Comments on the Criterion of Spontaneity This note is to draw attention t o the incomplete or misleading presentations sometimes made in connection with the
entha$y change for the reaction Pb(s)
+ HgzCIds) = PhClAs) + 2HgU
is -22.730 eal/mole a t 298'K, yet, when the reaction is run reversibly in a n electrochemical cell, 1980 callmole is actually absorbed.' The criterion of spontaneity and the related statement that the maximum amount of useful work a system can do in an isothermal and isobaric reaction is equal to the drop in its Gibbs energy (1 W,.M 5 1AG1),are sometimes proven by evoking the principle that maximum work is done in a reversible process. This principle is a consequence of the Second Law. But on oecasion it is presented prior t o the Second Law and justified with mechanical models (gas e x p a n s i ~ n )If. ~the proof of thecriterion ofspontaneityii.$tdbebased an I W,& W,I ,{itshouldlbe stressed that(this,equality;arises from.the Second Law. Otherwise the criterion of spontaneity may appear to he derivable from the First Law alone and the alert student may be led to believe that the Second Law is unessential for much of chemical thermodynamics.
' Lewis, G. N., Randall, M., Pitzer, K. S., and Brewer, L., "Thermodynamics,"
In five of the 25 general and physical chemistry hooks surveyed. California State University Long Beach. CA 90840
McGraw-Hill, Ine., New York, 1961.
N. M. Senozan
Volume 56. Number 6,June 1979 1 381
