In the Laboratory
W
Simulating How a Virus Spreads through a Population: An Introduction to Acid–Base Chemistry in the Organic Chemistry Laboratory Ronald M. Jarret Department of Chemistry, College of the Holy Cross, Worcester, MA 01610;
[email protected] Separation of a mixture by chemically active extraction (1–3) is commonly used in an introductory organic chemistry course to introduce or review acid–base chemistry, as well as common lab techniques. At Holy Cross College, the separation of 4-aminoacetophenone and benzoic acid is the first experiment performed in all sections of Organic Chemistry I. Most students take this course during the spring semester of their first year, after one semester of discovery-based general chemistry with lab (4–6 ). In keeping with our guided inquiry or “discovery” format (5, 7–9), students make solubility–miscibility observations that will be used to determine which materials are needed for the extraction. Recently, student participation has been further increased by the addition of an optional exercise that uses acid–base chemistry to simulate how a virus spreads through a population. Versions of this simulation have been used in other science departments (10). Now it has been adapted as a means to help introduce acid–base chemistry in an organic chemistry laboratory. Chemically Active Extraction Students are given about 1 gram of a solid mixture having varying ratios of benzoic acid and 4-aminoacetophenone (and possibly sodium chloride). In the interest of time, they are instructed to isolate and purify only one of the organic components. Isolation is performed by extraction of the sample (dissolved in ethyl acetate) with the appropriate aqueous acid or base solution, using a separatory funnel. The extracted salt is neutralized. The precipitate is filtered and then recrystallized with water. The melting point of the dried material is used to establish the purity of the sample. This portion of the three-component exercise is quite traditional. It takes up the bulk of a 3-hour lab period. Even with careful instruction, many students go through the motions without an appreciation for the chemistry that is going on. In an attempt to promote better understanding of the extraction procedure, each student is asked to contribute one solubility– miscibility observation to the pre-laboratory discussion.
Table 1. Solubility–Miscibility Data Compound
Water
AH
HCl(aq) NaOH(aq)
i
i
B
i
NaCl
s
A᎑ Na +
Ethyl Methanol Acetate
s
s
s
s
i
s
s
s
s
i
i
s
i
s
i
i
Water
–
–
–
i
s
HCl(aq)
–
–
–
i
s
NaOH(aq)
–
–
–
i
O
O
s O O− Na+
CH3
OH H2N
AH: benzoic acid
B: 4-aminoacetophenone
A− Na+: sodium benzoate
The solubility–miscibility observations are used during the extraction prelab discussion. Students provide the information to complete a table that lists the organic solids to be separated against solvents and aqueous solutions (see Table 1). It quickly becomes easy to see why one would dissolve a mixture of benzoic acid and 4-aminoacetophenone in ethyl acetate (not methanol), and remove the carboxylic acid with NaOH(aq) and the amine with HCl(aq). The chemical reactions (including subsequent neutralization reactions) are then listed and discussed. Sodium benzoate is a recent addition to the solubility– miscibility list. Its behavior in the various solvents and aqueous solutions helps students to clarify the chemistry and location of benzoic acid and its conjugate base during the extraction process. It also facilitates discussions of neutralization reactions. To test the students’ understanding of acid–base chemistry, they are asked to partake in an exercise that uses the solubility– miscibility materials to simulate how a virus spreads through a population. Simulation of a Virus Spreading through a Population
Solubility–Miscibility During a brief orientation meeting before the first fulllength laboratory experiment, the lab supervisor asks each student to perform a simple mini-experiment. Each student mixes specified amounts of two chemicals or solutions (as identified by the instructor) and records her or his observation. The materials and combinations are relevant to the acid–base extraction lab. The list includes 4-aminoacetophenone, benzoic acid, sodium benzoate, and sodium chloride, to be mixed with water, HCl(aq), NaOH(aq), ethyl acetate, and methanol (solvent combinations are also assigned).
