Studying Intermolecular Forces with a Dual Gas ... - ACS Publications

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Studying Intermolecular Forces with a Dual Gas Chromatography and Boiling Point Investigation William Patrick Cunningham,* Ian Xia, Kaitlyn Wickline, Eric Ivan Garcia Huitron, and Jun Heo Department of Science, Claudia Taylor Johnson High School, 23203 Bulverde Road, San Antonio, Texas 78259, United States S Supporting Information *

ABSTRACT: A procedure for the study of structural differences and intermolecular attraction between ethanol and 1-butanol based in laboratory work is described. This study provides comparisons of data retrieved from both a determination of boiling point and gas chromatography traces for the mixture. The methodology reported here should provide instructors with an investigation that functions within a molecular structure unit, and provides the students with valuable insight into intermolecular forces. KEYWORDS: First-Year Undergraduate/General, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Gas Chromatography

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through a long capillary tube coated with a nonreactive polymer or liquid.5 Compounds present in the sample of matter are isolated after traveling through the coated tube based on the substance’s size and intermolecular interactions.6 A sensor at the far end detects the vapor, and sends the readings to a computer which graphs it as a function of elapsed time. The trace generally consists of several peaks indicating the time at which a specific chemical has separated from solution, traveled through the column, and hit the sensor. This is typically used to test the purity of a particular substance, and, in some cases, may help in identifying a compound.7 Traditionally, these are its sole purposes.8 However, a gas chromatograph may also be used to study intermolecular forces. The sizes of said peaks do not increase proportionally with the percent composition of their particular chemical in solution. Peak size depends on the gas chromatograph and specific sensor type being used. This holds true for the Vernier Mini GC, or Gas Chromatograph Plus, that is used in this laboratory experiment. A sample of matter that consists of 1 μmol of ethanol and 1 μmol of 1-butanol will not generate a peak for ethanol that is half the size of a peak for a sample of matter consisting of 2 μmol of ethanol. This is due to the different structures of the two molecules. Solutions of small or less-polar molecules and larger or more-polar molecules will require more energy to vaporize than pure samples of the smaller molecule. The same effect appears when attempting to boil the solution. A sample of chemical A mixed with chemical B may boil at a different temperature than a sample of chemical A alone. In this laboratory experiment, students will conduct both a boiling point investigation and gas chromatography run on ethanol, 1-butanol, and an equimolar mixture of both.

ne of the critical learning skills in introductory chemistry involves students understanding the difference between intramolecular forces (chemical bonds between atoms) and intermolecular forces (forces that hold molecules together in solid or liquid phases).1 Although they exist, supplementary activities for intermolecular forces are mostly limited to teacher demonstrations. Boiling point and gas chromatography procedures have been used to study intermolecular forces,2 but never in parallel. The combination of such activities would ideally allow students to learn that the London dispersion forces between molecules in a homologous series increase as the van der Waals surface area of the molecules becomes larger.3 The reason for this is that the number of possible instantaneous dipoles increases as molecular size increases.4 Being able to understand two different properties depicted through two complementary investigations improves the students’ understanding of the effects of molecule size on boiling points. Traditionally, students are taught that molecular size has a direct correlation with boiling point in most cases. However, students should also be able to explain alterations in intermolecular forces found when a mixture of similar substances with different molecular masses is investigated. Using basic analytical instruments such as temperature probes and the more complex gas chromatograph, students will visually and analytically understand the underlying definition of a solution’s boiling point and the effects of molecule interaction with different molecules in a mixture such as changes in boiling point. When using this laboratory experiment, students are expected to already understand the basics of intermolecular forces such as the differences between dipole−dipole, London dispersion, hydrogen bonding, and ion-dipole, and what causes them. Ethanol and 1-butanol were used to portray such effects described because of their ideal difference in intermolecular attraction. Ethanol, a two carbon alcohol, proved very similar to any three carbon alcohol in terms of intermolecular attraction, so a readily available four carbon alcohol was used. A gas chromatograph is an analytical instrument which slowly heats a small sample of organic matter and blows the vapor © XXXX American Chemical Society and Division of Chemical Education, Inc.

