ELECTROCHEMISTRYPHYSICAL CHEMISTRY

B.Sc FIRST YEARSECOND 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 effectElectrophoretic 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]√CWhere 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.

Equ

ival

ent

cond

ucta

nce

√concentration c

HCI ACID

KCl

AgNO3

NaCl

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.

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

----------------------------------------------------------

------

------

------

------

------

------

------

------

------

----A C

a b

I

II

III

IV

ANODIC COMPARTMENT

CENTRAL COMPARTMENT

CATHODIC COMPARTMENT

+ + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + + + + + +_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

+ + + + + + + + + + + + +_ _ _ _ _ _ _ _ _ _ _ _ _ 2

+ + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _

2

2

2

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 con. around anode =Speed of cation

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

CASE IV:- WHEN ELECTRODES ARE ATTACKABLE

----------------------------------------------------------

------

------

------

------

------

------

------

------

------

----A C

a b

I

II

III

IV

ANODIC COMPARTMENT

CENTRAL COMPARTMENT

CATHODIC COMPARTMENT

+ + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + + + + + + +_ _ _ _ _ _ _ _ _ _ _ _

+ + + + + + + + + + + + + + + + +_ _ _ _ _ _ _ _ _ _ _ _ _ 2

+ + + + + + + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _

2

2

2

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

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

--- ---

• (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 NH4• + + Cl- + H2O

(i) Strong Acid with a Strong BaseWhen 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- + H2O