An APPROACH to the MODERN THEORY of SOLUTIONS JOHN R. SAMPEY Howard College, Birmingham, Alabama
The approach to the theory of solutions i s a problem The author of one will remind the instructor of the conwhich presents itself to mery instructor of elementary fusion of thought he experienced in dealing with anomaanalytical chemistry. Recent textbooks rejfect radically lous strong electrolytes, and follow this with a plea different points of view, ranging from pleas not to teach not to teach the classical theory of ionizatiou, since he the classical theory of ionization to frank statements of will have to uuteach part of it to reach the newer conomission of all reference to modern concepts of strong cepts. On the other hand, another text will state electrolytes. The present article directs the student's at- frankly that no effortwill be made to present the modem tention to some of the more significant limitatias of theory of solutions, since this material belongs more Arrhenius' theory, and then suggests several avenues of properly to a course in physical chemistry. approach to the ion-attraction theory. To one who has attempted to teach the usual one-year
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NE of the most difficult problems facing the instructor in qualitative analysis, or an elementary course in quantitative analysis, is the approach to the theory of solutions. The radically different points of view on the question are well reflected in the prefaces to recent texts of analytical chemistry.
course in physical chemistry for undergraduates, any effort will be most welcome which can be made to strengthen the background for the wide range of topics covered in that course. The analytical courses should be peculiarly adapted to the teaching of the important branch of physical chemistry covered by solutions, for a more exact knowledge of this field will not only enable the student to obtain better analytical results
but, what is equally significant, an introduction to the development of this very modern phase of chemistry wiU help him maintain a keenly critical attitude of mind through a year or two that all too often degenerates into the dull routine of mastering the tedious technic of that division of chemistry which has been termed the "handmaid of the science." The writer h d s himself equally unsympathetic toward the attitude that the classical theory of ionization should be ignored. In the first place, the student through his high-school and first-year college chemistry is already acquainted with Arrhenius' theory. In the second place, the fundamental concepts of Arrhenius hold for the behavior of weak electrolytes in very dilute solution. And finally, the growth of the modern theory can be traced most logically by beginning with the limitations of the classical theory, and from these proceeding to outline the points of view emphasized today. At the risk of being pedantic, let us present a brief of the case. The anomaly of strong electrolytes is the most serious limitation of the Arrhenius theory. This is made more striking after it has been shown how the degree of ionization may he determined for a weak electrolyte by measurements from the three fields of chemistry represented by electrical conductivity, osmotic pressure (and the related phenomena of freezing points, boiling points, and vapor pressure), and chemical activity. The concordance of results from these independent methods, after accounting for certain disturbing influences, makes a remarkable argument for the validity of the Arrhenius assumptions concerning partial ionization of weak electrolytes. The anomalous behavior of strong electrolytes, on the other hand, can be demonstrated from numerous attempts to determine the ionization constants of strong acids and bases. The classical theory may he shown to remain valid only for very dilute solutions of weak electrolytes. The seriousness of this limitation may be seen from data on solutions approaching the concentrations of those used frequently in qualitative and quantitative analysis. Perhaps the most interesting limitations from the student's point of view may be drawn from the rich field of hydrogen-ion catalysis. The surprising effects of neutral salts on the rates of such familiar reactions as the inversion of cane sugar and the formation of an ester are sufficient to stimulate further inquiry. Why should the presence of a tenth-normal aqueous solution of a neutral salt increase twofold the dissociation of a weak acid catalyzing these reactions? And when ethyl alcohol is employed as the solvent, why should the same concentration of a neutral salt cause the dissociation of the acid to be about ten times as great as calculated from the classical law of mass action? Furthermore, if dilution increases ionization (as stated in one of the postulates of the classical theory), why should the rate of a reaction catalyzed by hydrogen ions be decreased on dilution of the acid? Goldschrnidt in 1895 was the first to observe the anticatalytic effect of water; the interested student will
find many other illustrations of this phenomenon. To undertake anything approaching a survey of the whole battlefield of hydrogen-ion catalysis would prove too ambitious a program for even the superior student during his year or two of analytical chemistry, but an acquaintance with the rise and fall of some of the more important theories advanced in answer to the above questions will serve as one avenue of approach to the modern theory of solutions. It was a long road from Arrhenius' original assumptions of "activated molecules through the theories of Armstrong and Jones on hydration of ions, of Acree and Senter on the dual theory of catalysis, of Lapworth and Rice on unhydrated hydrogen ions, of Stieglitz and Kendall on intermediate compound formation, down to the thermodynamic points of view of Br$nsted, La Mer, and Hamed. The rival claims made by some of the proponents of the several theories will enliven for the student more than one otherwise dull article. In scarcely any other field of chemistry today will one find such accusations as the misrepresentation of data in thirty-nine out of seventy-five cases, or the charge that one of America's leading physical chemists has not yet familiarized himself with modern conceptions in the field of solutions. Another avenue of approach to the modern theory of solutions is through the stressing of the additive properties of strong electrolytes. The additive nature of the chemical properties of the ions has been the basis of every scheme of qualitative analysis. The additivity of such physical properties as color, specific volumes, heat capacities, heats of reaction, refractive indices, and viscosities are not so well recognized. And it is only recently that emphasis has been laid upon the close agreement of the observed values for the conductivities of strong electrolytes and those calculated from the principle of additivity. Final acceptance of the complete dissociation of strong electrolytes was brought about through the X-ray study of salts and the formulation by Debye and Hiickel of a theoretical foundation for the ion-attraction theory. The classical discoveries of Laue and W. H. and W. L. Bragg on the arrangements of the atoms in the crystal lattice of a salt left no ground for the former assumption that a crystalline true salt contained molecules of that salt. It remained for Dehye and Hiickel to give the mathematical interpretation for the attraction and repulsion of the ions in a solution of a strong electrolyte. The actual experimental determinations of the activities of different ions under varying conditions and concentrations in aqueous and non-aqueous solutions are today being actively prosecuted, and the formulation of a comprehensive theory of hydrogen-ion catalysis is by no means the least triumph of the ion-attraction theory. While a fuller appreciation for these contributions can come to the student only after a course in physical chemistry, the approach to the modern theory of solutions through these avenues has served the student of elementary analytical chemistry in more ways than one.
