Early Exposure to Chemical Reactions in General Chemistry W.S. Richardson and J. E. Teggins Auburn University at Montgomery. Montgomery, AL 36193 I t is not uncommon for students entering general chemistry to lack a high school course in chemistry. Among our students who have completed a course, we notice that their laboratom exoerience is freouentlv meaeer. This situation can be eiplaihed, a t least partiall;, by skortages of equipment and the limited time available for high schoollaboratory procedures. Thus, in a time of renewed interest in descriptive chemistry and the tendency of textbookauthors to place more and earlier emphasis on the nature of chemical reactions', our students bring little laboratow into . experience . the chemistry sequence. cobsequently, we have developed an experiment that introduces students to a variety of actual chemical reactions a t the beginning of the course. Besides the advantage of introducing the students to reaction chemi s t when ~ we are simultaneouslv .exolorine related tooics in the-classroom, these students are e x p o s 2 to a laboratory exoerience involvine dramatic chemical reactions that tend tostimulate their interest in the subject. The emphasis of the exoeriment. comine earlv in the course. is on reasonable interpretation df chemkal eients based primarily on a careful record of detailed observations. The thrust of the exercise is to encourage students t o propose reasonable explanations for observed chemical phenomena. Such a procedure does not require prior, detailed chemical knowledge. The experiment is conducted over two three-hour lahoratory periods. During the first laboratory session time is devoted to faculty demonstration followed by student duplication of a series of flame tests and chemical reactions that will be used during the next laboratory period. Firstly, the instructor demonstrates common flame tests for a selection of metal ions in solution. After each demonstration, every student duplicates the test. We have noticed that it is particularly important for students to recognize that the flame test detects the presence of a specific metal in a compound (suhstance) and that the test is not affected by the presence of water or other elements in the compound. We believe that one of the main problems encountkred in the teaching of general chemistry involves the assumption by instructors that all students have achieved the level of chemical intuition that the instructor possessed a t the same stage in his or her career. Secondly, a similar procedure is followed for several chemical reactions that proceed with noticeable results. Suitable types of reactions include precipitation of sulfates with barium ion, evolution of carbon dioxide when acids react with carbonates. production of color as ammonium hydroxide is added to copper solutions, production of ammonia by the reaction of hydroxide with an ammonium salt, and change in color of aci-d-base indicators. Again, it is necessary t o explain carefully, for example, that carbonates produce carbon dioxide with acids, and that the other substances such as chloride in the case of hydrochloric acid for the evolution of carbon dioxide, do not affect the test. We take care to select a series of reactions in which distinctive changes take place. For example, in the above situation, the A
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Journal of Chemical Education
carbonate unknown would he the only material to produce a gas when reacted with an acid. At the next laboratory period, students are given several unknown solutions. A flame test is performed on each solution. Then each possible pair of reactants is mixed. Five unknowns reouire 10 different mixtures: six reouire 15. Detailed reportsbf the ohservations made during each phase of the flame and chemical tests are recorded in the laboratory notebook and compared to the previous experiments. A final report on each unknown concludes with the attempted identification of the compound in solution. Many common and cheap reagents are suitable for this experiment, and the unknowns actually employed can be chosen to change the degree of difficulty of the experiment. With appropriate reagents this exercise can be made very challenging for high-quality students or simple enough to be performed by nontechnical majors in an introductory course. Usine these experiments we stress the f a d that detailed reportsof all ohs&ations are important. A comment from one student's report on the reaction of barium chloride with silver nitrate exemplifies the type of observations made: On mixing, a white solid appeared. No gas was evolved. Solution remained colorless. No noticible [sic]odor or temperature change. First solution gave green flame. Note that several different factors may be mentioned. Even negative observations can be valuable if, for example, the unknown is water, or if other unknowns given to the student produce positive results for the particular test. Twicallv. most students in the class are able to make accu,;I d e g l e d observations. Conclusions concerning somewhat more than half the results will he correct. and more than half the incorrect conclusions will be based on satisfactory reasoning. Since we have introduced this experiment early in our freshman sequence, we have noticed an increase in the interest of the students in their laboratory work. Students are experiencing, observing, and doina chemistry. right . away. he labora6ry is husy,~livelydiscuisions are numerous, and worthwhile questions are being raised. This level of enthusiasm was not apparent when traditional experiment was used a t the beginning of the general chemistry sequence.
a
Presented, in part, at the 22nd Midwest Regional Meeting of the American Chemical Society, Wichita, Kansas. November 1987. (a)Brown, T. L.: LeMay, H. E.. Jr. Chemistry: JheCentralSclence; Prentice-Hall: Engiewood Cliffs. NJ. 1988; pp 134-163. (b) Holtzclaw, H. F.. Jr.: Robinson, W. R. CoNege Chernistry;Heath: Lexington. MA. 1988; pp 203-235. (c) Kotz, J. C.: Purceil K. F. Chemlstryand Chemical Reactivity: Saunders: Philadelphia. 1987; pp 79-114. (d) McQuarrie, D. A,: Rock, P. A. General Chemistry: Freeman: New York, 1987: pp 43-81. (e) Whilten. K. W.: and Gailey, K. D. General Chemistry: Saunders: Philadelphia. 1984: pp 222-258.
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