Debye-Hiickel, interionic attractions

According to the interionic attraction theory of Debye and Hiickel... 

P. Debye and E. Hiickel, The Interionic Attraction Theory of Deviations from Ideal Behavior in Solution, Z. Phys. 24 185 (1923). 

Since the activities of ions and other solutes approach their concentrations as the solutions are made more and more dilute, activity coefficients must approach unity as a limit, However, to adjust the scale of activity coefficients so that they approach unity at infinite dilution, it is necessary to make non-thermodynamic assumptions as to the trend below the concentrations at which they can be measured. A basis for such assumptions is given by the Debye-Hiickel theory of interionic attractions which will be discussed in the next chapter. 

The Electrophoretic Effect. According to the Debye-Hiickel theory an ion is surrounded by an ionic atmosphere distributed with radial symmetry around the ion as center. This ion atmosphere, it will be recalled, is due to the fact that interionic attractions and repulsions tend to produce a slight preponderance of negative ions in the vicinity of a positive ion, and vice versa. Although the ion atmosphere is treated as a reality in mathematical discussions it actually is the result of a time average of a distribution of the ions. Each ion serves as a center of an ion atmosphere, and the relative position of each ion with respect to the other charged bodies in the solution influences the atmospheres of all the other ions. 

Debye-Hiickel theory /de-bay-e hoo-kSl/ A theory to predict the conductivity of ions in dilute solutions of strong ELECTROLYTES. It assumes that electrolytes in dilute solution are completely dissociated into ions but takes into account interionic attraction and repulsion. Agreement between the theory and experiment occurs only with very dilute solutions (less than lO M). See dissociation. 

In 1923, Peter Debye and Erich Hiickel presented their theory of interionic attraction which has been the foundation for the work done since then. Since strong electrolytes are highly dissociative in solution, the ion concentration is higher with the resulting distance between them smaller than for weak electrolytes. This increase in concentration results in the tendency towards an orderly distribution of ions as the electrostatic forces cause mutual attraction between oppositely charged ions. The potential energy of the ionic attraction must therefore be accounted for in considering electrolyte solutions. 

The interionic attraction theory provides a complete explanation for the concentration dependence of conductances in very dilute solutions. Debye and Hiickel s original treatment was improved by Onsager in 1926 and his equation has been convincingly tested over a wide range of conditions. 

The symbol A (or A°) represents the maximum theoretical value that the molar conductivity of an electrolyte will approach when diluted indefinitely with an inert solvent. At the beginning of this century Kohlrausch found that the molar conductivity of salts in very dilute aqueous solutions showed a linear relation with the square root of the concentration. This, Kohlrausch s square root law , was incompatible with the Arrhenius electrolytic dissociation theory (q.v.), but it has since been justified by the Debye-Hiickel-Onsager theory of interionic attraction effects, which have been shown to have a dependence. 

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