QS Study

Debye-Huckel-Onsager Theory

A theory providing quantitative results for the conductivity of ions in dilute solutions of strong electrolytes, which enables the Kohlrausch equation to be derived. Based on his analysis of the data available and his own experimental results the following empirical equation relating molar conductance (he was using equivalent conductance) with concentration was put forward by Kohlrausch-

Λ = Λ0 – A√C … … … (1)

where A is a constant. The constant A is characteristic of the electrolyte. The first satisfactory theoretical interpretation of the equation, however, was given in 1920 by Debye, Huckel and Onsager, and the constant A in Kohlrausch’s equation became meaningful.

Development of the study of crystals by X-rays and the electronic theory of valency led to the view that when a salt, strong acid a strong base is dissolved in water these are completely ionized. In the Debye-Huckel-Onsager theory this is accepted as a fact and the increase of molar conductance with dilution is explained in terms of decreasing inter-ionic attraction at low concentration.

Debye-Huckel-Onsager Theory is based on the following Assumptions;

  • The solute is completely dissociated; it is a strong electrolyte. Strong electrolytes are completely dissociated into ions.
  • Due to Coulornbic forces between the charges of the ions, they do not behave like molecules in their transport and thermodynamic properties.
  • Since the solution as a whole is neutral, the total number of positive charges must be equal to the total number of negative charges.
  • A given ion will have more ions of the opposite sign close to it than ions of the same sign. This cluster of ions is called the ionic atmosphere.
  • Ions are spherical and are not polarized by the surrounding electric field. Solvation of ions is ignored except insofar as it determines the effective sizes of the ions.
  • There is no electrostriction.