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 + - - - _ - - - + - + - - - - - - - - (a) (b) 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 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

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.

_ + 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

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.

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

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

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

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

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

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

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)

(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)

(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

(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

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