A LECTURE DEMONSTRATION OF DYNAMIC EQUILIBRIUM
It has been the experience of teachers of elementary chemistry that the presentation of the idea of dynamic equilibrium is one of the most difficult problems to be faced in the first year's work. In spite of the attention that is paid to this presentation i t is not uncommon to find second-year students who have a very hazy idea of this fundamental concept. Many word illustrations mav be resented which serve to guide the student in a correct line of thought but, as a rule, the illustrations which appeal t o t h e visual sense, whether they are of the graphic sort or of mechanical contrivance, tend to firmly imbed the static idea in the student's mind. Demonstrations such as a poised system in balance, while showing how we may have an equilibrium with a large amount of one product and a small amount of the other, f a i l t o show t h e s t u d e n t two active reactions, one forming the .products used by the other, the amount of product existing a t a given instant depending on the relative speeds of the reactions themselves. The apparatus illus-' trated here was designed for the purpose of presenting the idea Fro. 1.-APPARATUS POR DEMONSTRATING DYNAMIC EQUIL~BR~UM of dynamic equilibrium to lecture groups. The results obtained in the class-room seem to indicate that it is a valuable aid to instruction. The apparatus (Fig. 1) consists of two vertical tubular vessels mounted on a board. The upper left-hand vessel is so arranged that its liquid contents may be allowed to drip into the lower vessel a t a rate which may be governed by a stopcock. This passage of liquid from the upper vessel to the lower one has been called reaction number two and is so labeled on the apparatus. The designation, reaction number one, has been applied to the passage of liquid from the lower to the upper vessel. This is caused to take place by the operation of a simple air-lift pump (Fig. 2)
whose speed of operation may be regulated by a stopcock through which it is connected to a constant pressure device and the compressed air line. For the convenience of the operator and to call to the student's attention the importance of the speed of the reactions, pointers, which indicate whether the reactions are being accelerated or retarded, are fastened to the stopcocks. The speed of the reactions may also be noted by the student from the rate of drip from the upper vessel or from the frequency of pulsation of the pump. The a t t a i n m e n t of equilibrium is , automatically regulated by a given settmg of the apparatus, for reaction number one slows down as the level in the lower vessel falls and reaction number two speeds up as the level in the upper vessel rises. When the a p p a r a t u s is i n operation the stopcocks may be set for certain speeds for the two reactions a n d i n a few minutes a n equilibrium, the position of which will be shown by the relative amounts of liquid in the two vpssels, will be reached. The equilibrium may now be shifted by altering the speed of either reaction and the plainly indicated by the Srm change in the liquid levels. If it is so desired, both reactions may he made to ~roceed .-___.' very slowly and in such case FIG. 2.-DETAIL OP LIFT PUMPAND PRESsome time is required for the SURE REGULATOR attainment of equilibrium. In contrast, if both reactions are speeded up as in the case of a catalytic reaction, the equilibrium point will be reached in a much shorter time. The effect of removing one of the products of reaction, as would be the case when a volatile gas, a precipitate, or a slightly ionized substance is formed, may be illustrated by collecting the product of one of the reactions in a beaker and the reaction will proceed immediately to completion; that is, one of the vessels will be completely drained. The dimensions of the apparatus may be varied to suit the conditions under which it is to be used. The apparatus illustrated here is about
1,;I 1
eighty-five centimeters in height over all, with vessels about thirty-six centimeters in height. The vessels used should preferably be tall and narrow, as less change in volume is then required to give a marked change in liquid level. Prussian blue solution can be used very conveniently as the liquid.