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LABORATORY INVESTIGATION There were two laboratories occurring at once in this laboratory experiment: one to find the boiling point of the alcohol Received: January 25, 2017 Revised: November 13, 2017

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DOI: 10.1021/acs.jchemed.6b00992 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

solutions, and one to generate GC traces. To find the boiling point of the alcohols, about 10 mL of each alcohol was placed in a test tube and submerged in a 400 mL beaker of cooking oil. A temperature probe was clamped and lowered into the alcohol solution in the test tube. The beaker was slowly heated using a hot plate while the temperature probe tracked the temperature as a function of time. The point where the graph plateaued indicated the boiling point of the alcohol or solution being heated. Pure 1-butanol, pure ethanol, and an equimolar solution of the two underwent the same procedure in order to determine their boiling points. This data was compared to the accepted boiling points, which were given to the students, allowing students to discover that the boiling point of ethanol in the equimolar solution was significantly higher than the boiling point of the pure ethanol.

to boiling point investigations used to study intermolecular forces until their second year due to their complexity.15 However, students are taught intermolecular forces through lectures in first-year undergraduate chemistry, using boiling point investigations elsewhere.16 The present investigation uses the fact that the boiling point of a substance directly correlates with the strength of the intermolecular forces between its molecules, allowing for first-year undergraduate students to objectively study changes in intermolecular forces through changes in boiling points. Studying Intermolecular Forces With a Gas Chromatography Investigation

The second portion of this lab required students to analyze ethanol, 1-butanol, and an equimolar mixture of the two using a gas chromatograph. They began with a basic understanding of how the gas chromatograph works, and how to use it. After the traces were made, students analyzed the particular shapes of each peak. A 0.2 μL portion of ethanol generated a thinner peak than 0.2 μL of 1-butanol, and 0.2 μL of equimolar solution generated a peak for ethanol that was one-seventh the size of the previous ethanol peak. When presented with these results the students were met with several questions. Why was the 1butanol peak thicker than the ethanol peak? Why did half the amount of ethanol generate a peak that is one-seventh the size? The latter could be answered using the same intermolecular forces explained during the boiling point portion of the lab. The prior was explained by the boiling point of the 1-butanol, which is much higher than ethanol. Because 1-butanol is a larger molecule, there are more possibilities of instantaneous dipole formation causing greater intermolecular forces and a corresponding increase in boiling point. A higher boiling point requires more energy to vaporize the molecules in the capillary tube, resulting in less kinetic energy in its vapor phase . Since time was plotted on the horizontal axis, a slower moving particle resulted in a wider peak on the graph. Notwithstanding that none of the GCs generated inconsistent traces, only one-half of the students successfully used their knowledge of intermolecular forces to explain the trends found in the gas chromatograms. These students’ reports were considered to have a “thorough understanding” or “incomplete understanding”, as shown in Figure 1. This was to be expected because it was the first time that any class used a gas chromatograph. Students were more focused on caution than analysis during the investigation because of their inexperience and the expense of the equipment. A thorough review of the Journal of Chemical Education yielded numerous laboratory experiments which use gas

