Problem-solving laboratory exercises

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. Problem-Solving Laboratory Exercises Harold Wilson John Abbott College, Ste. Anne de Bellevue, PQ, HQX 3L9, Canada Chemistry is an experimental science. Any person claiming expertise in such a field should he able to design experiments that test the hypotheses and theories currently in vogue. I t therefore seems very strange that conventional lab oroerams ( I ) aive little training in this skill, which is so k ~ s ~ n tti i athescientist. ~ We are all familiar with the traditional situation in which students are given a detailed procedure for an experiment, which they slavishly follow. These experiments usually attempt to illustrate concepts discussed in lecture and give training in practical techniques, but the students give very little thought to the actual design of the experiment. This deficiency has been noted by several authors ( 2 4 , but the maioritv of teachers seem loath to rectify the situation. The problem-solving lahoratory (PSL) approach has been develooed to give students practice in experimental design while still meeting the objectives of a traditional laboratory program. This method has now been used for five years, and its use in the manner described in this article has restored laboratory work to its proper pre-eminent position in the chemistry curriculum. The Method

Students entering John Abbott College are traditionally eraduates of Quebec hiah schools with quite varied backgrounds in chemistry. ~ i are eyoung, ~ 16br 17 years of age, and not vet readv to design experimental tests of current theories.~husthey are askkd todevise and carry out practical solutions to oosed problems. These problems' increase in complexity as they advance through the various sequential one-semester courses and eventually become of almost research level. T o illustrate this gradation a list of PSL previously done in our laboratories is given: 1. 2.

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Determine the density of an irregularly shaped piece of cork (Introductory). Determine the oercent of KNOAs) in a mixture of KNOds) and I). LiBr(s) using c h m e t r y ( ~ e n kChemistry l Determine the second electron affinity of oxygen using calorimetry (General Chemistry 11). Determine the stereochemistry of the product of bromination of rmne-cinnamicacid(21\Organic I). Invvrt~g;,te the rffrrt of sdvent on the NMR of ethyl-2.R.dibrumo-.l-phenvlprup~noate tJl torganlr 111.

The prohlem to he solved is assigned one week before the exneriment is to be attempted. A tvpical handout is shown in ~ p ~ e n d i1.x This defines the pioblem, indicates which chemicals, apparatus, and instruments will be available, and sometimes gives a hint to aid in experimental design. During the week the students design a procedure that will solve the problem and prepare a laboratory notebook. They are aware that the problem can he solved using theories and techniaues with which thev are familiar and that material currentlv bring presented in lectures may he parricularly relevnnt. A student euide ( 7 ) .supplemented wirh A N materials. is available thatoutlines theeorrect use of the apparatus and instruments provided. During the experiment the instructor

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I Sample PSL assignments and tests are available from the author on request.

provides technical assistance and answers any questions relating to laboratory technique. The recommended procedure for preparation of the lab notebook is outlined in the student guide. Laboratory periods are typically two hours long ingeneral c h e m i s t j and three hours in orpanic. During this time the student is expected to solve the problemand complete the notebook, which is handed in on leaving the lahoratory. I t has been found that this ensures excellent preparation for laboratory work and effectively prevents the "cooking of results". Occasionallv the situation arises where.. despite . their best endeavors, some students are unable to devise a procedure. These students are allowed to attempt a technique exercise. The exercise used with PSL 1 is shown in the Appendix 2. The instructions aiven are not detailed as it is assumed that thestudenr will 1,;; familiar with the appropriate trchniques. After all students haveattempted the experiment a "posfmortem" is held. This discuss&, usually held in a subsequent lecture, highlights all the errors made in technique and experimental design. By the end of this litany of errors most students are convinced that their solution must be wrong and the sigh of relief when the correct solution is revealed is invariably quite audible. PSL at the Various Levels

The introductorv course is designed for students with little or no backgiound in chemistry. Consequently, the orohlems oosed are quite simple, hut thev invariably produce a variety of methods and-results t h a t vary fromkxcellent to atrocious. The poor results in some cases are due to had experimental techniques, but in the main they aredue to poor experimental design. Many will maintain that this should he expected as the students had no previous training in science. I t has been found, however, that prior experience in conventional lahoratorv, nroarams . .. does not seem to improw matters. Thusstt~drntsin &nerd Chemistry, most ot' whom have surcrssfullv comoleted a CHKM Studv Drwram, . tend to make the same errors as the beginners. The errors made are stuoefvina. . . .. revealina- conceptual problems that went unnuticrd whtm traditional laboratory rxrrcises were used. l'hii alone indicates a -ereat need for lnlx~ratoryexperience of this type. Oraanic chemistry is notorious for its "cookbook" approach to laborator; instruction. All students entering the organic courses at John Ahhott have been previously trained in the PSL approach. This prior experience, combined with an excellent text (5)that emphasizes a problem-solving approach, has allowed successful implementation of the PSL format to organic. ~

Evaluatlon All efforts to emphasize the experimental nature of chemistry are unfortunately doomed to failure if laboratory work is not made a maior component of the course evaluation. I t has been found that for