APPARATUS FOR ILLUSTRATING LE CHATELIERW PRINCIPLE W. C. TUCKER, b. University of Florida, Gainedle
IT HAS been found that students frequently have trouble in visualizing and understanding the workings of Le Chatelier's principle. I n an attempt to help stndents visualize this principle, the apparatus as described below has been assembled. The apparatus consists essentially of a very broad, but low, H constructed of glase tubing (approximately I-inch inside diameter) with the horizontal piece (B) approximately 30 inches long fitted with the outer member of a 329/42 joint a t each end. The upright pieces (R, P ) are about 18 inches long each fitted with a T connection, near the bottom, equipped with the inner member of the standard taper joint so they can be connected to the horizontal piece, and closed with 4-mm.-bore glass stopcocks at the b t t o m ends. The upright pieces are open a t the top. A small spool made of light gauge aluminum or stainless steel, with a "flag" hanging loosely from its axle to make it more readily visible, is inserted in the horizontal tube. This spool should be sized so that it will slide freely in the glass tube (B) and yet will very nearly fill the opening of this tube. I n operation, one starts with the horizontal tube (B) filled with water, the spool (D) being approximately midway between the tubes R and P, and with the tubes R and P about two-thirds full. This represents the system in equilibrium and an equation representing a chemical equilibrium is written on the blackboard. The concentration of the materials on the left (the reactants) is represented by the length of the column of water in R and that of the products by the water in P, (the system is a t equilibrium and no movement of water is observed). A quantity of water sufficient to increase the length of the column in R by a certain amount (for example, four inches) is put into a container the same diameter as the tube R, and then added to R explaining that this represents an increase in the concentration of one of the reactants (a stress or strain placed on the system). As this water is added, it is noted and explained that: the spool moves toward P showing that the change takes place which tends to use reactants;
VOLUME 35, NO. 8, AUGUST, 1958
the water level in R did not rise the expected four inches; and the water level in P was increased. All of these facts indicate that the change (reaction) takes place which tends t o minimize the stress or strain, resulting in a decrease in the concentration of any other reactants and an increase in the concentration of the products. These changes are explained in terms of the equation on the blackboard. These same analogies can be made adding water to P or withdrawing water from either R or P. With the concentration of the reactants being represented by the height of the liquid in tube R and that of the products by the height in tube P ; and since, with the apparatus filled with a single liquid the two heights will always be the same, students may get the impression that all chemical equilibria result in the concentrations of the products being equal to the concentrations of the reactants. To help avoid this misconception, ether, or any other liquid immiscible with water and of lower density than water, may be added to either tube R or tube P. The same manipulations as previously described may now be carried out without resulting in equal heights of liquid in the two vertical tubes. Liquids heavier than water cannot be used in this way as they would get into the horizontal tube and affect both sides in the same manner.