Studying Intermolecular Forces with a Boiling Point Investigation

The attraction between the molecules of 1-butanol and ethanol, called intermolecular forces, determines their physical state. In order to melt solids or vaporize liquids, enough energy must be present to overcome the intermolecular forces holding the molecules together.9 1-Butanol has a higher number of carbon atoms than ethanol. This increases the length of the carbon− carbon chain. The boiling point also increases due to the force of attraction between the molecules, which increases as the molecule gets longer and has more electrons. It takes more energy to overcome the force of attraction, and so the boiling point rises. Since ethanol vaporizes completely before reaching 1-butanol’s boiling point, the equimolar mixture becomes a 1butanol solution. The 1-butanol is then left to boil at its theoretical boiling point. The laboratory procedure intentionally does not state this information, making it necessary for the students to create their own theories based on the data they observe. Traditionally, a large part of a student’s understanding of chemistry comes from visual observations.10 Two substances mix and suddenly react. Students rarely consider the interactions between molecules, instead viewing chemical processes as a sort of magic.11 Ethanol, 1-butanol, and water appear as colorless liquids at first, but exhibit different physical properties. When by itself, ethanol boils close to the accepted value of 78.29 °C. But in solution with 1-butanol, ethanol boils 10 °C above the accepted value. However, 1-butanol’s boiling point at 117.73 °C12 remains constant either when heated alone or in an equimolar mixture. Students need to think at the molecular level in order to propose molecular models which explain such macroscopic observations.13 Without doing so, one cannot explain why one liquid boils at a higher temperature when mixed with a similar liquid. Most students managed to generate boiling point graphs which clearly indicated a higher boiling point for the ethanol in solution when compared to the pure ethanol. Nearly all of the students recognized this, and approximately three-fourths of those students could correctly explain the reason behind the trend. When paired with an introductory lecture on intermolecular forces, this hands-on activity gave a slight (though not statistically significant due to an inconsequentially small sample size) improvement to students’ performances on test questions regarding intermolecular forces. The intermolecular forces between the molecules of a substance determine its physical properties;14 therefore, intermolecular forces comprise a critical chemistry concept. Undergraduate chemistry students tend to remain unexposed

Figure 1. This chart shows the various levels of understanding displayed in student submitted lab reports. B




chromatography in conjunction with mass spectrometry.17 However, the majority of high school science departments do not have an extra $400,000 to spend on a GC−MS. Thankfully, there exist less costly alternatives in the form of homemade gas chromatographs or the Vernier Mini GC Plus used in this laboratory experiment. Existing investigations which use a gas chromatography system without mass spectrometry only display the concept of gas chromatography.18 This laboratory investigation provides an avenue for which institutions may study intermolecular forces using a cheaper gas chromatograph.

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EXPERIMENTAL PROCEDURES

Materials and Equipment

Each station was equipped with a Vernier Mini GC Plus, a microsyringe for injection, an equimolar mixture of ethanol and 1-butanol, acetone, ethanol, 1-butanol, and a computer interface; alternatively, a station was equipped with a ring stand, hot plate, 400 mL beaker, a graduated cylinder, test tube, test tube clamps, cooking oil, boiling chips, temperature probe, ethanol, 1-butanol, an equimolar solution of ethanol and 1butanol, and a computer interface.

Figure 2. shows standard peak shapes and times in a standard gas chromatograph trace of a 0.2 μL equimolar mixture of ethanol and 1butanol.

Brief of Gas Chromatograph Investigation Protocol

making sure to avoid contacting the glass. The data collection program was set up to plot temperature as a function of time, recording points every second. The data collection was started 10 s before the hot plate was turned on to record the initial temperature of the mixture, and ended 1 min after the graph plateaued. This process was repeated until all three alcohol solutions were analyzed. More thorough protocols for both parts of this investigation can be found as Supporting Information in the ancillary procedures file.

Use a gas chromatograph to run samples of the ethanol butanol mixture using the parameters provided in Table 1. Table 1. Gas Chromatograph Parameters Parameter Name

Parameter

Initial temperature Hold time Ramp rate Final temperature Hold time Total time Pressure

35 °C 1 min 5 °C/min 75 °C 5 min 14 min 7.0 kPa

Sample Student Boiling Point Data

When conducting the boiling point investigation on an equimolar mixture of ethanol and 1-butanol, students produced the data shown in Figure 3. More boiling point graphs are available in the sample graphs file.

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HAZARDS Students must wear aprons and goggles during the entire procedure of the experiment. Due to the presence of glass equipment, heat sources, and volatile solutions and mixtures, students must not wear baggy clothing or loose jewelry, and

The microsyringe was rinsed with acetone three times, then filled with 0.2 μL of an equimolar solution of ethanol and 1butanol. When the gas chromatograph indicated that it is ready for data collection, the syringe was inserted, and simultaneously, the syringe was depressed and the “collect” box was clicked. The data collection ran for 14 min. The process was repeated for 0.2 μL of pure ethanol. Then, the process was repeated for 0.2 μL of pure 1-butanol. The syringe was cleansed by rinsing three times with acetone until the plunger depressed smoothly. The needle was dried with a cleaning tissue, before it was returned to its case. Sample Student Gas Chromatograph Data

