JChemEd Activity uses Boiling Point Investigation and Gas Chromatography to Teach Intermolecular Forces in AP Chem

By W. Pat Cunningham on April 12, 2018


AACT members now have access to 25 ACS Publications articles each year. We are highlighting one article or activity each month that you might consider downloading through this member benefit. This month we are featuring an AP level “dual” investigation from the February, 2018 issue of the Journal of Chemical Education, titled “Studying Intermolecular Forces with a Dual Gas Chromatography and Boiling Point Investigation” by AACT member W. Patrick Cunningham and four members of his 2016-17 Scientific Research and Design class at Johnson High School, San Antonio.

The investigation, structured as an AP dual lab experiment to be done in conjunction with the study of bonding forces, answers a number of needs. First, it helps students clarify through a hands-on investigation the difference between intramolecular forces (covalent bonds, in this case) and intermolecular forces (polar-polar and London dispersion forces, in this case). Second, it gives students the opportunity to work with chromatography beyond the simple paper chromatograph usually encountered in middle school or first year chemistry. Third, it prompts students to visualize and draw representations of inter and intramolecular forces, in this case for homologous alcohols, ethanol and butanol.

The boiling point investigation utilizes temperature probes and graphing software generally available to science classrooms (cf. Vernier: https://www.vernier.com/products/sensors/temperature-sensors/; Pasco: https://www.pasco.com/prodCatalog/PS/PS-2125_pasport-temperature-sensor/index.cfm) (Accessed February 25, 2018)

The gas chromatographs are not commonly found in science classrooms, because the kind used in university and commercial research facilities typically run over $10,000 per unit, and the carrier gas is usually helium, which may be in short supply in some areas. However, in recent years, student-use GC apparatuses, such as Vernier’s mini-GC plus, have become more affordable. Our first Vernier GC was purchased with funds from an ACS-Hach grant matched with money raised by the school’s ACS Chem Club. After the first year of using GC in three AP chemistry sections, the students strongly requested more GC units, and Toshiba provided a grant for two more of the devices. They are used in this investigation and again used late in the AP year, after the AP chemistry exam, to analyze organic compounds synthesized by the AP students.

The boiling point v time graphs and gas chromatography traces provide students with complementary empirical evidence of the larger intermolecular forces shown by larger molecules. Typically the students use equimolar mixtures of ethanol and 1-butanol, since these two alcohols give good GC trace separation, and show distinct boiling points if boiled separately and unique boiling point graphs when boiled as a mixture. Moreover, the boiling points are within the temperature probes range and only require a cheap cooking oil heating medium on a hot plate.

The graph shown here is reproduced from the article by permission.

Chromatography

The supporting information for this activity includes detailed lab procedures, a student lab report, and sample data, which can be downloaded as a PDF file or Word Document. If you have a favorite ACS Publication article that you use in your classroom, please share it with us at AACTconnect@acs.org.