ELECTROCHEMISTRY PHYSICAL CHEMISTRY B Sc FIRST YEAR SECOND

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 : v. Relaxation effect or Asymmetry effect v. 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 = Λ 0 -[82. 4/(DT)1/2 ή +8. 20 X 105/(DT)3/2 λ 0]√C Where Λc =Equivalent conductance at concentration c. Λ 0 =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= Λ 0 -(A+BΛ 0)√c where A and B are constants for a particular solvent

VERIFICATION OF THE ONSAGER EQUATION v The plot of Λc vs √c should be linear. v. The slope of the line should be equal to A+B Λ 0, 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: - Ag. NO 3 Na. Cl √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. KNO 3 SOLUTION IN JELLY CHARCOAL POWDER Cu. Cr 2 O 7 SOLUTION IN JELLY (GREEN) Cu 2+ (Blue) Cr 2 O 72 - (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 + _ b a CENTRAL COMPARTMENT + + + _ _ _ + + _ _ + + + _ _ _ ----------------------------- ANODIC COMPARTMENT ----------------------------- + A C CATHODIC COMPARTMENT _ + + + + _ _ _ _ 2 + + + _ _ _ + + + 1 FIG: 5 MIGRATION VELOCITY OF IONS AND CHANGE IN CONCENTRATION WHEN ELECTRODES ARE NOT ATTACKED

CONCLUSION v 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 v No. of ions liberated on both the electrodes is equal.

CASE IV: - WHEN ELECTRODES ARE ATTACKABLE I + + _ _ II + + + _ _ III IV _ _ _ + + + _ _ _ 2 + + + _ _ _ a CENTRAL COMPARTMENT + + + + + _ _ _ _ _ b ----------------------------- ANODIC COMPARTMENT ----------------------------- + A CATHODIC COMPARTMENT + + _ _ 2 + + _ _ 2 + _ + + + + + _ _ 1 FIG: 6 MIGRATION VELOCITY OF IONS AND CHANGE IN CONCENTRATION WHEN ELECTRODES ARE ATTACKED C _

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 u a +u c Transport number of cation n. C= u. C 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- + H 2 O • • • (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 NH 4 OH. H+Cl- + NH 4 OH NH 4 + + Cl- + H 2 O