When running an equimolar mixture of ethanol and 1-butanol through the gas chromatograph, students produced the data shown in Figure 2. More gas chromatography traces are available as Supporting Information in the sample graphs file. Brief of Boiling Point Investigation Protocol

The hot plate was placed on the ring stand base, and the beaker was placed on top of the hot plate. The beaker was filled with 250 mL of cooking oil. A test tube was filled with 10 mL of one of the three different solutions of alcohol(s). A boiling chip was placed inside the test tube. The test tube was then clamped onto the ring stand and lowered into the oil so that all the alcohol was submerged in oil. Then the temperature probe was clamped and lowered into the alcohol solution at least half way,

Figure 3. Temperature of an equimolar mixture of ethanol and 1butanol as it is heated at a steady rate in an oil bath atop a hot plate. C



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must wear closed-toed shoes. Boiling flammable oil and alcohol over a hot plate is safer than using a Bunsen burner, but the risk of a fire is still present due to the vapors emitted, necessitating the presence of fire extinguishers in the classroom. While heating, transferring, and measuring alcohols, students must avoid inhaling the vapors released by the solutions and mixture. The laboratory must be properly ventilated to ensure that alcohol vapors are not present in dangerous concentrations. Ventilation reduces the chances of unexpected explosions, alcohol irritation, and alcohol poisoning. If students have chemical sensitivities, the boiling must be performed in a fume hood.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00992. Sample boiling point and gas chromatography data (PDF, DOCX) Student lab report (PDF, DOCX) Student lab report (PDF, DOCX) Detailed lab procedures (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

STUDENT RESULTS

ORCID

A class of 36 AP chemistry students, working in groups of two to four, ran this experiment a total of 12 times, over a two-day period in the lab with 80 min used to complete the experiment. The main assessment of the knowledge gained by the students came in the form of lab reports that each student was required to write individually. On the basis of analysis of the student submitted lab reports, the majority of students developed theories similar to the ones presented in this article, as shown in Figure 1. Of the 36 submitted lab reports, 7 lacked basic insight, 8 mentioned underlying concepts derived from reading or lecture, 18 displayed an incomplete, but developing understanding, and 3 displayed a thorough understanding. The reports that lacked basic insight had brief conclusions that did not address why the boiling point of ethanol changed nor the shapes of the peaks. The reports that mentioned underlying concepts had varying theories that mentioned intermolecular forces as the cause behind the data. The reports that displayed an incomplete understanding related results to London dispersion forces, and surface area, but did not display a complete understanding of the concepts. The reports labeled with a thorough understanding displayed a thorough understanding of both the intermolecular forces present during the boiling point lab and the data generated by the gas chromatograph. For greater detail, refer to the provided teacher resource document. Included as Supporting Information are two exemplary student lab reports deemed to have a thorough understanding of the concepts.

William Patrick Cunningham: 0000-0002-7172-1556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the AP chemistry students who performed this lab and Toshiba for providing us with a grant that allowed our research team to purchase gas chromatographs.

