LE CHATELIER, SCIENTIFIC PRINCIPLE, OR "SACRED COW" 1. de HEER University of Colorado, Boulder
THE editor of THIS JOURXAL has asked me to comment upon the preceding article by Anthony Standen (1). I shall devote the main portion of this reply to Standen's scientific arguments, which are mostly misleading or irrelevant, and in part plainly incorrect. As for the more general implications, I am equally critical of some of his statements. although here I admit that our disagreement is largely due t'our widely different views on the relation between science and philosophy. Since I barely touched upon this topic in my original paper (2), I welcome the opportunity t o elaborate upon my ideas concerned in the latter part of this note. Let me first comment upon the use of the words perturbation or stress in the formulation of the Le Chatelier-Braun principle in its usual vague form (I) (1, %).I I n other branches of science these terms often refer to well-defined notions. We might mention, in this connection, "perturbation theory" in quantum mechanics and the study of "stress" in applied mechanics. But within the framework of "principle (I)" the use of these words is unfortunate, because here "perturbation" or "stress" suggests an organismic association with "something unpleasant." This, in turn, partially accounts for the psychological satisfaction wh~ch we find in this "principle," since it tells us that Mother Nature is always ready to resist any such discomfort. [We shall return to this point later.] It would he much better in this case to speak of a "change," a word without such emotional content ("it is time for a change" has not yet been accepted as a scientific principle!). But whatever word one chooses, one has to consider changes in any of the thermodynamic state variables of the system under consideration, changes imposed in a specific and unambiguous way. Confusion does not enter till we talk about opposing or relieving such changes. Let me clarify my position by returning to the first two examples in the qualitative discussion of my paper (2), the isobaric temperature increase and the adiabatic volume increase. Standen misunderstands me completely when he says (1) that I consider "principle (I)" invalid in the former, valid in the latter case. I tried to emphasize that, since the system cannot in any direct way undo the change in the state variable concerned, upEinciple (I)," taken literally, is nonsense in both cases. The system can only react indirectly and I agree entirely with Standen that the use of the words "oppose" or "relieve" is ambiguous here. As a matter of fact, the same relation: ( 1 referring t o the isobaric temperature increase (2) is 9 > PO
' To be referred to hereafter as "principle VOLUME 35, NO. 3, MARCH, 1958
(I) "
called upon by some authors to show that systems "oppose a change" and by others to confirm that systems "relieve a change." It all depends on what semantic rules one arbitram'ly adopts in linking these expressions to the induced secondary changes in the conjugate state variables of the systems. A similar ambiguity exists with resuect to the result:
I 6P 1 < 1 6Po 1
(b)
referring t o the adiabatic volume increase (2). The important point is that (a) and (b) are mutually contradictory with respect to "principle (I)." That is, either (a) or (b), by a proper selection of terminology and semantic rules, can be interpreted t o validate the principle, in an indirect way, but then automatically the other equation contradicts it. This is the Ehrenfest dichotomy with respect to changes in intensive or extensive variables, as frequently referred to in my paper (2). One simply cannot get around this dichotomy and a salvation of "principle (I)" as a universal lam is only possible if one couples its arbitrary interpretation, as explained above, with an elimination of changes in either intensive or extensive state variables, justifiable within the framewmlc of the formulation of the principle itself. Apparently Standen is fully aware of this predicament for, notwithstanding his earlier remarks, he proceeds to present arguments intended to show that changes in extensive variables are really not " stresses" p r ~ p e r . ~ Now I simply fail to see that relating some of the changes concerned to the enuironm a t (obviously itself a rather vague notion in Standen's vocabulary) bears any relevance to the prohlem whatsoever. Moreover, to say that "the volume change musta have been brought about by a pressure change in the environment" (I), is entirely incorrect. Thus, in order to study a specific volume increase upon the equilibrium of a gas reaction, we can connect the reaction chamber with an evacuated space and break the partition.911 order to maintain his position, Standen would also have to postulate that entropy changes of a system must be brought about by a temperature change in the environment. Again this is (Continued on page 13.5)
'It is amusing t o note that Standen first observes that "perturbation" and "stress" are no easy words to interpret, and subaesuently considers it nearly obvious that a pressure change is, a volume is not 5 "stress." a Italics mine. ' We hope that no one will maintain that cresting this vacuum in the first place will change the barometric pressure of the environment. If one is bothered by this, one may think of all gases, originally present in the space to be evacuated, aa being either bound chemieelly or adsorbed physically by proper reagents. 133
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possible that they may pull in different directions. When nitrogen is added a t constant pressure, the increase in partial pressure of nitrogen tends to displace the equilibrium to the right, but the decrease in partial pressure of hydrogen tends (proportionately t o the cube of the fugacity) to displace the equilibrium to the left. It is interesting to see that the net result may be either one way or the other, according to the original percentage of nitrogen, but it is not a contradiction of a principle which, even if it can be applied at all (which is doubtful), does not give a univocal indication. What, indeed, can be changed in the environment besides temperature and pressure? It is hard t o say. To change the volume of the environment obviously makes no difference at all. A case in point would be moving a small sealed tube from a 2-cn.ft. thermostat to a 3-cu.ft. thermostat at the same temperature. Nor does it make any difference to the system within the sealed tube whether the fluid in the thermostat is air, water, or oil. Of course, we might bombard the system, from the environment, with protons, deuterons, any other 'ons, or even plain ordinary light, hut then we would set up conditions that are generally regarded as without the field of classical thermodynamics. de Heer arrives at two much more precise formulations, to replace (I). The change of an intensiue variable caused by changing the emresponding ezlnsive variable is smaller if chemical epuilibrium is maintained than if no reaction could take place i n the system +Ira)
The change the eorre, o.f an ezlensive va~iablecaused bs ehanoino " , ~ p r n d z w ! znleoshe 3 n r m I h is h r p r ? I ~ h t w d q u d d m u m 8s n.oznluzad Ihon z f m rm
