NOVEMBER, 1951

SOME LECTURE DEMONSTRATIONS IN GENERAL CHEMISTRY

THE following demonstrations have been found of value in presenting to beginning students concepts of the atomistic nature of matter, the controlled experiment, and the solubility product.

(1) The concept of the atomistic nature of matter can be demonstrated by (a) peeling an orange, sepa- rating the segments, seeds, etc., and observing the internal structure of particles within particles, (b) comparing the pouring of sand (with its visible particles rolling over one another) with the pouring of m t e r (with invisible particles), and (c) comparing the de- crease in total volume resulting on admixture of equal volumes of absolute alcohol and colored water with a similar change in volume resulting from pouring sand into a container holding oranges.

(2) The value of a control or blank experiment and the importance of introducing singly, as variations from the blank, only those factors under investigation can be illustrated by means of the Landolt (or iodine clock) reaction. A blank Landolt experiment is carried out a t room temperature according to customary laboratory procedure for the reaction. Two subsequent runs are made a t about 35'C. with (a) minor variations in ap- paratus (e.g., method of stirring, size of beaker) plus 10 g. of anhydrous sodium sulfate and 3 ml. of oil of wintergreen added per liter of total solution, and (b) no variations from the blank other than temperature. Experiment ( b ) illustrates the regular positive tem- perature coefficient of rate of reaction.' The former ex-

' Convenient reaction times (about 20, 38, and 17 seconds for the three runs, respectively) are obtainable from mixing equal volumes of the following two stack solutions: (a) 25ml. of 5 N sulfuric acid, 25 ml. of 1 per cent (by weight) soluble starch solution, and 400 mg. of c.p. potassium iodate per liter of solu- tion; and (b) 620 mg. of c.p. anhydrous sodium sulfite per liter of eolution.

I. H. KLEMM Indiana University, Bloornington, Indiana

periment, however, exhibits a lower velocity than for the blank due to an overwhelming retardation of the reaction by the sodium sulfate, which not only decreases the rate of the Landolt reaction at constant tempera- tnreZ but also produces a negative temperature coef- f i ~ i e n t . ~ The oil of wintergreen and changes in appara- tus serve only as extra variables and appear to have no effect on the rate of reaction. By performing the multi- variable experiment immediately after the blank, one can provoke some student discussion on the validity of the result as a measure of the effect of increased temperature on the rate of reaction. ,/(3) That the concentration of bromide ion enters the solubility product expression for lead bromide to a higher power than the concentration of lead ion does can also be effectively illustrated by means of a lecture demonstration. A saturated solution of lead bromide is prepared by mixing 250 ml. of 1 M lead nit,rate with 500 ml. of 1 M sodium bromide and removing the pre- cipitate by suction filtration. Treatment of one-half of the filtrate with 100 ml. more of the lead nit,rate solu- tion produces almost no additional precipitate, mhereas addition of 100 ml. more of the sodium bromide soh- tion to the other half of the filtrate gives extensive additional precipitation within a matter of seconds. This result proves rather surprising to students who are encountering chemical equilibria for the first time, inasmuch as they almost invariably predict that the greater amount of precipitate should form from the added lead nitrate solution due to the smaller volume of this solution required in the original stoichiometric mixture. :, -"

1 Unpublished results from' the Chemical Laboratories of Indiana University. '

SKRABAL, A,, Z. Elektmchem., 21,461-3 (1915).