A Thermodynamic Study of Electrolytic Solutions (1920)(en)(12s)

i86

MA THEMA TICS: F. L. HITCHCOCK

PRoc. N. A -S. -

A THERMODYNAMIC STUDY OF ELECTROLYTIC SOLUTIONS By FRANK L. HITCHCOCK DIPARTMENT OF MATHUMATICS, MASSACHUSETTS INSTITUTI oF TECHNOLOGY Communicated by Edwin B. Wilson, January 30, 1920

1. Scope of the investigation.-It is well known that aqueous solutions of strong electrolytes do not conform to the requirements of the usual mass law. Measurements of freezing-point and of electric conductivity unite in showing a marked rise of the "ionization-constant" with increase of concentration. The accumulation during recent years of a considerable body of accurate data renders a theoretical examination of these relationships highly desirable. The theory of chemical potentials, due to Willard Gibbs, offers a ready tool for examining what the rigorous consequences of the dissociation hypothesis must be. From this theory may be deduced, for example: 10. An expression for the heat of dilution as a function of temperature and of concentration. 20. A rigorous equation for determining the freezing-point in terms of the concentration-nothing being assumed as to the "ideal" character of the solution. 3°. A generalization of the usual mass law. These three equations will contain certain constants, dependent on the thermic properties of the solution. Some of these constants can be calculated from specific hejLts, or from latent heats of melting or of evaporation. Others, in the present state of our knowledge, must be determined empirically. The constants in the three equations, in so far as they are generalizations of the usual equations, are the same. Hence any results obtained under one head can be checked by the others. 2. Heat developed by chemical or physical changes at constant temperature and pressure.-It is well known that when a system at constant temperature and pressure undergoes any change, the heat Q emitted during the change is connected with, the total free energy sp of the system by the differential equation 6(1) dQ d(TaT This equation is quite general. It is not necessary that the change in question be reversible. If the system consists of several phases it is not necessary that the pressure be the same for all, provided it is constant for each.3. The general coAdition of equilibrium.-It is also well known that for a system of several phases whose composition is determined by masses Mop Ml,m2, ... n, the existence of a state-of equilibrium implies the equation

Vol. 6, 1920

x87

MA THEMA TICS: F. L. HITCHCOCK