In the following table, the relative activities of acids, in accelerating the decomposition of methyl acetate by water, are contrasted, in a similar fashion, with their relative conductivities. The conductivities of acids depend to such an extent on the concentration of hydrogen-ion, which moves five times, or more, faster than the anions and carries therefore the greater part of the current, that the conductivities of acids, in equivalent concentrations, may be considered an approximate measure of their relative degrees of ionization and of the concentrations of hydrogen-ion. For the purpose of comparison the activity and the conductivity of molar hydrochloric acid are both represented by 100. All the acids were used in normal solutions.
Chemical Activity of Acids and Their Ionization.[A]
| Acid. | Activity. | Conductivity. |
|---|---|---|
| Hydrochloric acid | 100 | 100 |
| Nitric acid | 92 | 100 |
| Sulphuric acid | 74 | 65 |
| Acetic acid | 0.3 | 0.4 |
| Formic acid | 1.3 | 1.7 |
| Chloracetic acid | 4.3 | 4.9 |
| Tartaric acid | 2.3 | 2.3 |
[A] Whetham, Theory of Solutions, p. 338.
In the following chapters and, indeed, throughout our further work, we shall continually meet additional instances of the quantitative relation between chemical activity and ionization. In fact, the results obtained in the field of quantitative measurement of chemical action, of which the above are single instances, have demonstrated, more than anything else, the value of the theory of ionization to chemistry and the necessity of taking it into account in expressing the results of chemical action in mathematical terms. [p083]
Summary.
Chemical Activity of Non-ionized Molecules.
On the other hand, there are large numbers of compounds, especially among organic substances, which do not appear to ionize to a measurable extent and whose actions, in large part at least, appear to be the actions of non-ionized molecules. It is characteristic that most of these actions take place at slow, very frequently easily measurable, rates of speed. Critical study shows that even for such actions ionization often plays a very important rôle, at least in some of their stages, and throughout the field of organic chemistry the intimate relations between electrical phenomena and chemical activity can be readily recognized.[146] But these relations are not obvious ones and do not, as yet, play a dominant rôle. It is because analytical chemistry deals predominantly with the reactions of ionogens, that the study of the reactions of ions will demand our extended attention.
Reactions in Non-aqueous Solutions.
It is evident,[150] from Walden's equation showing the relation between the ionizing power and the dielectric constant of a solvent (p. [63]), that the presumption is that hydrogen chloride in benzene solution is not absolutely non-ionized, but rather that it is ionized in traces.[151] No exact measurements of the [p085] degrees of ionization of hydrogen chloride in benzene solution have been made; that the solution shows an enormous resistance to the passage of the electric current and can be, at best, very little ionized, is all that has been established. In default of exact data, the semiquantitative determination by Kablukoff, showing that a 0.25 molar solution of hydrogen chloride in benzene has a resistance of 120 × 106 ohms, is of interest. From the meager data concerning the dimensions of the electrodes used, one may calculate (with the aid of a not unreasonable assumption as to the limiting value of the conductivity, at infinite solution) that the degree of ionization of the acid in the solution is perhaps of the order 5E−9, and the concentration of hydrogen-ion,[152] consequently, roughly 10−9. Now, the evolution of hydrogen by means of zinc, in aqueous solutions, takes place according to the equation Zn ↓, + 2 H+ ⇄ Zn2+ + H2 ↑, and depends on a ratio of the concentrations of zinc-ion and hydrogen-ion[153] (Chapters XIV and XV, q.v.). Even if the concentration of hydrogen-ion is very small, zinc will liberate hydrogen, provided the conditions are such that the concentration of zinc-ion cannot reach a large enough value to satisfy the equilibrium ratio, and stop the action. Now, in an alkaline solution, zinc-ion is converted into zincate-ion (Zn2+ + 4 HO− ⇄ ZnO22− + 2 HOH) and a large concentration of zinc-ion cannot accumulate. The consequence is that zinc liberates hydrogen freely even from alkaline solutions, for instance from molar solutions of potassium hydroxide, in which the concentration of hydrogen-ion, roughly 10−14, is very much smaller than that calculated for the benzene solution of hydrogen chloride (namely, 10−9). Now, although the values of the solution-tension constants of elements change most decidedly with a change of solvent, it seems likely[154] that their ratios, on which their mutual displacement depends, will not be found materially altered. Zinc chloride being insoluble in benzene, the ratio for equilibrium may not be fulfilled for zinc in contact with a benzene solution of hydrogen chloride. Hence, with that solvent, the evolution of hydrogen may, so far as it goes, very well be due to precisely the same machinery as that operating in aqueous solution. The liberation goes on until the metal is protected against any further action by a film of the solid chloride. It seems, therefore, at least possible, that the evolution of hydrogen observed by Kahlenberg and his collaborators[155] is a purely ionic action, the same as the similar [p086] action in aqueous solution has been proved to be by quantitative measurements.[156] Only exact measurements, comparable with those made in aqueous solutions, can settle the question at issue, and until such quantitative evidence is forthcoming, a definite conclusion that the action of hydrogen chloride in benzene solution is or is not an ionic action is not warranted by the facts.