Students are told that they are part of a culture that exchanges fluid (carried in a test tube) when greeting others. Unfortunately, one of them is infected with a virus. There are many variations that can be performed. The one most relevant to the extraction lab involves a society that has developed a vaccine against contracting the virus. Sadly, safe doses will not cure a person already infected with the virus, and a certain percentage of the population has an allergic reaction to the vaccine. Each student is given a numbered test tube and disposable pipet. Most have water, some have a concentrated solution
JChemEd.chem.wisc.edu • Vol. 78 No. 4 April 2001 • Journal of Chemical Education
525
In the Laboratory
of sodium benzoate, and one has aqueous sodium hydroxide (the virus). Each is given a few drops of the vaccine (aqueous hydrochloric acid, in the same concentration as the sodium hydroxide used). A heavy white precipitate (allergic reaction) forms in tubes containing sodium benzoate, and students having these tubes sit out. The rest are instructed to exchange 1 drop of fluid with another person, recording the time and tube number of the exchange. With a group of about 24 students, repeating the exchange with two additional people (for a total of 3 exchanges) works well. After the exchanges, each tube is tested with phenolphthalein to see who was originally infected. Knowing only that phenolphthalein was used as the indicator (and that other materials were part of their solubility– miscibility list) students are asked to determine how the simulation “works”. After being reminded of the pH range at which the indicator changes color, students are able to identify what is used for the virus, vaccine, and allergic reaction. They are then asked to think of another set of chemicals to perform the simulation if phenolphthalein is replaced by an indicator that changes color under acidic conditions. Once again, students are usually able to logically work through the problem and suggest that HCl(aq) could be used as the virus, NaOH(aq) as the vaccine, and the conjugate acid of 4-aminoacetophenone as the allergic reaction. Time permitting, a variation on the exercise is conducted with a second set of samples. While it does not contribute directly to the concepts of the acid–base experiment, this version requires group effort to solve a problem. In this society, there is no vaccine but one’s original fluid is protected from exchange. That is, the original fluid is given to others but it is received in a second test tube that is carried in addition to the one containing the receiver’s original fluid. After the exchanges are completed, the receiver tubes are tested. The class is asked to work together to determine the identity of the originally infected person. Since the exchanges are only in one direction, this is a relatively easy task (if good notes are kept). Verification is readily achieved by testing the original fluid of the suspected person. In the final variation (with a third set of samples), no precautions are taken by the members of the society. Exchanges are in both directions and the analysis becomes more difficult because of the large number of infected individuals. Given enough time and guidance, students can usually narrow down the identity of the originally infected person to two possibilities (those involved in the first exchange). This exercise demonstrates the importance of keeping good records and working together to solve a problem. Hazards Standard precautions should be taken when handling acids and bases. Ethyl acetate, benzoic acid, and 4-amino-
526
acetophenone are all skin irritants. Avoid contact. Ethyl acetate is a flammable liquid. The chemicals (benzoic acid, 4-aminoacetophenone, and ethyl acetate) should be disposed of by incineration. Conclusion Given the importance of student participation in our discovery approach, the virus simulation exercise gives students a chance to more quickly feel comfortable with one another. It also helps to reduce the tension that many students feel their first week in an organic chemistry lab. Further, it reinforces the concepts of the acid–base extraction exercise and the need to carefully make and accurately record observations in the lab. The set of virus simulation exercises has been used with a variety of groups ranging from middle school children to parents of prospective college students. Here, the focus tends not to be on the chemistry behind the exercise. An emphasis can be placed on group problem solving. Alternatively, the exercise can be used to evoke discussions on various social issues, including the relationship of science and society. Acknowledgments Jamie New provided valuable assistance in the preparation of the exercises. Mary Morton graciously shared her knowledge of the virus simulation exercise in its simplest form. The work was supported by a grant from the CocaCola Foundation and by College of the Holy Cross. W
Supplemental Material
Detailed instructions for these exercises are available in this issue of JCE Online. Literature Cited 1. Amsterdamsky, C. J. Chem. Educ. 1998, 75, 219. 2. Weichman, R. L. J. Chem. Educ. 1974, 51, 589. 3. O’Hara-Mays, E. P.; Yuen, G. P. J. Chem. Educ. 1989, 66, 961. 4. Ditzler, M. A.; Ricci, R. W. J. Chem. Educ. 1994, 71, 685. 5. Ricci, R. W.; Ditzler, M. A., Jarret, R. M.; McMaster, P. D.; Herrick, R. S. J. Chem. Educ. 1994, 71, 404. 6. Ricci, R. W.; Ditzler, M. A. J. Chem. Educ. 1991, 68, 228. 7. Jarret, R. M.; New, J.; Karaliolios, K. J. Chem. Educ. 1997, 74, 109. 8. Jarret, R. M.; New, J.; Patraitis, C. J. Chem. Educ. 1995, 72, 457. 9. Jarret, R. M.; McMaster, P. D. J. Chem. Educ. 1994, 71, 1029. 10. Kerrigan, P. K. CHED Newslett. 1999, Fall, 10.
Journal of Chemical Education • Vol. 78 No. 4 April 2001 • JChemEd.chem.wisc.edu
