LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Implementation of Gas Chromatography and Microscale Distillation into the General Chemistry Laboratory Curriculum as Vehicles for Examining Intermolecular Forces Clifford M. Csizmar, Dee Ann Force, and Don L. Warner* Department of Chemistry and Biochemistry, Boise State University, Boise, Idaho 83725-1520, United States
bS Supporting Information ABSTRACT: As part of an NSF-funded Course Curriculum and Laboratory Improvement (CCLI) project that seeks, in part, to increase student exposure to scientific instrumentation, a gas chromatography experiment has been integrated into the second-semester general chemistry laboratory curriculum. The experiment uses affordable, commercially available equipment and materials. It also parallels the corresponding lecture topics, reinforcing the concept of intermolecular forces (and exposing the structural basis of these forces) through the drawing of Lewis structures, microscale distillation, and gas chromatography. Group presentations end the laboratory session, exposing all students to the entirety of analytical data obtained by their peers. Students respond favorably to the exercise, citing use of the gas chromatograph as interesting and the group presentations as highly beneficial to their understanding of the material. KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Communication/Writing, HandsOn Learning/Manipulatives, Inquiry-Based/Discovery Learning, Gas Chromatography, Lewis Structures, Microscale Lab to GC,14,16,19 only spans a single, 2 h and 50 min lab period,15,19 and introduces a new topic and set of techniques to the students.16 20 The content and reception of this experiment are described herein.21
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eginning in 2007, the chemistry and biochemistry department has made a concerted effort to acquire new scientific instrumentation for both teaching and research purposes. As part of this endeavor, a successful NSF-Course, Curriculum, and Laboratory Improvement (CCLI) proposal allowed us to purchase new gas chromatograms and infrared spectrometers. Prompted by the CCLI project, we endeavored to expose students to instrumentation throughout the undergraduate chemistry laboratory curriculum. Further, we also sought to assess the impact that the increased familiarity and exposure to instrumentation had on students’ technical competency as well as on their ability to both understand a chemical problem and design an instrument-based strategy to address it. This article describes the integration of gas chromatography (GC) into our second-semester general chemistry laboratory course; our assessment of student learning and other aspects of the entire CCLI project will be described in a future publication. This GC experiment was developed with the intent that it would aid in student understanding of the associated topics (i.e., intermolecular forces and the structural basis of these forces) and spark an interest in the chemistry professions by exposing students to modern techniques. Whereas previously reported experiments have included GC in first-year laboratory courses, they often fall into one of the following categories: introduction of the instrumentation;1 analysis of a mixture to detect, quantify, or identify the presence of a specific component;2 11 evaluation of the success of a synthesis;12,13 or introduction of a general topic or problem (e.g., isotopes).14 20 This laboratory falls into the latter category, but is unique in that it does not require previous exposure Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
’ ORGANIZATION OF LABORATORY SECTIONS General chemistry laboratory sections at this institution consist of 24 students and last for 2 h and 50 min. For this experiment, students are divided into eight groups of three. The section is outfitted with four Vernier mini GC units and associated Vernier LabQuest devices. The units are highly compact, portable, use room air as the carrier gas, and a MEMS chemi-capacitive detector (MCCD). They are also user-friendly as well as affordable, which makes them ideal for the undergraduate laboratory setting. Eight microscale distillation apparatuses are also provided, giving each group access to their own equipment for the three distillations they will perform. A projector and laptop computer are placed in the laboratory to collect and display the students’ data. ’ OVERVIEW OF LABORATORY EXPERIMENT This exercise centers on the fundamental idea of intermolecular forces, which ties in closely with the material that is covered in the general chemistry lecture curriculum. Throughout the experiment, students draw Lewis structures, operate a gas chromatograph, and perform a microscale distillation for each of their compounds. The data obtained from each of these Published: May 25, 2011 966
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Table 1. Example of Compound Sets and Student Data Entered into Class Data Table
a
A total of 8 sets of compounds were used in this experiment. The complete list of all compounds used in all sets is included in the Supporting Information. b Students enter their results into these columns. c Distillation ranges are all slightly lower than the boiling point value reported by the manufacturer (Sigma-Aldrich) for each compound; this is attributed in part to Boise’s elevation. d Students are given the Lewis structures for these compounds to serve as a starting point for the remaining two in each set.
activities illustrate how the structure of a molecule affects its interactions and thus its physical properties.
group is prepared to present their data. Students are informed of this selection process and that the last half hour of class will be dedicated to these presentations, allowing them to plan accordingly. The presentations consist of students identifying how the structural differences among the molecules in their set result in the experimental data that they observed.
Introductory Discussion and Lewis Structures
Following introductory remarks by the instructor (focusing on Lewis structures, the theory of distillation and gas chromatography, and proper use of the equipment), each student group is given a unique set of three compounds, with each set containing an obvious “trend” to the structure of the compounds (e.g., increasing molecular weight, increasing polarity, etc.). Two example sets of compounds are shown in Table 1. Students determine the Lewis structure for each chemical prior to proceeding to the next stage of the experiment.