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REFERENCES

(1) The College Board. AP Chemistry Course and Exam Description; http://media.collegeboard.com/digitalServices/pdf/ap/ap-chemistrycourse-and-exam-description.pdf (accessed Jan 2018). See p 62 of the document available for download. (2) Csizmar, C. M.; Force, D. A.; Warner, D. L. Implementation of Gas Chromatography and Microscale Distillation into the General Chemistry Laboratory Curriculum as Vehicles for Examining Intermolecular Forces. J. Chem. Educ. 2011, 88 (7), 966−969. (3) Chang, R.; Goldsby, K. Intermolecular Forces and Liquids and Solids. In Chemistry, 12th ed.; McGraw-Hill Education: New York, 2015; p 469. (4) Tan, D. K. C.; Chan, K. C. An Analysis of Two Textbooks on the Topic of Intermolecular Forces. Asia-Pacific Forum on Science Learning and Teaching 2004, 5, Article 3. (5) Hall, J. F. Resolution of Mixtures II: Chromatography. In Experimental Chemistry, 4th ed.; Stratton, R., Stephanian, M., Eds.; Brooks/Cole: Belmont, CA, 1996; p 78. (6) Nam, E.; Hill, M.; Randall, J. Experiment 8: Investigating Gas Chromatography. In Organic Chemistry with Vernier; Vernier Software & Technology: Beaverton, OR, 2012; p 8-1. (7) Pavia, D. L.; Lampman, G. M.; Kritz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques; Thomson Brooks/Cole: Pacific Groves, CA, 2006; Vol. 4, pp 797−817. (8) Cunningham, W. P.; Pallesen, K. T. Analysis of AlcoholsGas Chromatograph. Vernier Innovative Uses, in process for publication. (9) Zumdahl, S. S.; Zumdahl, S. A. Liquids and Solids. In Chemistry, 9th ed.; Brooks/Cole: Belmont, CA, 2014; pp 455−457. (10) Cook, M. P. Visual Representations in Science Education: The Influence of Prior Knowledge and Cognitive Load Theory on Instructional Design Principles. Sci. Educ. 2006, 90 (6), 1073−1091. (11) Hanson, R. H. Chemistry Is Fun, not Magic. J. Chem. Educ. 1976, 53 (9), 577−578. (12) Lide, D. R. CRC Handbook of Chemistry and Physics: A ReadyReference Book of Chemical and Physical Data, 88th ed.; Taylor & Francis: Boca Raton, FL, 2008. (13) The College Board. AP Chemistry Course and Exam Description; http://media.collegeboard.com/digitalServices/pdf/ap/ap-chemistrycourse-and-exam-description.pdf (accessed Jan 2018). See pp 19−37 of the document available for download.

Teaching and Learning Outcomes

In accordance to Learning Objectives 2.1, 2.3, 2.7, 2.10, 2.11, 2.13, 2.16, and 6.1 of the AP Chemistry Course and Exam Description, this investigation requires students to take into account concepts such as London dispersion forces, polar molecules, gas chromatography, molecular structure, and the experimental process in order to generate an accurate conclusion. These concepts are also crucial in understanding objective HS-PS1-3, “Matter and its Interactions”, of the Next Generation Science Standards.19 Students studied the physical properties of a homologous solution in order to better understand intermolecular forces. Of the first 36 AP Chemistry students who conducted this laboratory experiments, 29 generated accurate conclusions. Considering the broad spectrum of ideas needed to do so, an 80% success rate is judged to be quite substantial. D




(14) Wilcox, C. J. An Inquiry-Based Classroom Activity on States of Matter. The Chemical Educator 2004, 9 (5), 270−271; http:// chemeducator.org/bibs/0009005/950270jw.htm (accessed Jan 2018). (15) Struyf, J. An Analytical Approach for Relating Boiling Points of Monofunctional Organic Compounds to Intermolecular Forces. J. Chem. Educ. 2011, 88 (7), 937−943. (16) Wolthuis, E.; Visser, M.; Oppenhuizen, I. Molecular Weight Determination by Boiling-Point Elevation: A Freshman Research Project. J. Chem. Educ. 1958, 35 (8), 412−414. (17) Rosenfelder, N.; Van Zee, N. J.; Mueller, J. F.; Gaus, C.; Vetter, W. Gas Chromatography/Electron Ionization−Mass SpectrometrySelected Ion Monitoring Screening Method for a Thorough Investigation of Polyhalogenated Compounds in Passive Sampler Extracts with Quadrupole Systems. Anal. Chem. 2010, 82 (23), 9835− 9842. (18) Mclean, J.; Pauson, P. L. A Gas Chromatography Demonstration Apparatus. J. Chem. Educ. 1963, 40 (10), 539−540. (19) Next Generation Science Standards. High School Structure and Properties of Matter; http://www.nextgenscience.org/topicarrangement/hsstructure-and-properties-matter (accessed Jan 2018).

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Studying Intermolecular Forces with a Dual Gas ... - ACS Publications

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