Entropy Crlterlon tor Equlllbrlum To the Editor:
In "The Entropy Criterion for Chemical Equilibrium" Norman 0.Smith has made some interesting and important points (I). However, he missed a good opportunity to show that the entropy criterion applies to equilibrium under both sets of constraints. Appropriately, Smith has shown that the entropy Sof the system for the reaction of H2 with 1 2 to form HI goes to a maximum for equilibrium when the volume and internal energy of the system are constant. As he notes, such entropy calculations are rarelv illustrated. Then. under the different. but more common, constraint of constant temperature and Dressure for the svstem. he shows that the Gibhs function (: for the system goes through a minimum at equilibrium. The point of the maximum in the e n t r o ~ of v the svstem does not coincide with equilibrium under these condi1) depending on the hardness of the-solvent relative to the constant softness of HgZ+.Water (protonic part) is harder than acetonitrile and dimethyl sulfoxide and, thus, the reaction goes more to the right for water than for the other two, as indicated in the article. Thus, the aoftness of the Hg2+ ion does not need to he varied to account for the positions of the equilibria. muglas S. Rustad Sonama State University Rohnelt Park. CA 94928