Post-Laboratory Exercises
An electronic copy of the class data is e-mailed to all students at the end of the lab to aid in completion of their post-laboratory assignment. The post-laboratory exercise provides additional exposure to the data trends observed during the experiments and queries the student’s understanding of their relationship to molecular structure. This exercise also gives the student an opportunity to interpret and extrapolate the data by applying the observed trends to new, yet structurally related, molecule sets. In this manner, the student is exposed to the concepts on multiple levels to deepen and reinforce comprehension.
Microscale Distillation
Students assemble a microscale distillation apparatus and then distill separate aliquots of each of their compounds, thus, conducting three individual distillations. Because they are working in groups of three, this provides each student with the opportunity to perform this technique. The distillation ranges for each compound are recorded in the appropriate place in the Class Data Table (see Table 1). Gas Chromatography
Students set up the gas chromatograph according to given parameters and inject a single mixture of their three compounds. Upon elution of all compounds from the GC, students use the instrument’s software to obtain retention times for each peak. They readily correlate the peak that elutes first with the lowestboiling compound, thus, reinforcing the observations and conclusions made in the distillation section. These retention times are also recorded in the Class Data Table (see Table 1). Culminating Laboratory Activity: Student Presentations
’ HAZARDS The students use hot-plates to distill flammable organic compounds, so care should be taken to avoid direct contact with the surface of the hot plate. Further, the distillation should not be completed to dryness. The organic compounds in the laboratory are flammable and may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Direct contact should be avoided. ’ RESULTS AND DISCUSSION
Once all groups have finished their experiments and filled in their portion of the Class Data Table, the document is projected onto a screen (or blank wall) in the lab. A member of each group is chosen at random by the instructor to present their data to the class. The random selection helps ensure that each student in the
Sampling of Student Results
Students typically generate experimental results that are consistent with literature expectations. Table 1 shows a sample of representative student responses for two of the eight compound sets and, as indicated, the distillation ranges reported 967
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are reasonably consistent with the boiling points obtained from Sigma-Aldrich for each respective compound. Additionally, elution of the compounds from the GC correlates well with the boiling points and students are easily able to obtain retention times from their chromatograms. An example of a studentobtained chromatogram for compound set 7 is provided in Figure 1. The chromatogram, which is provided without annotation to demonstrate what the students are asked to interpret, indicates that 1-propanol and propionaldehyde have similar retention times, whereas the propionic acid is easily resolved. In spite of the incomplete separation, students were readily able to distinguish between these two peaks and correlate them to the proper compound.
We solicited comments from students concerning the experiment, and select responses are shown below. These are representative of the collective opinions of the students that participated in the laboratory: • “In this one the structures really affected the boiling points and it was very interesting to see that relationship. Most labs you don’t get a relationship that clearly presented.” • “This lab helped me in my understanding about the topic that we were studying. I was able to go hands on and see what I was learning, not just read it.” Despite minor criticism that the lab was too lengthy, no lab section extended beyond the allotted 2 h and 50 min time slot. We therefore attribute those complaints to an unwarranted student expectation that the procedure for all lab sessions will allow them to “leave a little early” as had often been the case in the first-semester general chemistry lab. Having a culminating lab activity (the end-of-lab student presentations) precludes this, with the advantage of “spending the full lab time” being that each student group will benefit from the efforts of the entire class, not just their partners. Additionally, the perceived value of the student presentations depends largely on the level of instructor guidance. If the instructor allows students to merely repeat their numerical data, the value of the presentations does decrease; the instructor must actively guide the students toward the physical meaning behind the data. In sections where the instructor assumed a more active role, student opinions of the presentation portion increased significantly, one commenting that “...I appreciate the end of class presentations and wish all labs had them because they are very beneficial.” Therefore, adjustments have been made in the instructor notes in the Supporting Information to direct the instructors to take an active role in the presentations.
Student Response to the Laboratory Exercise
After the laboratory session, students were instructed to complete a brief survey to express their opinions of the experiment. Overall, student responses were favorable, and the majority of students indicated that they found the laboratory to be highly informative, beneficial, exciting, and enjoyable. For example, when asked on a five point scale the extent to which they agree with the statement “the GC distillation experiment helped me understand the relationship between molecular structure, intermolecular forces, and boiling point,” 133 of the 149 respondents either strongly agreed or agreed (only 4 students disagreed, while none strongly disagreed and the rest neither agreed or disagreed). A total of 116 students strongly agreed or agreed with the statement “this experiment helped me increase my chemical knowledge and skills” (5 students either disagreed or strongly disagreed). Clearly, most students perceived that the experiment was beneficial to their learning.
