[113] Cf. Walden, Z. phys. Chem., 55, 683 (1906).
CHAPTER V THE THEORY OF IONIZATION. II Ionization and Osmotic Pressure. Ionization and Chemical Activity
We will turn now to the consideration of evidence bearing on the theory of ionization, found in the data on osmotic pressure. The apparent molecular weight of hydrogen chloride is found to be smaller than 36.5, when determined in aqueous solution (p. [37]), and it is found to approach the limit 18.25 as a more and more dilute acid is used.[114] The value found represents the average molecular weight of all the molecules in any solution, the osmotic pressure, freezing-point or boiling-point of which has been taken. It is evident that, if there is dissociation of hydrogen chloride into hydrogen and chloride ions, the average values found for the molecular weight must be lower than 36.5, must be variable, and must approach the limit 18.25, as the dissociation into the smaller molecules becomes more and more complete. Such a result is, therefore, what we would anticipate on the basis of the theory of ionization. For a salt like potassium chloride KCl, a similar tendency toward a minimum, average molecular weight of (K+ + Cl−) / 2 or (39.1 + 35.5) / 2 = 37.3 would be anticipated, and, as a matter of fact, molecular weight determinations with potassium chloride in aqueous solution give results agreeing with such a tendency.[115] For a salt like calcium chloride, on the other hand, we would expect that its ionization into three ions, according to the equation CaCl2 ⇄ Ca2+ + 2 Cl−, would give a minimum, not of one-half the formula weight, but of one-third, viz., (Ca2+ + 2 Cl−) / 3 or (40 + 71) / 3 = 37, when the molecular weight determination is carried out in aqueous solution. As a matter of fact, with salts of this type, the determinations, by osmotic pressure methods, indicate a dissociation into three smaller components, as required by the theory. It may be added that, for [p068] a salt, sodium mellitate, Na6(C12O12), the salt of a hexabasic acid, Taylor found average molecular weights tending to a minimum of one-seventh of the formula weight, as we should expect from the ionization of the salt into seven smaller molecules, (C12O12)Na6 ⇄ 6 Na+ + (C12O12)6−.
Quantitative Evidence.
Loomis[119] found in a similar way a ratio of 3.61 for HCl, 3.71 [p069] for KOH, 3.60 for KCl, 3.67 for NaCl, 3.73 for HNO3, etc., when 0.01 molar aqueous solutions were used. For similar solutions of calcium chloride CaCl2, magnesium chloride MgCl2, and sodium sulphate Na2SO4, the value 5.07 was found as the ratio between the depressions of the freezing-point and the concentration of the salts in extremely dilute solutions—a result showing, plainly, a dissociation of each salt into three smaller molecules. The limit 5.67 for such a dissociation is not quite reached in these cases, because salts of the types Me″X2 and Me2′Y″ ionize less readily than do the electrolytes Me′X′, a fact also shown by their conductivities.
We thus find that the most exact work on molecular weight determinations in dilute aqueous solutions agrees excellently, as does the conductivity of such solutions, with the demands of the theory of ionization, a fact which is particularly impressive because osmotic pressure and electrical conductivity are in no wise fundamentally related phenomena, and yet each, as a measure of ionization or electrolytic dissociation, leads independently to the same conclusion.[120]
The Chemical Composition of the Ions of Electrolytes.
The compounds which are dissociated into ions, by solvents which cause ionization (p. [62]), comprise the salts, the acids and the bases; chemists are, in fact, more inclined now to invert the statement and say that those substances which have long been known as salts, acids and bases, owe the essential characteristics, which led to their classification, to the fact that they are ionizable (see pp. [72]–[82]). The composition of the ions, formed from the simpler of these compounds,[121] may be expressed by saying that the metal component or metal-like component (hydrogen, ammonium) forms the positive ion (cation, metal ion) and all the rest of the [p070] molecule forms the negative ion (anion, acid ion[122]). Thus, sodium chloride, nitrate, sulphate, phosphate yield the sodium-ion, Na+, and the chloride (Cl−), nitrate (NO3−), sulphate (SO42−), or phosphate (PO43−) ions; cupric nitrate Cu(NO3)2 dissociates into the cupric ion (Cu2+) and the nitrate ion, calcium sulphate CaSO4 into the calcium-ion (Ca2+) and the sulphate-ion, aluminium sulphate Al2(SO4)3 into the aluminium-ion (Al3+) and the sulphate-ion.
But the question arises, as to how we know that the salts mentioned produce ions of the given composition; why, for instance, should sodium nitrate be considered to dissociate into sodium, Na+, and nitrate ions, NO3−, the nitrogen atom carrying all of the oxygen atoms with it in the negative ion?