LE CHATELIER, COMMON SENSE, AND "METAPHYSICS" ANTHONY STANDEN Interscience Publishers, Inc., New York
following notes are suggested by the recent article by J. de Heer (THIS JOURNAL, August, 1957, p. 375) which was critical of the well-known principle of Le Chatelier and Braun, and have the purpose of suggesting that, when properly understood, this principle is entirely valid. de Heer first gives the principle in the form: T H E
I: a chemical system i n equilibrium is subjected to a perturbation (stmss), the epilibrium will be shifted (a reaction o i l 1 occur) such as to portidly undo this pwzurbation (oppose the stress).-(I)
Following Ehrenfest, he then gives two examples, in which he says that the principle is invalid in one case, valid in the other. The case where the principle is said to be invalid is that of heating, through an interval 6T, a gaseous mixture which was in equilibrium a t the original temperature, T. If the chemical equilibrium were "frozen" (perhaps by the absence of a suitable catalyst), the gaseous mixture would take up from the surroundings an amount of heat, qo. But if the reaction can proceed with appreciable velocity, the equilibrium is displaced, the reaction takes place in the endothermic direction, and an amount of heat q is absorbed by the system. We a11 know that and therefore more heat is transferred from the surroundings than when the equilibrium was frozen. de Heer concludes: "The temperature increase induces an endothermic reaction t o take place, so that more heat is taken up from the thermostat than would have been the case if all gases had been inert. Thus the equilibrium shift does not 'oppose' the heat transfer; t o the contrary, it 'aids' it.' Now just exactly what, according to the principle, is the system supposed to he either "opposing" or "aiding?" According to formulation (I), taken entirely literally, the system should "partially undo this perturbation," but since "perturbation" is not an easy word to interpret, there is added the explanatory expression "oppose the stress." And what is the stress in this case? The increase in temperature in the surroundings. The environment (I propose to use this word, because l'surroundings" is singular in grammar, although ending in "s," and leads to expressions like "the surroundings is. . ." or "the surroundings were. . ." which are inelegant)-the environment underwent a rise in temperature. What was the reaction of the system (supposed embedded in the environment) to this perturbation or stress? It absorbed heat, thereby re-
ducing the temperature of the environment. That is to say, it acted to "partially undo this perturbation (oppose the stress)." But the stress was the change in temperature of the environment, not (as de Heer misreads) the heat transfer, which was an exchange of energy between the environment and system, resulting from the perturbation or stress. It may be noted that Le Chatelier's principle is valid here even if the only change wrought upon the system is a physical change. If there is no chemical reaction, the system absorbs heat when 'the environment increases in temperature. If a chemical equilibrium is displaced as a result of the temperature change, the system absorbs even more heat, in other words, it opposes the perturbation (stress) by a greater amount than if there were no chemical change. The second case discussed by de Heer is an increase in volume of the system from V to V 6V. There is a corresponding change in pressure, 6Po if there is no chemical reaction, 6P if a chemical equilibrium is displaced. Then 8P < P o , and de Heer concludes "The reaction indeed, in this indirect way, 'opposes' the volume change." But the volume change of the system is not a stress imposed upon the system, which it could either "oppose" or "aid." The volume change must have been brought about by a pressure change in the environment. The principle cannot be applied a t all unless the question is reformulated, speaking not of a change in volume of the system, but of a change in pressure of the environment from P to P -6P. The system expands, so as t o counteract this change. It expands by an amount 6Vo if there is no chcmical change. If equilibrium is displaced, the reaction takes place in the direction giving an increase in volume, and 6V > 6V0. de Heer discusses an interesting case in the equilibrium system N2 3H2 s 2NH3. If the system contains more than 50 mole Oj, nitrogen, then the addition of more nitrogen a t constant pressure results in a displacement of the equilibrium from right t o left, with the formation of even more nitrogen. I n attempting t o apply the Le Chatelier principle here, it is difficult t o see how the addition of a material substance to the system can be construed as a perturbation of the environment. But if an attempt be made to do this, it must be noticed that such a change can never occur singly. I n this example, the addition is to be made a t constant pressure; this implies that the volume must change. And if the addition were made a t constant volume, the pressureimhst change. I n either case, there are two kinds of changes to be considered, and it is
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(Continued on page 134)
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