The Experiment in the Context of the Chemistry Curriculum
To increase student exposure to and competency with instrumentation, it was our intent to develop simple instrumentbased experiments for students in general chemistry, and then to increase the complexity as the students advanced through the undergraduate laboratory curriculum. Currently, within the context of gas chromatography, we use the experimental pedagogy outlined in Table 2 to instill a thorough understanding of the technique. We rationalize that early and frequent access to the GC, as well as other instrumentation, should result in rich laboratory experiments that are interesting to the students and should also provide exposure to the tools and techniques chemists employ to solve scientific problems. Whereas the student feedback to the GC distillation experiment, presented above, suggests that this experiment is contributing to this goal, the overall impact of our concerted efforts to expose students to instrumentation is as yet unknown. To determine the overall impact of this and other
Figure 1. Student-generated gas chromatogram of compound set 7. This unannotated chromatogram is an example of the data students obtain and are asked to interpret upon use of the mini GC. Whereas propionic acid is well separated (the large peak with 6.5 min retention time), there is incomplete separation of 1-propanol and propionaldehyde (1.3 and 0.8 min, respectively). Students were readily able to correctly assign all peaks.
Table 2. Summary of Laboratory Curriculum as it Pertains to GC Laboratory Curriculum Year
Laboratory Course
Topics Introduced
First
General Chemistry
Injection, obtaining a chromatogram, measuring retention time
Second
Organic Chemistry
Sample preparation, correlation between peak area and composition, various detectors,
Third
Advanced Synthesis
Characterization of synthetic products, use of a chiral columns to determine enantiomeric excess, quantification of kinetic reaction products in substrate-competition experiments
Fourth
Advanced Analytical Chemistry
Separation and quantification of complex mixtures, impacts on detection limits and sensitivity
structural influences on retention time
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similar experiments with respect to students’ ability to problem solve, we have developed and implemented an assessment strategy that looks at student learning in all of our laboratory classes. The process and results thus far will be presented in due course.
(21) We would like to point out that Frank Creegan, Emeritus Professor of Chemistry at Washington College, has developed an unpublished POGIL-based laboratory that involves boiling points and intermolecular forces. This laboratory experiment served as the inspiration for the experiment presented herein.
’ CONCLUSION This laboratory experiment successfully integrates gas chromatography and microscale distillation into a general chemistry course in a manner that is both informative and interesting to students. It is readily completed within a standard time slot of 2 h and 50 min, utilizes affordable and commercially available instrumentation and chemicals, and is met with positive reviews. On the basis of such positive response to the pilot project, this lab is currently incorporated into the chemistry curriculum. ’ ASSOCIATED CONTENT
bS
Supporting Information Laboratory procedures; instructor notes; compound set handout sheets; and chromatograms. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the NSF (DUE #0737128) for support of this project. The contributions of Morgan Davis, Christopher Siepert, and Autumn White are immensely appreciated. Boise State University is also thanked for providing instrumentation, funding, and other materials necessary to complete this laboratory experiment. ’ REFERENCES (1) Nowak-Thompson, B. Chem. Educ. 2005, 10, 179–180. (2) Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056–1058. (3) Burns, D. S.; Berka, L. H.; Kildahl, N. J. Chem. Educ. 1993, 70, A100–A102. (4) Cassidy, R. F.; Schuerch, C. J. Chem. Educ. 1976, 53, 51–52. (5) Donahue, C. J. J. Chem. Educ. 2002, 79, 721–723. (6) Elderd, D. M.; Kildahl, N. K.; Berka, L. H. J. Chem. Educ. 1996, 73, 675–677. (7) Fong, L. K. J. Chem. Educ. 2004, 81, 103–105. (8) Lokken, D. A. J. Chem. Educ. 1975, 52, 329. (9) Mayotte, D.; Donahue, C. J.; Snyder, C. A. J. Chem. Educ. 2006, 83, 902–905. (10) Neuzil, E. F.; Stone, D. J. J. Chem. Educ. 1993, 70, 167. (11) Tackett, S. L. J. Chem. Educ. 1987, 64, 1059–1060. (12) Sadoski, R. C.; Shipp, D.; Durham, B. J. Chem. Educ. 2001, 78, 665–666. (13) Van Ryswyk, H. J. Chem. Educ. 1997, 74, 842–844. (14) Berka, L. H.; Kildahl, N. J. Chem. Educ. 1994, 71, 613–616. (15) Cody, J. A.; Wiser, D. C. J. Chem. Educ. 2003, 80, 793–795. (16) Kildahl, N.; Berka, L. H. J. Chem. Educ. 1995, 72, 258–260. (17) MacNeil, J.; Volaric, L. J. Chem. Educ. 2003, 80, 302–304. (18) McKay, S. E.; Lashlee, R. W., III; Petrie, G. A.; Moody, S. M. Chem. Educ. 2006, 11, 319–320. (19) Reeves, F. C.; Pamplin, K. L. J. Chem. Educ. 2001, 78, 368–370. (20) Wedvik, J. C.; McManaman, C.; Anderson, J. S.; Carroll, M. K. J. Chem. Educ. 1998, 75, 885–888. 969
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