USABLE ANALOGIES IN TEACHING FUNDAMENTALS OF CHEMICAL EQUILIBRIUM W n m m E. CALDWBLL, OREGONSTATE COLLEGE.CORVALLIS, OREGON
Three w b l e and interesting analogies are suggested in presenting the subject of chemical equilibrium before students jirst considering this subject matter.
. . . . . .
Many instructors have their own favored analogies in the teaching of students who are beginning with the study of chemical equilibrium. Some of the analogies that instructors present are better than others while some are not suitable in that they are cases of static instead of dynamic eqnilibrium. Certain instructors might like to know of analogies used by others in connection with this subject so herewith are presented a few particularly usable ones. A small boy playing on a moveable stairway and waking down the escalator as fast as the moving stairs pass upward presents an analogy of two reverse actions in which the position of the boy remains fixed, much as at equilibrium in a chemical reaction two reverse reactions result in constancy of proportions of reactants. The analogy may he expanded upon as to position of the boy when he started his play, corresponding to point of equilibrium, and result of hastening one action more than another. Suppose one considers a large number of mounted rookie cavalrymen who enter an enclosed field for practice in mounting and dismounting their horses. If the speed of mounting is equal to that of dismounting an average of half of the men will be mounted at oqe time. Let us consider that the horses are activated by some factor and the proportion of mounted and unmounted horsemen will change. This analogy permits of further elaboration. The analogy which is thought particularly fitting for consideration we will term the "dance-floor analogy." I t is best considered in conjunction with some simple equilibrium reaction such as: NaCl
+ KBr
KC1
+ NaBr
Consider that a large dance is being given and a thousand couples enter the floor wearing dark clothes and an equal number wearing white clothes. As the music starts the couples begin dancing, and couples may he designated as MbL6 and MwLw (Mastanding for man in black, L, being lady in white). This dance is unusual in that couples exchange partners as they collide instead of scowling at each other as usually occurs. After the music has gone on for some time and couples have exchanged partners on collision, some couples with light and dark clothes wiU be dancing together. The conditionmay best he presented by equation: MbLs
+ MA. * MbLm f M A & 2079
2080
JOURNAL OF CHEMICAL EDUCATION
DECEMBER. 1932
Four types of couples now exist on the dance floor much as four molecules in a solution (disregardingionization in this preliminary discussion). One may well consider relative speeds of reactions in forward and reverse reaction; the forward reaction being rapid at first and diminishing in speed; the reverse exchange of couples being zero at first before such couples were formed, and attaining the speed of the forward reaction to establish an equilibrium. Other conditions being constant a numerical equality in number of couples would result, corresponding to fifty per cent. forward reaction. Increase in the speed of the music, "hot music," enhances possibilities of collision and both reactions hasten. Relative speed of music at first would govern speed in the attainment of equilibrium. The analogy between temperature or catalyst and kind or tempo of the music is then apparent. One may impose the restriction that a man dancing with his own partner tries to avoid collision or oppositely dressed couples steer into them. In this manner the relative numbers of the different kinds of couples will change. The effect of relative affinities between atoms and resultant change in point of equilibrium is then similar. It is suggested a "dark" man prefers a "blonde" so consider that MbL, couples stroll out into the garden. If all such couples remove themselves as do insoluble gases or solid molecules in chemical reactions, our dance floor reaction goes to completion in,one direction. These few paragraphs suggest how the analogy may be expanded to consider many of the factors involved in*instilling an adequate concept of chemical equilibrium in a student's mind. It is realized that analogies are seldom perfect but these few have been found especially usable in classroom discussion in instilling interest, and by association with the known, facilitating an understanding of the fundamental factors of reverse reactions.
