Lauren R. Wilson
Ohio
A Research Approach in the
Wesleyan University
Delaware,
Ohio 43015
Introductory Laboratory
A s freshmen enter college with better preparation in science and mathematics, two trends have developed in the teaching of freshmen chemistry: increased emphasis on principles in lecture material and incorporation of more quantitative exercises in the lahoratory. As theoretical concepts receive increased treatment, the time devoted to the study of descriptive chemistry decreases. Many current laboratory programs are centered around exercises which merely demonstrate well-tested laws or involve analyzing classical quantitative 'Lunknowns." These conventional laboratory programs usually provide a new experiment each week. Each experiment involves a whole new set of chemicals and theories and it is usually independent of the previous week's experiment. Consequently, the student never really identifies with a compound or a reaction but instead must dash into the lahoratory, mix together chemicals, gather data by some previously devised procedure, write up the report and get ready for a new experiment next week. Such an approach creates an unnatural chemical situation and usually leaves students at the conclusion of the chemistry course wondering why chemists are excited and stimulated by their profession. On the other hand, a practicing chemist works with a system for a prolonged time and soon identifies the system as "his own." He studies the literature pertinent to his problem and carefully plans his experiments in advance, without the assurance of his instructor that he can finish in one laboratory period. Why then do we set up such an artificial system for our undergraduates and then say, "Isn't chemistry fun? See how excited I am that your results match those which I already know." At Ohio Wesleyan freshman students have responded enthusiastically to a lahoratory program which better parallels the actual research procedures that provide the excitement and challenge of chemistry. I n a program initiated with freshman honor students, the fall term begins with this unified lahoratory. The program is designed to be a free.standing teaching-and-learning device and no great effortis made to correlate the lecture material with the laboratory work. However, frequent overlaps in content do result and consequently provide a reinforcement process. Several laboratory lectures are required. I n the implementation of a unified lahoratory program the instructor must realize that a change need be made from the philosophy of the lahoratory as Rased on a paper present,ed before the Division of Chemical Education at the 155th National Meeting of the American Chemical Society, San Francisco, April, 1968.
developed in the conventional approach. No longer is the laboratory employed to expose students to all lecture topics. (Did it ever?) Instead it is intended to build independence, confidence, experimental adventurousness, and to apply these qualities in meaningful chemical situations. The first laboratory period is devoted to discussion of the philosophy and objectives of the laboratory part of the course and the class is introduced to the reference materials and research journals which make up the chemical literature. Each student is then given a homework assignment dealing with use of the library. The lahoratory experience begins with the synthesis of a cobalt compound. A procedure which describes the preparation of an unidentified material is given to each student. At first glance all procedures are similar. They utilize comparable reactants and conditions. However, in reality four different compounds are involved and different synthetic routes to the same product may also be used. The order of assignment to a compound is arranged so that adjacent laboratory students do not have the same compound, thus minimizing the tendency of students to "check" their neighbors' data and promoting a healthy growth of self-reliance and confidence. When different procedures are used, inevitably the amount of time required for the syntheses will vary. However, if the initial explanation of the philosophy and objectives of the program is sound, student objections to unequal time demands are minimized. I n fact an esprit de corps which is contagious to the rest of the class develops among those students who have difficult and time consuming syntheses. After the compound is identified, (see "Analytical Determinations and Results") each student is encouraged to pursue additional experiments of his own initiation. He must submit his proposed procedure