LECTURE DEMONSTRATIONS FOR GENERAL CHEMDTRY C. ? SORUM I. University of Wisconsin, Madison, Wisconsin
I. CHEMICAL EQUILIBRIUM
Maten'als. Two glass crystallizing dishes 8 inches diameter and 4 inches deep. One 3-inch casserole, one 31/2-inch casserole, one 4-inch casserole and one 1-inch casserole. Enough colored water to fill one crystallizing dish to within '/z inch of top. Procedure. Place the two crystallizing dishes close together on a small table or on the lecture demonstration desk. Fill one dish to within '/a inch of the top with the colored water. The lecturer takes the 3-inch casserole and stations himself beside the dish containing the solution. The lecture assistant takes the 3l/&1ch casserole and stations himself beside the empty dish. The lecturer explains to the students that the colored water and his casserole are to represent the reactants A and B in a simple double decomposition reaction. The products B and C will be represented by the assistant's casserole, which a t the moment is kept out of sight, and whatever solution accumulates in the second dish. The simultaneous movement of liquid from one dish to the other, and back, by dipping, will represent the two reactions that take place. The students are asked to note: (a) The change in the amount of liquid that moves from left to right and right to left as the reaction proceeds. (b) The change in the levels of liquid in the two dishes as the reaction proceeds. ( c ) That there soon comes a &me when the amount of liquid moving from left to right eqbals the amount moving from right to left and, by that time, that the levels of the liquid in the two dishes have become constant; a state of equilibrium has then been reached. (d) After equilibrium has been reached the reaction goes round and round, from left to right and right to left with no change in the rates of the two reactiom and no change in the amounts of liquid in the two dishes. (e) That time is required for the attainment of equilibrium. (I) That, a t equilibrium, the rates of the two reactions are the same but that the amounts of liquid in the two dishes are not necessarily the same. While the process is going on the lecturer quickly substitutes the 4-inch casserole for the 3-inch casserole, the dipping operation being continued without interruption. The students are asked to note that an increase in the concentration of one of the reactants, symbolized by increasing the size of the casserole, causes a sudden speeding up in the rate of the reaction ip
to the right. This results in a change in the water levels in the two dishes. A new equilibrium is eventually reached. The levels of the liquids in the two dishes are again constant, but they are not the same as they were a t the previous equilibrium. The operator with the larger casserole has the lesser amount of liquid; what he has gained in casserole he has lost in liquid. The equilibrium point has been shifted to the right. Again, without interrupting the dipping operations, the lecturer substitutes the smallest casserole for the 4-inch casserole. The dipping continues until a new equilibrium has been obtained. The students are asked to note that a decrease in concentration of a reactant, symbolized by injecting the small casserole, shifts the equilibrium point to the left, that the levels of thesolutions in the two dishes are again constant but different and that what the lecturer has lost in size of casserole he has gained in amount of solution. Comments. This experiment enables the student to see what, in the ordinary equilibrium reaction, he must imagine or visualize. He notes the process by which equilibrium is attained. He notes that the rate of one reaction starts out a t a maximym and gradually decreases to an equilibrium value. He notes that the reverse reaction starts out from zero rate and gradually increkes to the equilibrium value. He observes that a t equilibrium, the rates of the two reactions are the same but the quantities of reactants and products are not necessarily the same. The effect, upon a reaction which has reached a state of equilibrium, of changing the concentration of a reactant is shown. The experiment is a very simple one and, in the experience of the author, very popular and very effective. 11. THE COMMON ION EFFlECT
Materials. Two glass cylinders 4 inches in diameter and 19 inches tall. Two 150-ml. beakers level full of dry precipitated CaCOs. About 35 ml. of reagent quality sodium acetate crystals. 400 ml. of 2 N acetic acid. Procedure. Place one 150-ml. portion of dry CaCOx in each cylinder. Divide the 2 N acetic acid into two 200-ml. portions; dissolve the sodium acetate crystals in one of the 200-ml. portions. Add the two portions of acetic acid to the two cylinders a t the same time and note, first, the rate at which the foam rises in the two cylinders and, second, the amount of foam formed in each of the two cylinders. Comments. The foam rises more rapidly in the cylinder to which the pure acetic acid solution is added than it does in the cylinder to which the acetic acid-
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sodium acetate solution is added. However, the total amount of foam formed when all reaction has ceased (in about 3 minutes) is the same. Since the rate of the reaction of an acid with a carbonate is directly proportional to the concentration of hydrogen ions, it is evident from this experiment that the concentration of hydrogen ions in 2 N acetic acid has been reduced by the presence of sodium acetate. Since the total volume of carbon dioxide foam produced in the two cylinders is the same, it is evident that, whereas the sodium acetate has reduced the concentration of hydrogen ions by decreasing the ionization of the acetic acid, it has not altered the total amount of available hydrogen. I t follows, therefore, that the ionization of acetic acid
JOURNAL OF CHEMICAL EDUCATION
should be represented by an incomplete, equilibrium reaction, that the equilibrium point in this incomplete reaction can be shifted in the direction of more acetic acid molecules and fewer hydrogen ions by the addition of acetate ions and that the removal of hydrogen ions by reaction with carbonate to produce carbon dioxide and water causes this incomplete ionization reaction to be made complete. The experiment is simple and very easy to. follow. In addition, it has the advantage over other experiments of this type, all designed to show that the decreased ionization of a weak acid by addition of a salt of that acid does not alter the total amount of available . hydrogen, in that it takes place in a short time.
