In the Laboratory
Intermolecular Forces in Introductory Chemistry Studied by Gas Chromatography, Computer Models, and Viscometry Jonathan C. Wedvik, Charity McManaman, Janet S. Anderson, and Mary K. Carroll* Department of Chemistry, Union College, Schenectady, NY 12308
The important concept of intermolecular forces can be difficult for students to grasp. We teach about intermolecular forces by integrating traditional viscometry-based physical measurements with modern chromatographic analysis and use of computer-based molecular models. This experiment also introduces the theory of chromatographic separations and incorporates relevant “real-world” samples, methods, and techniques. The three-hour experiment is used in our firstterm introductory chemistry laboratories, in which the majority of enrolled students are science or engineering majors. Gas chromatography (GC) is a widely employed separation technique in industrial and research laboratories. There are numerous GC experiments in the literature for the college chemistry laboratory. Historically, most of these were intended for mid- to upper-level classes, such as organic chemistry and instrumental analysis (1–3). Although the initial expense of buying GCs is significant, the small sample volumes required for GC analysis result in lower student exposure to potentially hazardous chemicals, and the short analysis time required for GC renders the technique applicable to introductory laboratories that have relatively large numbers of students. The number of published articles referring to use of GC in the introductory chemistry laboratory is small but is increasing (4–11). This experiment demonstrates how differences in intermolecular forces allow chemists to separate and identify mixtures. As part of this experiment, students perform qualitative GC analyses of mixtures of n-alkanes and identify a liquid sample that simulates one collected at an arson scene. The task of solving a “crime” keeps the students interested and focused on the experiment; this type of determination of flammable mixtures is used in forensic chemistry laboratories and in other laboratory experiments (10, 12). The separation of the n-alkanes and “arson samples” in the GC column is based primarily on differences in London forces. To complement these data and to help students understand better how intermolecular forces affect the properties of compounds, a viscometry experiment and computer models are used. Students relate the observed viscosity of a few organic liquids to their structures and intermolecular forces in the liquid state. Computer modeling allows students to envision, on a molecule-by-molecule basis, three-dimensional structure and polarity. Experimental Conditions
Materials All chemicals were purchased from Aldrich or Fisher Scientific and used without further purification. Gasoline, paint thinner, and charcoal lighter fluid were purchased from local merchants. *Corresponding author.
Gas Chromatography The two gas chromatographs used for this experiment are identical Perkin Elmer Autosystems; each is equipped with a flame ionization detector and a 15-m fused-silica capillary column with an inner diameter of 0.32 mm and a methyl silicone film thickness of 1.0 µm. Both GCs have been adjusted to yield consistent and comparable results. The temperature program used begins at 70 °C for 1 min, ramps at 20 °C/min to 100 °C and holds at 100 °C for 1 min, and then ramps at 20 °C/min to 160 °C and holds at 160 °C for 3 min. The flow of helium carrier gas is regulated at 7 mL/min. The same temperature program is used throughout the experiment. The sample injection port is kept at a constant temperature of 200 °C. The flame ionization detector is kept at a constant temperature of 250 °C. The GC instruments we use in the course are not equipped with laser printers. In order to get print-quality chromatograms for Figure 1, GC analysis of the three accelerants was performed using a similar temperature program on a Hewlett-Packard model 5890 gas chromatograph equipped with a 30-m column (0.25-mm diameter, 0.25-µm phenyl methyl silicone stationary phase) and a Model 5971A mass-selective detector. Modeling We use the MacSpartan computer program (Wavefunction, Inc.) to visualize the three-dimensional structure of a homologous series of organic compounds and molecules used in the viscosity experiment and to calculate dipole moments and electrostatic potentials. Files containing the structures and dipole moments of the n-alcohols (methanol to hexanol), npentane, 2-pentanone, and water were constructed and saved before the student laboratory period. Students are not expected to have prior experience with the MacSpartan program and are provided with a brief introduction to the use of the software for viewing information saved in files. Viscometry Cannon-Fenske type viscometers are used in fume hoods. No attempt to regulate the temperature of the liquids is made. Students use pipet pumps to draw the liquid up to the first line on the viscometer and use laboratory timers to measure the time for the liquid level to drop to the second line. Although we use commercial viscometers, such measurements can be performed with simpler, and correspondingly less expensive, equipment (13, 14). Laboratory Experiment Overview This experiment is divided into four sections: (i) prelaboratory reading and problem-set assignment, (ii) GC analysis of an unknown mixture of n-alkanes and of a sample that simulates arson evidence collected at the scene of a crime,
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Figure 1. Gas chromatographs of (A) unleaded gasoline, (B) charcoal lighter fuel, and (C) paint thinner.
