The dissociation of acids in methyl and in ethyl alcohol

Abstract

A comparison-of the electrical conductivities of aqueous solutions of salts and acids reveals a striking contrast between these two classes of substances as regards their electrolytic dissociation. In water almost all salts are largely dissociated into ions and, when no complications are introduced by hydrolysis, the conductivities of dilute salt solutions show remarkably good agreement with the theory of Debye and Hüekel as modified by Onsager. Acids, on the other hand, show a wide variation in their degree of dissociation and very few behave as strong electrolytes. This difference in behaviour between salts and acids is usually explained by the properties of the proton, which, unlike other cations, has no screen of electrons round the nucleus, and therefore is assumed to be incapable of existing uncombined in solution, There is much evidence direct and indirect in support of this view, and it seems probable that in water the proton is united with one molecule of solvent to form what Brönsted calls the oxonium ion OH 3 + . In fact, the degree of dissociation of an acid probably depends on the relative affinity of the proton for the anion and for the solvent molecule. Brönsted has developed this idea in a series of recent papers on acid and basic catalysis and considers both the anion and the solvent molecule as bases in consequence of their power of combining with protons. If the ionisation of acids is dependent on the chemical attraction of the solvent molecule for the proton, it is of interest to know how the ionisation varies in different solvents. Goldschmidt has made a detailed study of the conductivity and catalytic properties of certain acids in methyl and ethyl alcohol, and finds that the halogen hydracids behave as strong electrolytes both in aqueous and alcoholic solutions, whereas weak acids such as salicylic acid become much weaker in alcohol, their dissociation constants in methyl alcohol being smaller than in water by a factor of 10 -5 . Small additions of water to the alcoholic solutions of the halogen hydracids cause a relatively large diminution in their conductivity and catalytic properties in consequence of the reaction:— H + . ROH + H 2 O ⇆ H + . H 2 O + ROH the water molecule having a much stronger affinity than the alcohol for the proton. This accounts also for the dissociation of weak acids being greater in water than in alcohol.