Experimental Design in the General Chemistry Laboratory Margaret V. ~erritt',Marilyn J. schneider2, and Jeanne A. Darlington ~ e i l e s l College, e~ Weliesiey, M A 02181 The "cookbook" nature of many general chemistry laboratory experiments has been criticized increasingly as an unrealistic portrayal of chemical experimentation. Many students are able to complete such experiments with little investment of thought, or interest, on their parts. I n an effort to provide a more active learning environment, we have elected to change the emphasis in our experiments and involve students in planning their experiments. This revised approach, based on student-designed experimental plans, rather than instructor-generated procedures, was designed to achieve the following goals: emphasize the many different and correct ways of doing chemistrv. ". rrnpha3ire the interactive nature of lnhoratory research, .give students a sense of what chemists do or at least how they do it, and facilitate =eater in-depth learning of the chemistry done in the lab. ~
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Our assumption in this revision was that students would learn more chemistrv bv their active involvement in planning the experimental approach for the following reasons: they would need to understand what they were doing before, after, and while they were in lab, they would have a sense of ownership in the experiment, and by working with other members of ther lab section, they would master the principles of the experiment in order w explain what they are doing to one another. ~
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Eight Solutions Experiment (two weeks) Absorption Spectraphotometry Concentration of a Dye in Solution Fading of Bromphenol Blue (kinetics) Determination of I t o f FeSCN2+(two weeks) Acetic Add Content of Commercial Vinegars (two weeks) Neutralizing Power of Commercial Antacids Preparation and Characterization of Buffers (two weeks), and Electrochemical Determination of Equilibrium Constants.
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The approach described here was used for the first time in the spring . - semester of 1991 a t Wellesley College in the second-semester general chemistry laboratory. The course focus is on properties of solutions, chemical equilibrium and kinetics, thermodynamics, and electrochemistry. Approximately 100 students, divided into lab sections of 1 6 16. are enrolled in the snrine semester offering of the course; about 30 students take ihe course in the fali semester. The laboratory of the first semester of general chemistry is a project-type lab in which transition metal complexes are synthesized and characterized. No separate credit is given for the laboratory portion of courses a t Welleslev. In terms of a final made, the lab counts approximately i0%. The three and &half hour laboratoly sections meet weekly for 13 weeks; students complete a lab practical during the last lab. Each section is taught by a single instructor, either the lecturing faculty members with doctorate degrees or full-time lab instructors, all having bachelor's or master's degrees in chemistry. Experimental Design Laboratories-Key Components Revised Lab Manual The goal was to alter the emphasis and experimental a o ~ m a c hrather than the course content. Conseauentlv, we dl2 not select new material but rewrote the labmanual to emphasize the principles and goals of the experiment and to eliminate experimental details. In general, each experip~
'Author to whom correspondence should be addressed. 'Current address: Department of Chemistry, Lafayetfe College, Easton. PA 18042.
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ment in the manual requires no more than two pages of text. Students are encouraged to consult other sources in planning their experiments. We also developed a new ex~ e r i m e n on t absorption spectrophotometry (given below) hone as the first 1:ih of thr semkter to int;od;ce students to the plannineand interactive approach ofthecourse. The reviseb experiments were evaliated by two student colleagues. They found that a list of specific materials m d glassware was essential in providing realistic and helpful constraints for student experimental plans. In our revisions. we relied heavily on experimentd procedures developed through a NECLJSE project !Yew Knglimd Consortium for Undereraduate Science I.:ducation~(l). The ~-~ ~ - - following is a list of the experiments currently donein this course:
Journal of Chemical Education
Pre-lab Experimental Plans The heart of this a p ~ r o a c h is the reauired pre-lab experto imental plan that stidents submit t ~to ;fa$ days the lab ibr instructor evaluation. The nssigtunent, in all cases, is the development of specific, detailed written experimental procedures to accomplish the desired goals of an experiment, such as the preparation of 250 mL of 0.1 M buffer of pH 9.2 and the determination of its buffer capacity. Grades on the pre-lab experimental plans are part of the over-all grade for the laboratory. Individual lab instructors have developed several ways of assisting in the planning process. Some instructors emphasize pre-lab meetings with students. Others spend more time discussing experimental plans, and their revisions a t the beginnine of the lab ~ e r i o d In . other cases. students work togetter, with instructor input, a t the end of the previous lab to develop their experimental procedure for the next week. Student-Student Interactions in Planning and Completing Labs Our experience in the firskernester of this approach indicated that the least successful and most frustrating labs were those in which each student worked completely alone. Instruetors have developed a variety of methods to encourage student-student interactions both before and during lab. In some cases, each student submits her own plan but is encouraged to discuss her plan wit hothers. One instructor assiens a different partner each week with explicit instructions to schedule'an appointment to develop and submit a single plan for evaluation. These students then work as partners in the lab. In other sections, stndents work alone prior to the lab, but then discuss their plans with an assigned partner before beginning the experiment together.