to the instructor for review of safety hazards and availability of chemicals and apparatus. At this point no attempt is made to discourage a student from trying an experiment which appears to be weak or to have little chance of success. For those who are unsure about what experiment to initiate, a list of broad ideas relating to appropriate material from the lectures and reference reading is distributed. Notification of this phase of the lahoratory is made in the introductory lahoratory lecture and students are encouraged to record side observations in their laboratory notebooks each day. Duplicate copy notebooks are required and the carbons submitted a t the end of each laboratory period. The instructor regularly reviews these notes and returns them with comments and suggestions for further study. Three formal laboratory reports are required, the Volume 46, Number 7, July 1969
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first a t the end of the synthetic phase, the second after qualitative and quantitative analyses are completed, and the final, a cumulative report, after the individual studies are completed. A full ten-week term involving two three-hour laboratory periods per week was used to complete the program as described. Freshman students who undertook this program in their third term of chemistry proceeded more rapidly and were able to complete essentially the same library experiment and synthetic and analytical work in a seven week program of one three-hour lab per week. These latter students had previous experience in qualitative, quantitative, and spectrophotometric analysis. The Library Experimenl
I n order to introduce students to the organization of chemical literature and to prepare them for suhsequent parts of the laboratory, a library experiment is used to initiate the entire laboratory program. Part of a laboratory period (1-2 hr) is devoted to discussion and illustration of the types of reference materials available, including journals, chemical dictionaries, handbooks, treatises, etc. Particular emphasis is placed upon journals and periodicals which can easily he read by freshmen (e.g., Chemistry, J. CHEM.EDUC., Sci. Am., etc.) hut research journals are also included. The Chemical Abstracts system is explained and examples of each type of index are used. Each student is then given a laboratory-homework assignment which requires him to look up data and references pertaining to questions of types which might arise during any part of his chemistry studies. Frequently this library experiment returns to haunt the instructor in a satisfying way. Once freshmen learn to overcome the mechanics of finding information in the library many stimulating and unexpected questions arise. Preparation of Compounds
A series of chloro-ammine and chloro-ethylenediaminecobalt(II1) compounds was chosen for this experiment because they are relatively easily prepared and purified, they are stable and easily handled, and they lend themselves nicely to the basic concept of the program which is preparation and subsequent identification of a series of related compounds. The wide variety of colored solutions and solids encountered sparks students' curiosity. The compounds used are identified in Table 1. I n order to stimulate each student to sharpen his powers of observation and chemical reasoning, precautions are taken to avoid giving obvious compound name or color clues in the directions for synthetic Table 1.
Cobalt Complexes Studied
Hexaamminecahalt(II1) chloride Chlmopmtsamminecobalt(II1) chloride
[Co(NHs)sl Cla
procedures. Questions are answered in terms of encouraging the student to investigate on his own. This approach allows each student to build a sense of identity with "his compound." A real bonus sometimes results in the synthesis of trans-dichlorobis(ethyleuediamine)cobalt(III) chloride. Overheating and subsequent overevaporation of the reaction solution during the synthesis of the green compound frequently results in partial conversion to the orange tris(ethylenediamine)cobalt(III) chloride compound. Overheating of a neutral solution of the green compound will also convert the trans-dichloro compound to the violet cis-dichlorobis(ethy1enediamine)cobalt(III) chloride complex. Upon isolation of their products, some students obtain a mixture of crystals. With this built-in surprise, i t is easy to stimulate ideas for separation, identification, and extension experiments with these compounds. The enthusiasm generated among the students with the mixed crystals is contagious to the rest of the class and soon everyone is