LE CHATELIER'S PRINCIPLE AND THE EQUILIBRIUM CONSTANT ARlLD I . MILLER Carleton College, Northfield, Minnesota
MANYstudents of chemistry have difficulty in understanding how the position of equilibrium iu a gaseous reaction can change when the pressure is altered, in accordance with Le Chatelier's principle, without causing a corresponding variation in the equilibrium coustant. This problem is common, not only among students in general chemistry and qualitative analysis, but also among advanced students who have perhaps never studied the problem in a quantitative way. Practically all students can predict correctly the effect of changing the pressure on the position of equilibrium, once the statement of the principle has been learned or memorized; hut when asked for an explanation of why the shift occurs, it usually proves impossible for them to develop a satisfactory theory. The following scheme has proved very useful to the author in helping students with this difficulty, and does not appear in the common texts and reference books. Consider the system represented by the equation Nz+ 3Hn 2NHa (1) to be a t equilibrium. The equilibrium constant for this reaction is expressed in the usual form,
Suppose now that we double the total pressure by compressing the volume to one-half its original volume. This momentarily doubles the pressure of each constituent of the reaction mixture. Thus, before the system can readjust to the new situation, we have:
Since the equilibrium constant is a constant a t constant temperature (except for deviations from ideality which occur a t high pressures and which need not he considered here), the equilibrium must shift in such a way that "K" will be increased until it is equal to K p . This occurs, of course, by the reaction of nitrogen with hydrogen to form more ammonia, which increases the value of the numerator and decreases the value of the denominator of (3), until "K" becomes equal to Ks, when the system is again a t equilibrium. This suggestion does not prove that the equilibrium will shift until "K" = K O ,but it does show that there can be a shift in the position of equilihrium without a change in the value of the equilibrium constant. It has been the author's experience that the use of this method of explanation clarifies the concept of equilibrium to many students who had difficulty in this area.
