edited by
J. DUDLEY HERRON Purdue University West Lafayette, Indiana 47907
Evolution of an Experiment (from molesl12to gaseous CaCI,) G. A. Dauphinee Dalhousie University Halifax, Nova Scotia, Canada An experiment originally designed to illustrate a simple problem in solution stoichiometry has produced a stimulus to student recognition of some applications of descriptive chemistry. The basic experimental design provides a short, inexpensive procedure for laboratory or demonstration use. Certain observations produce an apparent paradox, thus creating an "open-end" situation that can he resolved by interested students. This ontional extension can vary in depth and precision to fit the circumstances of a class. b n e idescapable and usually unanticipated ohservation during the experiment leads to a consideration of readily available information on the properties of several important materials. Response of many students in recent years indicates appreciable interest in descriptive chemistry when a "need to know" has been established. The experiment was originally designed to ensure that one of t,he first exneriments in the freshman lab would give stu(ltrnt9 a chanw tu relate stuichiometric problems to a real situation. If one notes that rcneent araric Cu(OH)1 us~lnlly . .. carries 005% Fe impurity, a pmsihle riesip appears. Students arr dirccted to dis;olvc 2.5 e ui Ca(OH,v in 50 ml f i A! HCI, dilute to 250 ml, and test a 5-ml samplewith 4 drops of 1M KCNS. The color developed is compared to similar tests with known 1W3, 10-4and 10-5 M F ~ C & solutions. A color match with 1W4M FeCI3 is found, and a calculation of Fe (present as iron(II1)) follows. Clearly the calculation a t one stage involves manipulation of l W 4 (molesh) and 0.25(1). It was hoped that visualization of data by a student would discourage answers such as 4 X l W 4 (molesfi2). This limited objective appears to have been achieved. Modifications in the design are open to the user. I t is convenient to have the unknown give a color test fairly close to l that seen with known 1W4M Fkc13. The amount o f ~ Cused should he sufficient to ensure that excess acid will he present in the unknown solution. Using the amounts specified, with additions of portions of the solid to the HCI contained in a 150-ml beaker, gives a rapid hut controlled reaction. Safety glasses would b e a routine precaution at this point. The potential of the experiment was then extended by adding a color test using SASSA (saturated aqueous solution of salicylic acid) and comparing with test results using this ~
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Editor's Note: Most teachers want to stimulate students to inquire ahout chemical problems on occasion. However, finding problems that are both intriguing and simple enough to be attacked by students who have not yet learned a lot of chemical facts is not easy. This article provides one possibility. The author suggests several possible investigationsof a system;the reader may think of others. If you have an "invitation to inquiry" which you have found effective,we would he delighted to consider it for publication in this column. Send three copies of your contributionsto the column editor. Contributions should be limited to four double-spaced pages. 116 / Journal of Chemical Education
and 1 0 - M ~ FeCla. Four drops reagent on known 1W3, of SASSA mixed with 5 ml of 10-4 M FeCla producC a definite color, hut when the unknown is tested no color is apparent because formation of the iron(II1) complex with salicylic acid is practically eliminated in the strongly acidic unknown solution. At this noint. the student is confronted with a situation where the undnown'matches 1W4M FeC13on the KCNS test hut certainlv not on the SASSA test. (The number of students who do not"respond to this paradox is distressing). The dilemma mav he resolved if students are encouraged to (1) calculate the excess acid present in the unknown s d u tion and (2) observe the effect when a few drops of 6 M HCI are added to the SASSA color test developed with known lW3 M FeCla. It is useful to have some CaC12 available to demonstrate that Ca2+(.,) does not interfere with the color develonment. This entirelv reasonable hypothesis is the probable first guess of alert students attempting to explaid the test re~ults. ' l ' h ~potrntial for further work by interested students on the eifect of \,arying pH in these tests ia evident. A goud stu~n ~ r o a r hto this 1 haldent ran IISII~II\,desire a i a t i ~ f a c t o.ra\.. lenge, and a pH meter (if availahle) can he used effectively to monitor pH change versus color development. These developments were planned, but about 5 years ago an unnlanned development produced a verv. ~rofitahle . learning experience. It happened, in order to solve a logistic problem not related to this discussion, that we asked our stockroom people to provide 2.5-gm samples of Ca(OH)2 in unstoppered vials. These are prepared during the summer and stand on the shelf for a few months before use. (Students can be informed of this fact a t the start of the experiment, hut one should defer any mention of the inevitable production of appreciable amounts of CaC03(,) due to the Ca(OH)2(,)-C02(,, reaction under these conditions). Accordingly, when such samples are added to HCI, vigorous evolution of a eas is observed. Few students are concerned hv this unauoida%le ohservation. When pressed to state thk identity of the gas they suggest Clz, 02, Hz, Hz0 vapor. Simple tests negate these guesses. It would he good practice to have available a separate apparatus for treatment of some "aged" Ca(0H)z with HCI and collection of gas samples by displacement of water. The high (or low) point in this sequence came when a student who performed very well on numerical exercises announced after 10 min consideration that the gas had to he CaC12. Teachers may be surprised a t the rarity of student suggestion that the gas could he carbon dioxide. It is possible, in this situation, to suggest consideration of standard reference material on the chemistry of limestone, slaked lime, limesand mortar, etc. (Rochow,' as you would expect, provides an excellent discussion of the relevant chemistry). The "limewster" test for CO? is g~.ncwllyrecognized, hut the mncept of m s - i d ~ drwctiun and the practical impliratims of the C ~ [ O H ) ~ ( . ) - C Osystem ~ ( ~ ) appkar to he generally unrecognized. By the end of this experiment, students may he willing to discuss t,he fact that LiOH(,) was the substance used in the Apollo 11 moon flight to remove carhon dioxide from the capsule atmosphere. Rachow, Eugene G., "Modern Descriptive Chemistry," W. B. Saundem Company, Philadelphia, 1977. p. 129.