looking into the Buchner funnels to see the two color precipitate. The synthetic procedures used in the preparation of the cobalt(II1) complexes are essentially those from the literature with modifications made in quantities of reactants and in the experimental procedures in order to fit the abilities and schedules of freshman students. The intent of the experiment is for each student to prepare 10-15 g of compound in order that sufficient material will be available for individual experiments once the characterization study is completed. Not all students achieve this goal. In some cases, the instructor provides additional product when yields are too low to provide sufficient compound to allow completion of the individual experiments. Synthesis1 of [Co(NHJs]Ch Dissolve 12 g of NH,CI in 27 ml of water. Cmefully bring the solution to a boil and dissolve 18 g of CoCI2.6H~Oin the solution. Place 1 g of activated charcoal in a flask and add the hot cobalt solution, then cool the flask and its contents under running water. Add 45 ml of concentrated aqueous ammonia. and cool the entire preparat,ion until its temperature is 1O0C or lower. Slowly add 24 ml of 30% H20zto the solution with a medicine dropper while gently swirling the Rerk and its contents. (CAUTION: Keep H202 off the skin and eyes.) Heat the solution on the steam hath at 50-60°C until the pinkish tint is removed from the solution (about 20 min). Cool the solution in an ice hath and filter through a.Buchner funnel. Transfer the product, filter paper and all, to a boiling solution of 150 ml of 0.5 M HCI. Heat with stirring until the solution is again boiling and filter hot in a Buchner funnel with suction. (Use a highly retentive paper to remove the charcoal.) To reprecipitate the complex, add 21 ml of conc. HC1 and cool. Filter the solution and dry the product at llO0C for an hour. Yield (class aver.) = 13.1 g, 65%. The overall reaction for the preparation may be written as
Synthesis2 of [Co(NHa)sCI]CIp Dissolve 18 g of CoCL.6H2O in 25 ml of water. Mske a slurry of 56 g NH,C1 in 56 ml of conc. aqueous ammonia. Add the cobalt solution to the slurry, stirring well. Very slowly (two drops at a time with caution) add 11 ml of 30% HIOl with a PALMER. W.
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medicine dropper while stirring vigorously. Heat the resulting mixture on the steam bath until a thick slurry is obtained. If the slurry shows a blue tint, either the evaporationis proceeding too fast or the H?O. was added too rapidly. Add 225 ml of 3 M HC1 and heat at 60°C far 10 min. Cool the solution to room temperature and filter in a Buchner funnel with suction. (Use a highly retentive paper to prevent the fine crystals from passing through.) Wash the product with three 20-ml portions of ice wat,er and then with three 20-ml partiom of acetoneand suckdry. Transfer the product plus filter paper to a solution of 680 ml of 2 M ammonia. Warm the resulting solution on a steam bath (60°C maximum) and, after all the solid has dissolved, filter the hot solution with suction. Reheat the filtrate on the stesm hath with stirring. A t five minute intervals slowly add three 170-ml portion8 of 12 A l HCI. Cool the solution to room temperature and filter. Wash the product with three 20-ml portions of water followed by three of acetone. Yield (class aver.) = 11.9 g or
this energy and to apply it in the library, each student is asked to submit a written proposal outlining a method by which he can qualitatively test his product for the presence of each of the reactants used. This proposal is submitted in advance of the laboratory period when the qualitative analysis will he done. The instructor reviews each proposal, commenting where necessary, and the revised proposals are returned to the students prior to the laboratory period. Accompanying the returned proposals is an outline prepared by the instructor which describes the qualitative analysis of all reactants used by the class. The students compare their proposals with the instructor's and then proceed with the analysis in the laboratory. 