(iii) examination of the relationships among a sample’s structure, boiling point, dipole moment, and viscosity, and (iv) post-laboratory questions. During the laboratory, students work in groups of three or four and share their data with other groups. Following the experiment, students submit laboratory reports individually.
Pre-Laboratory Work In preparation for the experiment, students read the experiment instructions and answer a series of questions about the relationship between a compound’s structure and intermolecular forces.1 They are asked to prepare a plot of boiling point vs molecular mass for the n-alkanes and to explain the trend they observe. Students bring their plots and answers to the other pre-laboratory questions to the laboratory. The laboratory period begins with a brief class discussion, led by the laboratory instructor, of the principles and theory behind the gas chromatograph. Chromatographic Analyses The laboratory instructor gives a brief tour of the instrument to two student groups at a time, demonstrating the proper sample injection technique while illustrating how to perform a typical GC analysis.2 Sample volumes of 0.25 µL are injected via a 1.0-µL syringe equipped with a Chaney adapter and guide assembly, which assures reproducible sample volumes and aids in preventing syringe damage.3 Four analyses are performed by each group: an “arson” sample, two pure hydrocarbon samples (n-pentane through n-nonane), and a mixture containing two to six alkanes. Each group collects all of its data during its time at the instrument. All samples are presented to the students in sealed vials with rubber septum caps, which results in minimal student exposure to the compounds and mixtures. Analysis of n-Alkane Sample After all of the groups have completed their GC analyses, students consult with other groups to pool retention times on the pure hydrocarbons and use this information to determine which components are present in their assigned unknown 886
mixture. Pooling of chromatograms is required, because no individual group collects data on all of the standards. This promotes cooperation among the students in the laboratory and also ensures that the GC analyses can be completed within the three-hour laboratory period. In this section of the experiment, students observe the individual peaks of the chromatograms and relate molecular weight and boiling point to retention time qualitatively. For a homologous series of organic compounds, larger molecules experience greater London forces;1 thus, they have higher boiling points and longer retention times than smaller molecules. Students identify the components of the unknown nalkane mixture by comparing the observed retention times with retention times of the standards. Analysis of a “Forensic” Sample Next, students analyze an unknown sample that simulates evidence collected at an arson scene. The sample consists of either unleaded gasoline, paint thinner, or charcoal lighter fuel, which are the most widely used accelerants in crimes involving arson (12). Others have demonstrated that it is possible to simulate more realistic arson analyses by soaking wood pieces in the accelerant, charring the wood, and having students perform headspace GC analysis (10). The chromatograms of these three flammable mixtures (Fig. 1) are very complex and distinct, so it is relatively easy to distinguish between accelerants; however, identification of each component of these mixtures would be difficult and tedious. A fingerprint method of sample identification in comparison with previously run mixtures whose identities are known is utilized to identify the unknown flammable hydrocarbon mixture. Students learn to recognize patterns in the chromatograms and quickly discover which flammable mixture was used by the “arsonist”. This section of the experiment is designed to pique the students’ interest by adding the element of mystery. It illustrates that it is possible to identify (“fingerprint”) very complex sample mixtures and also that simple chromatograms with fully resolved peaks are not generally the types of chro-
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In the Laboratory
matograms that chemists see in “real-world” analyses. The use of flammable hydrocarbon samples also gives students a look at the chemical complexity of everyday products.
Molecular Structures and Intermolecular Forces Students will further examine the relationship between a compound’s structure and its intermolecular forces in the liquid phase by performing other laboratory and computer experiments at times when they are not using the GC instruments. Relationship between Structure, Boiling Point, and Dipole Moment We provide the students with a table of boiling points and formula masses of several n-alcohols and ask them to examine how this homologous series differs from the n-alkanes. They open prepared MacSpartan files for each n-alcohol, study the shape of the molecule using real-time rotation with a Macintosh mouse, look at the electrostatic potential surface, and record the molecule’s previously computed dipole moment. Each member of the n-alcohol series has similar intermolecular forces (London forces, dipole–dipole interactions, and hydrogen bonding); students discover the correlation between dipole moment and formula mass for the n-alcohol series. Next, students examine the shape and record the dipole moment of water using a previously prepared MacSpartan file. We ask them to explain why the normal boiling point of water falls between those of propanol and butanol. Finally, using MacSpartan, students compare the shapes and electrostatic potentials and record and compare the dipole moments of n-pentane, 2-pentanone, and 1-butanol. Representative data are shown in Table 1. Relationship Between Structure and Viscosity Using a viscometer, students measure the time required to drain equal volumes of n-pentane, 2-pentanone, and 1-butanol; sample data are presented in Table 1. These molecules were selected because they have similar shapes. n-Pentane is nonpolar, whereas 2-pentanone has a permanent dipole moment, and 1-butanol can hydrogen bond to itself. Consequently, the flow time for n-pentane is the fastest and for 1-butanol is the slowest. Even though 2-pentanone has the highest molecular weight, it does not hydrogen bond to itself and flows faster than the n-butanol. We ask students to interpret the observed trend in flow times but do not require them to calculate viscosity. Students observe that, for molecules of similar length, the one that hydrogen bonds to itself will undergo the strongest intermolecular forces and be the most viscous in the liquid phase even if it does not have the largest dipole moment.
