A Comment on "Debye's Activity Coefficient: In Which Concentration Scale?" To the Editor:
Morel's article [J. CHEM.EDUC.,56,246 (1980)l reminded me of another reliable source, "TextBook of Physical Chemistry" by S. Glasstone, Second Ed., D. Van Nostrand Company Inc., 1954. I quote from page 960: RELATIONSHIP BETWEEN ACTIVITY COEFFICIENTS -The derivation of equation (131) is based on the assumution that an ideal solution~ohevsRaoult's Law or Henrv's L& (cf. equation (125)); hence, the activity coefficient derived from the Debye-HkkeI treatment is J, (cf p. 6851, sometimes called the rational oetiuity coefficient. In connection with solutions of electrolytes it is more convenient to use the activity coefficients f , and J,, referred to the ideal dilute solution; the latter, as already seen, is usually represented by the symbol y. [On pages 683-6116 and 960-961,) Glasstone discusses all pertinent points raised hy Morel. B. D. Sharma Los Angeies Pierce College Woodland Hill. CA 91371
To the Editor:
I n the paper entitled "Dehye's Activity Coefficient: In Which Concentration Scale?" [J. CHEM. EDUC.,56, 246 (1980)l I have tried to indicate one possible argument leading to the choice of a concentration scale for the activitvcoefficient of ions calculated from Dehye-Hiickel theory. There is not, however. a uniaue and final answer to the auestion asked in that title.
I n the scone of the teachine of classical thermodvnamics. w r w y : t n d I did n.jt ~ W ~ Pt r < ~ I h ImU~~. I L I Ilk+ . ~ I , , t.,kc. i h ~ ts~ ..i w < , r ~ ~ ~ hn ~i t, sw. \ c r 1, , )
clarify some points. When teaching classical thermodvnamics, we usually first look a t the mixtures thus allowing us-to introduce the concept of ideality based, implicitly or not, on the statistical model of the mixture of identical hut distinguishable particles. This leads to Raoult's law as the ideal law. When, later on, we look a t the solutions, it becomes necessary to make use of practical concentration scales: molality, molarity. We are then led to define various "ideal 1aws"for the variations of the chemical potential of the solute with the concentration
whereas, in fact, it is only a question of choosing various reference states. One must realize that the concept of ideality is not an absolute one: an ideal law is generally based on a precise model and must predict the behavior of the systems when the ideal conditions. defined hv the model. are filled (ex.. . -. the solute concentration tending toward zero). When we study dilute solutions, in particular the electrolytic ones, Raoult's law is surely not the best ideal law for the solvent; it has simply the advantaee of beine the most well known and the onlv one which is d&ussed in classical textbooks. Consequently, Henry's Law.
w,
= wLe
+ R T In x ,
is not the best ideal law for the solute. Statistical mechanics oronose a better model than the "ideal mixture" concept to describe the behavior of a solute in an infinitelv dilute solution. When we brine it hack to a classical form, thk expression for the chemical potential becomes
Volume 60
Number 1
January 1983
87