6.6 07 . ,". For the most part the qualitative analysis of each of The overall reaction involved in the preparation is the possible species in the compounds is rather straight 2CoCI9 2NH4CI+ 8NHa + H2Oz = 2 [Co(NH&Cl]Cl&) + forward. Since the students involved were first term 2H,O freshmen, only a few had any previous experience with qualitative analysis. All students were able to devise Synthesis3of [Co(CzHa(NH&)3]CIz some type of qualitative analysis scheme and most Mix 45.6 g of 30% ethylenediamine and 12.8 ml of 6 N HCl of them would work, although some rather exotic and add this solution to a flask containing 18 g of CoClr.GH1O reagents were proposed. dissolved in 55 ml of water. Equip the flask so that a vigorous All of these compounds are water soluble and in stream of air may be bubbled through the solution. After aerating the solution for three hours, pour the solution into s. each case the standard silver nitrate test for chloride large evaporating dish and evaporate on the steam bath until a. works well since each compound has at least one ionic crust just begins to form. Then add 11 ml of 12 N HCL and 22.5 chloride. Identification of cobalt can best be done ml of 95% ethanol. Cool the solution to room temperature and by initial reduction of the inert cohalt(II1) complex filter in a Buchner funnel. Wash the crystalline product with to the labile cohalt(I1) compound. The reduction 20-ml portions of ethanol until the filtrate is clew. Dry the product in the oven a t llO°C for one hour. Yield (class aver.) = proceeds easily when metallic zinc is added to an 20.8 g, 80%. aqueous solution of the cohalt(II1) compound. The The overall preparative reaction is cobalt(I1) complex readily exchanges ligands and cobalt may be identified by extraction of the blue tetrathiocyanato complex into an alcohol-ether layer. Ammonia is evolved from each of the ammine complexes when Synthesis4 of [Co(C~H~(NH~)z)zCIz]CI they are heated with 10% sodium hydroxide and its Dissolve 30.3 g of CoCb.6H20 in 95 ml of water. To this presence may be detected with litmus paper or, with solution, slowly add 140 ml of a 10% solution of ethylenedismine caution, using a calibrated nose. The presence of while constantly swirling the reaction flask. Equip the flask ethylenediamine is more difficult to confirm. Student with a bubble tube and an exit so that a vigorous stream of air can be passed through the reaction mixture. Aerate for 10 to 12 proposals usually contain the standard organic tests hours and then add 66 ml of conc. HCI slowly with stirring. for primary amines. A somewhat simpler method is Evaporate the resulting solution on a steam bath until a thin to first check for ammonia with sodium hydroxide. crust forms on the sorface. Remove the solution from the If this test is negative, and for the compounds consteam hath and allow it to cool slowly to room temperature. Remove the solid product from the reaction mixture by suction taining ethylenediamine it will he, a fresh sample of filtration and wash the crystals on the filter disk with four 20 ml compound can be tested by the Kjeldahl procedure. portions of methanol. Follow the methanol washing with four 20 A few milligrams of solid complex is added to 2-3 ml of ml portions of diethyl ether and air dry. The product obtained of sulfuric acid and the mixture heated . consists of bright green crystals of [ C o ( C ~ H ~ ( N H ~ ) ~ ) C l ~ I C I ~ H C Iconcentrated until it turns dark brown or black. The solution is The solvated HC1 molecule may be removed by heating theproduet in an oven a t llO0C for one hour. The desired product is then cooled and made basic with 30Y0 sodium hydull green and contains the stoichiametrio quantity of chloride. droxide. Upon heating, ammonia is detected a t the Yield (class average) = 13.5 g or 55%. mouth of the test tube with litmus. Those who suspect The preparative reactions may be summarized as water in the compound may be convinced that it can be obtained by diierence once the quantitative analysis is completed. I n preparation for the quantitative analysis of his compound each student is again required to submit, in Analytical Determinations and Results advance, a proposal outlining methods for determining each of the chemical species identified by the qualitative By this time the curiosity of most students will te\ts. Their proyowls are cherked by the it~strurtor have sparked them into using their new found abilities ~ n r d r t u r n d with ,r drtrlilrd ~~rocrdurt: for derrrminina in the library and they will begin to postulate names and chloride and cobalt. To emphasize that practicing formulas for their compounds. I n order to harness chemists frequently do not carry out total quantitative analyses in their own labs, each student is told that he will receive the percentage nitrogen in his compound a F ~ ~ ~W.~ C., ~ ~"Inorganic u s , Syntheses," McGraw-Hill once he has completed the chloride and cobalt analyses Book Company, New York, 1946, Vol. 11,p. 221. op. cit., p. 222. FERNELIUS, satisfactorily.
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Table 2.