Results The most sigruficant changes resulting from the altered cmphasis have bccn in thc improved student attitude toward the lab and in the laboratorv environment itself. Many students are initially uncertak and skeptical about their abilities to plan experiments themselves. As the semester proceeds, they begin to like the lab and take satisfaction in their masterv of the material. Student nleasure in coming to lab with fuller understanding of t i e material is manifested in increasine self-confidence in even the most timid and uncertain beginning student. The stress on eroun interactions leads to a more coneenial and oleasant atmosphere in the lab as well as more student&tudent and student-facultv discussions. The instructor does relatively little lecturing and serves more as a resource person and consultant (sometimes as a referee) as student partners work together in developing and executing theirown experimental procedures. Both student-student and stude&faculty discussions focus more on the chemical principles involved in experiments than those labs featuring the "recipe" approach. Students help one another and enjoy themselves. Most students, by the end of the semester, have developed a stronger sense of self-confidence and authority about the lab than seen in the more traditional instructional format. Students have been nearlv unanimous in their critical praise of the pre-lab experi6ental plan approach. A questionnaire was distributed and collected in the last lab meeting of the first semester of the revised approach. Approximately 90% of the written comments were completely positive. Most of the negative comments were directed a t the time commitment and frustration involved. Even the most critical comments were coupled with helpful suggestions for improvements, such as that more instruction in techniques and the use of instruments be given at the beginning of the semester. These changes have now been made in the course. A scheduling consequence of the pre-lab planning approach is that the unprepared student simply cannot do the lab work. Illness has become a more serious nroblem in that make-up time must include time for desigkng an experimental procedure. We also have found that upperclass students, unaccustomed to working with other students, have had ereater difficultv than first-vear students in making appointments withUassigned for pre-lab planning. Faculty members have been uniformly enthusiastic about their changed role in the lab as well as in their assessment of student performance. They have enjoyed the more challenging and interesting nature of interactions with their students. At the same time, the demands on the instructor are higher in the lab because of the variety of experimental approaches being pursued simultaneously. Students often propose and pursue imaginative experimental approaches that the instructors have not considered. Althoueh student ~erformanceon exams on lecture materials an; on an unrkvised final lab practical have not been noticeably better than in the past, students have all shown marked improvement in the practical skills of prenarine solutions a s well as with calculations and exnerimental operations associated with dilution. Our first semester of using this approach quickly demonstrated that it requires a greater time commitment by both student and faculty prior to the lab than the more detailed instructor-designed experiments. We have further revised the course and the manual to reduce the total amount of experimental work assigned. More laboratory time is now available for student revision of the experimental approach after initial experimentation. In addition, we have increasingly incorporated commercially available graphics
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soitware for data analysis. A mixture of PC's and MacIntosh computers are available for student use. We are currently using the programs "Graph " (MimMath Scientific Software, Salt Lake Citv, UT)on the PC's and StatView I1 (Abacus concepts, ~ e r k e l e CA) ~ , with the Macs. Linear least squares regression analysis with this software is now introduced in the first week of the semester. In summarv. ". the incornoration of a n exnerimental desim component has successfully increased student interest, euthusiasm, and active particination in the eeneral chemistry laboratory. This approach can be implemented readily elsewhere, as at Wellesley, by revision of existing experiments and lab manuals. Wholesale substitution of new experiments is not required. We now plan to extend this successful methodology to additional laboratory courses a t Wellesley
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Sample Laboratory Experiment: Absorption Spectrophotometry: Determining the Concentration of a Dye in Solution The following experiment was done in the third week of the semester to introduce the students to the interactive nature of the rest of the semester's work. The color of a solution results from the absorption ofcertam wavelennhs of white lixht fallinaon the solution. The colors depend, as you lem-ed last semester, on the electronic structure of the substance present. The wavelen&h of maximum absorption in theeabsorption spectrum is called L, (lambda max) and is characteristic of the material. The absorbance A can be used to determine the concentration of a colored material in solution. The absorbance is a function of three factors: (1) the path length, b, of the light passing through the sample
(usuallyb = 1em): (2) the molar absorptivity, a , which is characteristic of the
material studied; and (3) the concentration of the material, c. The relationship between these three parameters is expressed in an equation called Beer's Law: A = abc