Post-Laboratory Work In post-laboratory questions, we ask students about the concepts of intermolecular forces, experimental technique, and experimental error. Representative questions are shown in the Box. Answers to these questions must be incorporated into each student’s written laboratory report and questions of this type are considered “fair game” for the next in-class examination. Discussion This experiment incorporates a variety of measurements, thereby forcing students to integrate observations of several types, leading to a better overall understanding of intermolecular forces. It reflects the excitement of chemistry and its use in solving “crimes”. Students work cooperatively, which can produce a more effective learning environment. During the course of the experiment, students are introduced to GC analysis, computer models, and viscometry. We have designed the experiment to emphasize three-dimensional molecular shape and dipole moments in the computer modeling section, London forces in the GC experiment, and the effect of hydrogen bonding in the viscometry-based measurements. Acknowledgments We thank Douglas E. Tanner, Tara K. Morcone, Candace Foust, and Charles W. J. Scaife for assisting us in testing the experiment at Union College, and the members of the Union College General Chemistry Curriculum Committee for helpful discussions. J.C.W. would also like to thank Julie Shepelavy (Ballston Spa High School, Ballston Spa, NY) for her comments on the experiment and Linda C. Brazdil (John Carroll University, Cleveland, OH) for helpful correspondence. The gas chromatographs used in this research were purchased as part of our General Chemistry laboratory renovation, supported by a grant from the W. M. Keck Foundation of Los Angeles. C.L.M. acknowledges support through a Dow Chemical Foundation Summer Undergraduate Scholarship for Excellence in Chemistry.
Sample Post-Laboratory Questions 1. Do you think that there is significant intermolecular interaction between the stationary phase (methyl silicone polymer) and the mobile phase (helium)? Are there significant intermolecular attractions between the alkanes and helium? Explain. 2. Accurate viscosity measurements must be performed with the viscometer partially submerged in a temperaturecontrolled bath, to ensure that the liquid is held at constant temperature. Why is temperature control so important for viscosity measurements? 3. Would you expect n -hexanol or 2-hexanone to have a higher viscosity? Explain your reasoning. 4. A student compares a chromatogram obtained for a mixture in the laboratory with a reference chromatogram of pure compound “X”. In each chromatogram there is a peak at tR = 2.38 min. The student concludes that X is in the mixture. Further analysis shows that X is not a component of the mixture. Suggest two reasons that could account for the student’s incorrect conclusion.
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Notes 1. In the lecture/recitation portion of the introductory chemistry course, the topics of London forces, dipole-dipole interactions, and hydrogen bonding are covered in depth. 2. The viscometry and computer modeling portions of this experiment could be combined with a “dry-lab” approach to GC data collection and analysis at institutions that do not have GC instrumentation available in their general chemistry laboratories. 3. Even though we use syringes equipped with Chaney adapters and guide assemblies, the main source of error for the GC portion of the experiment is sample injection into the GC. The only way to improve injection reproducibility is to have students practice their injection technique and make replicate measurements; however, there is insufficient time to do this in a single lab period.
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2. Leary, J. J. J. Chem. Educ. 1983, 60, 675. 3. Harvey, D.; Byerly, S; Bowman, A; Tomlin, J. J. Chem. Educ. 1991, 68, 162. 4. Neuzil, E. F.; Stone, D. J. J. Chem. Educ. 1993, 70, 167. 5. Burns, D. S.; Berka, L. H.; Kildahl, N. J. Chem. Educ. 1993, 70, A100. 6. Garner, C. M. J. Chem. Educ. 1993, 70, 310. 7. Corkill, J. A.; Raymond, K. W. J. Chem. Educ. 1994, 71, 202. 8. Berka, L. H.; Kildahl, N. J. Chem. Educ. 1994, 71, 613. 9. Kildahl, N.; Berka, L. H. J. Chem. Educ. 1995, 72, 258. 10. Elderd, D. M.; Kildahl, N. K.; Berka, L. H. J. Chem. Educ. 1996, 73, 675. 11. Brazdil, L. C. J. Chem. Educ. 1996, 73, 1056. 12. Orna, M. V. ChemSource—Instructional Resources for Preservice and Inservice Chemistry Teachers; Forensic Chemistry: New York, 1994. 13. Sorrell, C. A. J. Chem. Educ. 1971, 48, 252. 14. Daignault, L. G.; Jackman, D. C.; Rillema, D. P. J. Chem. Educ. 1990, 67, 81.
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