Student Cobalt Analyses (Class Averages)
Compound [Co(NHa)slCla lCo(NH~)5CIlCL ICo(en)al Cln [C~(en)~Cb]Cl
Theoretical
Experimental
Standard Deviation
22.0% 23.5 17.1 20.6
23.6% 23.6 19.6 25.6
1.0 2.4 2.4 3.6
Frequently, several students synthesizing the transdichlorobis(ethylenediamine)cohalt(III) chloride succeed in preparing more than one colored compound. They are encouraged to separate the colored crystals by physical and/or chemical methods and then to split up the analyses with some students doing each separate color and some a combination. The quantitative determination of cobalt provides an excellent opportunity to introduce a modern analytical technique and the accompanying theory at an early stage in the student's career. Analysis for cobalt is done spectrophotometrically using the hexaaquocobalt(I1) ion and Bausch and Lomb Spectronic 20's. Reagent grade Co(NO&. 6H20 or CoC12.6Hz0 is used as a primary standard for the Beer's Law plot. The cobalt(II1) complexes are decomposed in strong base, the metal ion reduced to the +2 oxidation state with zinc and hydrochloric acid, and the resulting solution filtered into a volumetric flask and diluted. The cobalt analyses tend to rnn high by this method, hut the student can easily compute an empirical formula from his data. (See Table 2.) Recent experiments have shown that a major contribution to the deviation in the cobalt analysis results from the use of Co(NO& or CoC12 as primary standards. The data obtained by students is greatly improved if primary standard grade CoS04is used to standardize student reference solutions. The total chloride content is easily obtained by direct gravimetric analysis using a standard silver chloride procedure. Extension Studies
Once the empirical formula of the synthesized product is known, each student designs additional experiments to extend the study of his compound. These studies have included
a re-examination of the synthetic procedure initially used with emphasis on improved yield and purity examination of solvent effects upon the synthetic procedure and synthesis in nonaqueous solvents synthesis of similar compound^ using the same ligand but varying the central metal ion alternate methods of gravimetric and volumetric analysis far cobalt and/or chloride application of ion exchange resins in determining the nature of theionic species in solution magnetic susceptibility measurements by the Gouy method and comparison of the low spin, diamagnetic, eobalt(III) compounds with paramagnetic compounds molecular weight determination by freezing point depression qualitative spectroscopic comparisons of similar compounds in the visible, ultraviolet, and infrared regions
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Table 3.
Compound
Student Chloride Analyses (Class Averages)
Theoretical
Experimental
Standard Deviation
method3 of separation of the mixture of compounds ohtained in the synthesis of trons-dichlorobis(ethylenediamine)cobalt(III) chloride Errors
Standard deviations for the quantitative data are reported in Tables 2 and 3. These calculations are included to illustrate the scatter of individual measurements around the class averages. Since all the analyses are conducted on compounds prepared by students, no longer can the standards of accuracy and precision developed in classical analytical chemistry he applied. Statistically, the data obtained in the chloride analyses tend to be much better than those for cobalt. This is likely due to a superiority of the analytical method used for chloride as opposed to the one used for cobalt. In the case of the cobalt determinations incomplete decomposition of the complex in preparation for the analysis will result in a brownish colored solution which does not follow Beer's Law for the hexaaquocobalt(I1) ion. All chloride analyses were done in triplicate. Most students conducted only a single cobalt determination resulting in a total of 4 to 6 measurements on each compound. In only a few cases did students have difficulty in establishing the empirical formulas from their analytical data. In all cases of ambiguity, a second recrystallization produced a product which, when analyzed, gave data that clearly indicated the empirical formula. Conclusion
Students find this program interesting and challenging. The good agreement of analytical data is rewarding and students frequently repeat divergent analyses. The enthusiasm and self-reliance built in this program is carried by the students into subsequent courses. Enthusiasm of the freshmen is contagious and stimulating to the upperclass students who are bombarded with questions they are unable to answer. The net result of this program has been increased student interest in the laboratory part of freshman chemistry. Acknowledgments
I wish to thank Dr. Violet Meek for her valued suggestions and criticisms in developing this program, my able student assistants Wesley Cosand and Barry Wilson for their cooperation and inspiration and the students of Chemistry 15-16 for their patience and enthusiasm.
