Interventions Encouraging Students to Draw and Reason Regarding Intermolecular Forces Improve Ability to Predict Relative Boiling Points From Lewis Structures

Title: Investigating the impact of three-dimensional learning interventions on student understanding of structure–property relationships

Authors: Sonia M. Underwood, Alex T. Kararo, and  Gabriela Gadiaa 

Journal: Chemistry Education Research and Practice

Year: 2020

Chemists often infer the macroscopic behavior of compounds from the microscopic behavior of individual molecules. Lewis structures, like those shown for ethanol and dimethyl ether in Figure 1, in turn, communicate what forces individual atoms and molecules are subject to. Novice students, however, often struggle in translating from a Lewis structure to a pattern of forces acting on molecules and then from this microscopic environment to macroscopic behavior. Researchers from Florida International University recently explored whether a series of interventions could help students develop these skills. Specifically, they measured the students’ ability to go from the Lewis structure of a small molecule to a drawing of the intermolecular forces present to a prediction and explanation of the boiling points of similar compounds.

Figure 1: Lewis Structures of ethanol and dimethyl ether

            The researchers studied 1,000 students taking a two-semester course in a large southeastern research university taught by full time instructors. The students were split into four groups. Group 1 was enrolled in an active-learning chemistry class with no specific interventions to improve their three-dimensional learning. Three-dimensional learning (3dL) teaches students to connect the fundamental concepts of a scientific discipline, the crosscutting concepts that span different disciplines, and the scientific practices and reasoning that allow scientists to study their discipline. Group 2 was given an in-class worksheet structured to help them think about the intermolecular forces between ethanol, dimethyl ether, and ethane and predict how these forces would ultimately influence the boiling points of each molecule. The correct answer was the ethanol has the highest boiling point and ethane the lowest because of the presence of strong intermolecular forces that take high amounts of energy (and high temperatures) to separate liquid ethanol molecules from each other. Group 2 students were asked to make a claim, provide evidence, and justify the reasoning for their proposed boiling point trend. Group 3 was given the same worksheet as group 2 but was also given an additional open response exam question asking them to predict the relative boiling points of fluoromethane and propanol by reasoning through the intermolecular forces present (propanol is higher, again because of hydrogen bonding). Group 4 was enrolled in a General Chemistry course, using what is called the CLUE curriculum that had been redesigned to incorporate activities similar to the worksheet and exam question in Group 3 throughout the course of the semester. (Figure w).

Figure 2: Students were split into four groups with each receiving more intensive intervention. All groups were offered the same final extra credit assignment to measure their understanding of intermolecular forces, their relative strengths, and how this relates to the energy needed to boil a mixture.

            At the end of the second semester of General Chemistry, instructors gave students an extra credit assignment requiring them to apply their knowledge of how chemical structure influences the boiling points of ethanol and dimethyl ether. Students in Group 1 answered correctly that ethanol had the higher boiling point 35% of the time. In group 2 52% of students were correct, along with 80% in group 3 and 90% in group 4 (Figure 3).  In addition, students from groups 3 and 4 were more likely to realize that hydrogen bonds are present between, rather than within molecules. Group 4 also gave more sophisticated justifications of the boiling point trend, more often pointing to not only the presence of hydrogen bonds but also their relative strength and the corresponding increase in energy needed to break the hydrogen bonds and cause the liquid to boil.

Figure 2: Students in the groups that received more intervention focused on drawing and reasoning how intermolecular forces impact boiling points were more likely to correctly determine that ethanol has a higher boiling point than dimethyl ether.

            This intervention demonstrated that relatively small interventions significantly improved students understanding of fundamentally chemistry topics and their ability to use microscopic ideas to explain macroscopic phenomena. The best performing group, however, was enrolled in the group 4 course that allowed them ongoing, rather than one or two, opportunities to apply their developing knowledge to predict how chemical structure impacting boiling point.  The authors note that while they are harder to grade, assessments and assignments that require students to draw and explain their understanding on chemical phenomena are likely to help avoid students completing the class either lacking or misunderstanding certain concepts. While the study was conducted at only on large research university and only by about 40% of students who completed the extra credit final assessment, if the results turn out to be generalizable the interventions reported in this paper could aid students as they learn to translate from chemical structure to function.

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