When an Experiment Fails Laboratory exercises are assigned in general chemistry courses with the intention of illustrating a principle, albeit often with the expectation that students will acquire skill in the use of corresponding techniques. In introductory courses,especially, oversimplifications necessarily are made in the presentation of material, with the consequence that minor perturbations can produce a variety of seemingly anomalous results. This can he very frustrating t o both student and instructors; however, the alert teacher can turn an experiment that "fails" to advantage. Some years ago a colleague, Henry Heikkinen, and 1 were presenting the CHEM Study approach t o a group of high school teachers in aSummer Institute a t the University of Lucknow, in India. When the participantsperformed the exercise in which the 11-clock reaction is employed to demonstrate, among other things, that the rate of a reaction increases with increasing temperature, they in fact found the opposite. The higher the temperature the longer it took for the Iz color to appear. It turned out that the assistant charged with the preparation had used thiasulfate instead of hisulfate. The system with the thiosulfate had behaved in a manner not qualitatively distinguishable from the classical system. (On the basis of thermodynamics, a t least, i t is possible thaL the mechanism is the same in both eases. According to the original analysis given by Landolt in 1886, the time (8) required for the appearance of the color is given by 0 (ll(kL- kz))ln(kdkz), where k l and kzare pseudofirst-order rate constants for the oxidation of S 0 F by 103- and the oxidation of I- to 12, respectively. Thus R could vary with temperature in either direction, depending upon the values of k l and kn and the manner in which they vary with temperature. Actually I was reminded of the indicated experience by a passing statement in Pannetier's text' to the effect that in the presence of I- the reaction between lo3- and HSOX- has s negative temperature coefficient!) Mv ultimate reaction t o the "mishao" was one of satisfaction. The oartieioants had aooruaehed the exercise inanex.. tremrlv 1,la.e manner, n h s u l u t r l y ~ e r t . of ~ itheoutrcme. ~~ Swne werr ,c, irnhwd with n f d m y inrrccany rhat theirexpertatams h~ tultillrd rnat thty dud not want to t4ieve their own d , s ~ r v n t i ~ ,Inn f.w thr class learned murr h n the ~ mrurrrnw d t h e r w n t t r example than they would have if thing, had rune arrord~ngi n plan. They Imrwo the hazard, oiaccepting. categorically, generalizations such as "The rate of a chemical reaction always increases with increasing temperature." The consummation devoutly to he desired is that students achieve a level of understanding a t which the reasons for the rule and the exception have meaning. We do encounter studenb who appear to have considerable potential, hut who have yet t o realize that the real world is more complicated than our models of it. One way of providing a means for the development of this awareness would he the assignment of the task of measuring the rate of the reaction between I- and SzOp2- as a function of temperature. There isan implied moral for the teacher. It can never happen that every teacher will develop in its entirety each lahoratory exercise he or she assigns. It will continue to he advantageous to use more or less standard procedures that well exemplify a principle. Nevertheless all of us should make every possible effort to augment our knowledge of the exercises we assign. The greater that understanding the less likely i t is that one will he surprised by unexpected results and the more likely that one will find ways for improving the procedures or of discerning the reasons for the little anomalies that will appear and which often are wrongly assigned to ineptitude on the part of the students.
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Pannetier, G., and Souehay, P., "Cinetique Chimique," in the series "Ch~mieGgnCrale," Masson e t Cie, Paris, 1966.
UniversitB de Dakar Dakar-Fann, Senegal
528 i Journal of Chemical Education
L. F. Koons