Thus, for a known substance under conditions of fixed pathlength and wavelength, the absorbance is directly proportional to concentration. In this experiment, you will be given a dye solution of known concentration (your stock solution), and a solution of the same dye having a n unknown concentration. You will measure the absorption spectrum of your dye and will construct a calibration &rye (A versus concentration) by determining the absorbance of four solutions of the dve of known concentration (standards). These standardswill be prepared by dilution of the stock solution. You will then use the calibration curve to determine the concentration of your unknown solution. Experiment
1. Prepare four standard solutions (quantitatively)by dilution
of the stock dye solution you were given. Try ta prepare these solutions so that the absorbance of your unltnown solution will he in the range of your standards. (compare the intensities of color of
the solutions.) 2. Take one of your standard solutions and obtain an ahsorp., Is this consistent tion spectrum for the sample. Determine & with the color you observe? 3: Determine the ahsarbance of your four standard solutions at an appropriate wavelength. Your absorbance readings should be appmximately 0.2 < A c 0.8. If they are not, you will need to prepare more dilutions. Plot the absorbance of these solutions versus concentration. Do you get a straight line? Does it go through the origin? 4. Measure the absorbance of your unknown solution and determine its concentration. Volume 70 Number 8 August 1993
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Equipment 10-mL graduated pipets 100-mL volumetric flasks Spectronic 20 (single beam spectrophotometer) Scanning spedrophotometer Computer software for plotting data Notes to the Teacher (1)This experiment is used a t the beginning of the semester to intrbduce students to absorption spectrophatometry which is then used in two subsequent experiments (Kf of FeSCN2+andthe Fadine of Bram~henolBlue). No me-lab plan is required. Students are assigned to small groups to develop, test, and evaluate their procedure in the lab. (2) Three dyes are used per lab section: Alizarin Yellow R (stock:0.25 giL), Erythrosin B (stock:0.15 g/L),and Crystal Violet (stock: 50 mg/L). The stock solutions are stable far at least one week. Each student is given 20 mL of a separate unknown prepared daily via quantitative dilution of 4-10 mL of the stock solutions to 250 mL. (3, in-labromputing fmilitits, whde not ncrcssan, greatly farilitntc rapid ssdeisment of the rvperlmenral resuk3. M a n y students repent thew sulutivn preparation afwr viewing large scatter in their data. (4)The availability of a Hewlett Packard diode array spectrophotometer with its rapid scan capability has greatly simplified and speeded the acquisition of absorption spectra in this and other experiments. Buffers This lah is designed to give you expcricnce in preparinc buffers and rx;~mininnthem behavior. l'hr buffcrs to he studied a r e ones often used to study biological systems. The PK:~ of the weak acid component of these buffers a r e listed in the table. Acid Phosphoric Acid. H3P04
Carbonic acid, HzC03 Ammonium N H d Glycine, 'H~NCHZCO~H Tris(hydroxymethyl)aminomethane(TRIS or
2.15 7.20 12.15 6.35 10.33 9.24 2.35 (C02H) 9.78 (NH$) 8.08
THAM), (HOCHd3CNH3c You will he assigned t h e preparation of buffers suitable to study one of two enzymes: (1)Speedzyme with maxim u m activity a t pH = 9.2or (2) F a s t k e , maximum activity at p H = 7.2. Develop experimental procedures for the following assignments 1. Prepare 250 mL each of two different (different weak acid components) 0.10 M buffers suitable far studying your assigned
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Journal of Chemical Education
enzyme at the pH of its optimum activity. Remember that a 0.10 M buffer is one in which the total concentration of the components is 0.10 M:i. e., [HA] + [A-l = 0.10 M. Possible methods for preparing the buffers: (a) start with a measured portion of the weak acid, (h) start with a measured portion of the weak base, (4 start with 1M solutions of each component, (dl start with measured portions ofboth the conjugate acid and its conjugate base. 2. Using one of these buffer solutions, determine the effect of dilution on the pH of the buffer. 3. Buffer capacity is defined as the number of moles of strong acid or base needed to produce a unit change in pH of 1L of buffer. Determine the buffer capacity of both of the buffers prepared in part (1)for strong acids and strong bases. 4. Determine the effect of dilution on the buffer capacity of one of the buffers prepared in part (1). Do you expect the effectof dilution to be the same on the two buffers? If you have time, you may wish to test the effect of dilution of both buffers. 5. Repeat the preparation of one of the buffers in part f l ) using a different method of preparation. Optional Experiment Amphiprotic materials, often called ampholytrs, a r e sometimes used a s huffers. Devise and test a procedure for evaluating the huffering properties of a n 0.1 hl solution of a n ampholyte Reagents Available 0 . Solutions A. Solids 2 M ammonia sodium carbonate 0.2 M HCI sodium bicarbonate 0.2 M NaOH potassium hydrogen phosphate 6 M HCI potassium dihydrogen phosphate ammonium chloride 6 M NaOH glycine and glycine.HCI TRIS and TRIS.HCI D. Other C. Glassware 250- and 100-mL volumetric flasks pH meter magnetic stirrer 10. 25, and 50 mL volumetric pipets 10-mLgraduated pipets 50-mL burets Acknowledgment Adrienne Khol and Alice Y. Kim. both members of the Wellesley class of 1993, assisted i n -ting a n d testing the revised lab manual in Januarv of 1991. William F. Colem a n had used t h e developmeni of pre-lab plans previously i n teaching a one-semester introductory chemistry course for exceptionally well-prepared students. is experimental procedures were t h e basis for some of our revised experiments. Literature Cited 1. The Interaction ofLighfand MofferA Mod& far the Introduelov Chemlslv Loborotory: J. Darlington, A. Skinner, E. Weaver, and V. White Available through the NECUSE RoJerf Office, Science Center 123, Harvard Univemity, Cambridge. MA 02138.
