FOR THE DETERMINATION OF TRANSPORT NUMBER
MILLI-AMMETER
VARIABLE RESISTANCE
EXPERIMENTAL SOLUTION
VOLTAMETER OF COULOMETER
FIG:8 HITTORF’S TRANSPORT NO. APPARATUS FOR THE DETERMINATION OF TRANSPORT NO

19. 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)

20. (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.
CH3COOH+NaOH CH3COONa+H2O B
CONDUCTANCE
EQUIVALENCE POINT
VOLUME OF NaOH ADDED(ml)

21. (iii)Titration of a mixture of a weak and strong acids with a strong base:When 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

22. (iv) Titration of 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
CONDUCTANCE B
EQUIVALENCE POINT
VOLUME OF NH4OH ADDED

23. (v)Precipitation titration
When silver nitrate is titrated against potassium chloride solution. C A B
CONDUCTANCE
VOLUME OF KCL ADDED(ml)