The Ionizing Power of Solvents Related to the Unsaturated Condition of their Simple Molecules and to their Power of Association.
In liquid ammonia we might well have, for instance, the action +NH3-NH3− + HCl ⇄ +(NH3-NH3)H + Cl−, or +NH3-NH3− + H+Cl− ⇄ +(NH3-NH3)H + Cl−. Now, in liquid ammonia, the salts NH4Cl, NH4NO3 [or, more probably, (NH3)xHCl, (NH3)x,HNO3] have the functions of the aqueous acids[109]; that is, the hydrogen-ion of the acids is found combined with the solvent ammonia. The ion +(NH3-NH3)H, and similar ions in liquid ammonia, would correspond then to what is considered the hydrogen-ion in aqueous solutions[110] (formed according to HCl ⇄ H+ + Cl−, as ordinarily written), and the polarized charges on molecules like +NH3-NH3− appear thus as possible active agents in this dissociation of the hydrogen chloride molecules. [p066]
Now, it is a significant fact that all the best ionizing solvents are compounds whose simple molecules are unsaturated exactly like those of ammonia; this is true for water H2O, the unsaturated character of whose oxygen atom is now universally recognized. It is now a familiar fact that liquid water is not represented by the formula H2O but consists of more complex molecules (H2O)n. According to the most recent investigations,[111] while steam is H2O, or monohydrol, ice is trihydrol (H2O)3, and liquid water, at ordinary temperatures, a mixture consisting chiefly of dihydrol (H2O)2, some trihydrol, and very little monohydrol. The proportion of the last appears to increase with a rise of temperature; the proportion of trihydrol seems to increase with a fall in temperature. One can easily see how such aggregates would result from the saturation of the free charges on oxygen, by further molecules of water. One can also see that such an association of water molecules could leave a positive and a negative charge on the associated molecules, which would be polarized and more effective than the simple molecule would be.
That the molecule of hydrogen cyanide contains a similarly unsaturated atom was demonstrated by Nef.[112] He proved that the behavior of hydrocyanic acid agrees with the structure expressed by the formula HN═C═, which we may well write HN═C±. In sulphur dioxide, another good ionizing solvent, we have, similarly, unsaturated sulphur, the sulphur atom being here quadrivalent, whereas its maximum valence is six.
Now, the ionizing power of solvents like water, ammonia, etc., has been ascribed, by various chemists, not only to their dielectric properties, but also to the unsaturated condition of their molecules, and particularly to their powers of association into large molecules. The relations developed suggest that all three properties are most intimately related, the dielectric properties and the powers of association being consequences, possibly, of the fundamental condition of unsaturation, and of the great tendency toward self-saturation,[113] of the simple molecules of the best ionizing solvents. From Walden's work it appears that the dielectric constant finally determines the quantitative ionizing effect of a solvent.
Chapter IV Footnotes
[49] From the molecular weights of elements and compounds, the atomic weights of elements may be determined, with the aid of analysis. (Cf. Smith, Inorganic Chemistry (1909), p. 196, or General Chemistry for Colleges (1908), p. 130 (Stud.), or Remsen, Inorganic Chemistry, Advanced Course (1904), pp. 71–80 (Stud.).)
[50] The weight of a small volume of a gas or vapor, at any definite temperature and pressure, is determined. With the aid of Boyle's and Gay-Lussac's laws, this observed volume is then reduced to standard conditions. Finally, the weight of 22.4 liters, under standard conditions, is obtained by calculation.
[51] For the deduction of formulæ see Smith, Inorganic Chemistry, 196, 203; College Chemistry, 40; or Remsen, ibid., p. 79 (Stud.).
[52] Kopp, Liebig's Ann., 105, 390 (1858); Kékulé, ibid., 106, 143 (1858) (Stud.).