Dennition of Standard States
T o the Editor: Omer Robbins, Jr. showed conclusively (J. CHEM. Enuc., 48,737 (1971)) that the definition of E n in most freshman chemistry tcxts is incorrect. The question now is "What can the textbook writer do?" since thc definition proposed in the article is probably beyond the comprehension of freshman. The first point to realize is that it is the definition of the standard state, and not that of E " , that is wrong. "E" is the electromotive force of the cell in its standard state" is a correct definition and can be comprehended and remembered forever. But what is the "standard state?" is the question the textbook authors hide. The problem seems to be that the authors want to define the standard state once, and once only; and the students xant to accept and learn the definition only once. But this is impossible, as most authors state, and as good chemists realize. Thc chemistry books begin with a discussion of "The Scientific Method" emphasizing the tenative nature of the conclusions and the hypothetical nature of models, and the dangcr of accepting on authority xvhat can be tested in the lab or understood by reason. And G. K. Lewis ("Thermodynamics," Rew York, 1923) talks of making partial definitions, postponing discussion of topics until cxpedient, and choosing standard states for their convenience. I n his book the groxing nature of chemistry as knowledge is fully appreciated. Since chemical knowledge is growing and will continue to grow, it seems to me both wise and expedient to share this groxth vith the student by, in this case, letting him see the growing clarity in the definition of the standard state of electrolytic solutions. I suggest the following sequence of definitions of the standard state. then ex(1) All Concentrations One Molar-and amine the ease of making 1 molar solutions and the change of concentration with temperature, because of the expansion of water. (2) All Concentrations One Molal-as a more convenient scalc than 1 molar. Howver, this doesn't xorlc either, since solutions are not ideal. The text could stop here, and tell the student, "In all problcms or other calculations, assume the solutions arc ideal. Further discussion of non-ideal solutions will be given in later, physical chemistry, courses." OR, the text could introduce activity and activity coefficients. (3) T h e Activities and the Activity Coeficients are All I . This is a hypothetical state and the student must be shown h o a~ hypothetical state can give measurable values in the lab.
I think that if the texts did this, then the student would be prepared for definition #4 when it comes; be mould be part of the process that real chemists went 654 / journal of Chemical Education
through; and he would have done a great deal of u-ork on "the standardstate." Lastly, it should be emphasized that "the standard state" can be defined to suit the conveniencr of the rxperimenter. The error in the textbooks is that they do not faithfully report the definition of the standard stat? used by those who actually measured thevalues of E o that they report. This is not an error of clefinztzon, but an error of reporting. If they ~vishto say that the standard state is 1 molar they may, but the valucs of E o must bc measured a t that statc and not at some other.
MO's and Air Pollution
T o the Editor: I n the article "Molecular Orbitals and Air Pollution" [J. CHEM.EDUC.,49, 26 (1972)], B. M. Fung has an excellent idea. The use of symmetry rules to explain simple reactions is vell worth incorporation into undergraduate physical chemistry courses. Howcver, I have two objections to his presentation. One objection is t,he failure to clarify the point that a partially filled orbital can act either as a highest occupied molecular orbital (HOMO) or a lo~restunoccupied molecular orbital (LUMO). This concept is used in the NO-0% reaction but not specifically stated. If the U in LUh20 were changed to unfilled, there ~vould be no possible confusion. A second and more seriou~objection pertains to the molecular orbital designation for R\T~.Professor Fung shom the a, orbitals to be higher in energy than thc c, orbital. It seems ~ r e l lestablished from spectroscopic evidence, and has for several years, that the a, orbitals are l o w r in energy than the a, orbital. This would change the energy levrl diagram in Figure 1 and the draxings on molecular orbitals in Figure 2A but would not change the basic argument concerning the Nz-O2 reaction. Most recent physical chemistry texts now shov the correct energy level diagram for N2. In 02,it appears that there is a "crossover" and the r , orbital is lower than the r, orbitals, as shown in F i b r e 2B. I n fairness to Professor Fung, it should be noted that Professor Pearson in the references cited makes the same mistake concerning the energy level diagram of N?. The distinction concerning partially filled orbitals is made by Professor Pearson in these articles, but not very prominently.
