DEMONSTRATING CHEMICAL EQmLIBRmM Noms W. RAKESTRAW, OBERLINCOLLEOE. OBERLIN, OHIO The conception of chemical equilibrium as a condition of balanced velocities rather than one of rest is difficult for the mind of the beginner to grasp. To make this point impressive the writer has made use of a hydraulic analogy which is easy to demonstrate, simple to understand, and sufficientlylike the chemical condition to serve as a reasonable example. A diagram of the apparatus is here shown. The two sides of the "reaction" are represented by chambers A and B, into and out of which a liquid can be pumped to represent the two opposing re2ction velocities. A is made of a piece of 3/ciuch iron pipe about two feet long, with small holes drilled into it about two inches apart. A manometer, M,is fitted to the bottom of the pipe to serve as an indicator for the water level within and the pipe then enclosed in a large glass tube, open a t the bottom, to collect the water from the small apertures. This is delivered into a large funnel supported below, whence i t drains into the cylinder, B. From this i t is taken up by a power-driven pump, P, and delivered again into the top of A . An auxiliary water-circulation pump, such as is used on Fords, serves very well, and, to regulate the flow delivered a t D, a "by-pass" with a clamp, C, is most conveniently used. By closing or opening C the flow a t D is increased or decreased, respectively, while the pump is operating a t constant speed. The velocity with which water is delivered a t D determines the height in the manometer, M. But, on the other hand, the more water in A the more rapidly will it run out through the small holes and the more rapidly will i t be delivered through E into the "reaction chamber,'' B. Constant levels in M and B will result, corresponding to constant proportions between reacting constituents in a reversible chemical system. With the pump working a t maximum capacity the level in M should stand near the top, and in B near the bottom. On opening C the velocity a t D falls off, the level in M sinks and in B rises, reaching finally a new equilibrium when the flows a t D and E are equal. Every teacher will,
of course, devise his own "patter" to accompany the demonstration. The water is, of course, colored by a dye or by permanganate to render it visible at a distance and a white background is placed behind M and B. I t also makes the analogy more impressive to hang black or red arrows on the two tubes carrying the opposing "reactions." Another point which can be demonstrated with the same apparatus is the nature of the "reaction velocity curve." When A is filled to the maximum height the flow at D is suddenly shut off completely and the level in M observed. The latter will, of course, sink rapidly a t first and more slowly as it gets lower, corresponding closely to the course of a first order chemical reaction. A little experimentation will usually be necessary in choosing the size of tubing, etc., especially the length and size of the pipe in A and the diameter and number of the holes in it, according to the capacity of the pump used. The tube, E, must also he large enough to deliver into B as rapidly as the fullcapacity of the pump requires.
