Lesson 15. KOHLRAWSCH LAW ANS OSTWALD'S DILUTION LAW AND ELECTRIC CONDUCTANCE OF MILK

Module 7. Electrical conductivity

Lesson 15
KOHLRAWSCH LAW ANS OSTWALD'S DILUTION LAW AND ELECTRIC CONDUCTANCE OF MILK

15.1 Introduction

It is now evident from the earlier studies that electrolytes dissociate into charged particles known as ions and they play an important role in various chemical reactions. Their behaviour at different dilutions needs to be assessed to understand the degree of ionization and their mobility. The Ostwald’s dilution law and Kohlransch’s Law explain the behaviour of these ions and help in calculating their ionic mobility.

15.2 Ostwald's Dilution Law

This law is concerned with the relationship between the dissociation constant and the degree of dissociation of a weak electrolyte (acids or bases).

For any weak electrolyte Ostwald’s dilution law states that the degree of dissociation is inversely proportional to square root of the molar concentration and is directly proportional to the square root of the volume containing one mole of electrolyte

15.1

Where:
Kp = constant of protolysis
α = degree of dissociation (or degree of protolysis)
C(A-) = concentrations of anions
C(K+) = concentration of cations
C0 = overall concentration
C(KA) = concentration of associated electrolyte

Concerning conductivity, this results in the following relation:

Where:

Kc = constant of dissociation
Λc = equivalent conductivity
Λ0 = boundary conductivity
C = concentration of electrolyte


According to Arrhenius theory of electrolyte dissociation, the molecules of an electrolyte in solution are constantly splitting up into ions and the ions are constantly reuniting to form unionized molecules. Therefore, a dynamic equilibrium exists between ions and unionized molecules of the electrolyte in solution. It was pointed out by Ostwald that like chemical equilibrium, law of mass action can be applied to such systems also.

Consider a binary electrolyte AB which dissociates into A+ and B- ions and the equilibrium state is represented by the equation:

15.11

Thus, degree of dissociation of a weak electrolyte is proportional to the square root of dilution.


15.3 Limitation of Ostwald's Dilution Law


The law holds good only for weak electrolytes and fails completely in case of strong electrolytes. The value of 'α' is determined by conductivity measurements by applying the formula Λ/Λ∞. The value of 'α' determined at various dilutions of an electrolyte when substituted in Eq. (i) gives a constant value of K only in the case of weak electrolytes like CH3COOH, NH4OH, etc. the cause of failure of Ostwald's dilution law in case of strong electrolytes is due to the following factors"

  • The law is based on the fact that only a portion of the electrolyte is dissociated into ions at ordinary dilution and completely at infinite dilution. Strong electrolytes are almost completely ionized at all dilutions and Λ/Λ∞ does not give accurate value of 'α'.
  • When concentration of the ions is very high, the presence of charges on the ions appreciably affects the equilibrium. Hence, law of mass action its simple form cannot be strictly applied in case of strong electrolytes.

15.4 Kohlransch's Law


At infinite dilution, when dissociation is complete, each ion makes a definite contribution towards equivalent conductance of the electrolyte irrespective of the nature of the ion with which it is associated and the value of equivalent conductance at infinite dilution for any electrolyte is the sum of contribution of its constituent ions", i.e., anions and cations.

/\ = λa + λc

Where λ a is the equivalent conductance of the anion and λ c that of the cations.

The ionic conductances are proportional to their ionic mobilities. Thus, at infinite dilution,

λc = kuc and λa = kua

Where uc and ua are ionic mobilities of cation and anion respectively at infinite dilution. The value of k is equal to 96500 c, i.e., one Faraday.

Thus, assuming that increase in equivalent conductance with dilution is due to increase in the degree of dissociation of the electrolyte, it is evident that the electrolyte achieves the degree of dissociation as unity when it is completely ionized at infinite dilution. Therefore, at any other dilution, the equivalent conductance is proportional to the degree of dissociation. Thus,

Degree of dissociation α = /\ / ( /\ )

= (Equivalent conductance at a given concentration) / (Equivalent conductance at infinite dilution)

Last modified: Tuesday, 23 October 2012, 7:15 AM