Activity pubs.acs.org/jchemeduc
An Inquiry Experience with High School Students To Develop an Understanding of Intermolecular Forces by Relating Boiling Point Trends and Molecular Structure Melinda Ogden* Atlantic Community High School, Delray Beach, Florida 33445, United States S Supporting Information *
ABSTRACT: Knowledge of intermolecular forces (IMFs) is fundamental to comprehending chemical and biological systems and their transformations at a particulate level. Including instruction on IMFs in the high school curriculum allows for students to develop a conceptual understanding of the material presented in chemistry class as opposed to simply restating facts. Herein, a student-centered activity developed for high school students to discern relationships between molecular structure and boiling point as an introduction to IMFs is described. The lesson was developed in such a way that students must use data to support their conclusions, and the data employed were chosen specifically to prevent misconceptions or incomplete conceptions from forming. Details regarding the interactions between students and the teacher are provided to help those looking to employ this activity in their own class successfully. This activity and follow-up lesson were implemented with over 250 high school students, many of whom, even a year later, were able to apply their deep understanding of particulate level electrostatic interactions to new chemical systems, including solubility of molecular compounds. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Hydrogen Bonding, Noncovalent Interactions, Student-Centered Learning
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INTRODUCTION
This approach allows the teacher to interact with each student at their current level of understanding and bring all students to the same level of knowledge by providing different amounts of assistance. By having students develop the ideas themselves, they are able to truly understand IMFs and what governs their strength instead of memorizing a set of rules to follow to answer questions. The students also gain valuable practice using data to generate explanations,9 and grounding the activity with data prevents the students from making too broad of generalizations. This activity was implemented with tenth grade students in their first of two semesters of honors chemistry in an urban public high school with approximately 2500 total students. This experience was incorporated after students learned to draw Lewis dot structures and VSEPR shapes for covalent compounds but before any experience with IMFs. This complete lesson, from data gathering to teacher wrap-up, takes approximately 200 min on a block schedule in which a class period is 100 min and students meet every other day. On a traditional 50 min period schedule, I estimate the experience to take 150 min total.
Intermolecular forces (IMFs) are crucial to understanding many aspects of chemistry and biology, yet their abstract nature can pose problems for the concrete-operational students in a tenth grade chemistry course. There have been many studies showing students have deep misconceptions about IMFs and their role in phase changes1−5 and that having students draw the molecules and indicate the IMFs helps to resolve those misconceptions.6,7 Graphs of boiling points or boiling point trends have been described as instructional tools for IMFs8 and as a method to probe students’ understanding of the types of IMFs for different molecules.2 Here, I explain how these approaches were combined in an inquiry setting with high school students in a manner that avoids the ubiquitous misconceptions students have regarding IMFs. Using an inquiry approach maximizes student understanding, and this approach was thus chosen as the method to introduce students to IMFs. Students first measure boiling points of several compounds and are then provided with boiling points for additional compounds making up a homologous series of alkanes, ethers, alcohols, and several other molecules included to prevent formation of misconceptions. Students work to group the molecules into categories based on structure and boiling point until all molecules have a group with an explanation. Meanwhile, the teacher ensures the students are developing appropriate ideas by challenging the students’ molecular groupings if too broad. Once complete, the class shares their ideas and the teacher provides scientific terminology, organizes their ideas, and explains the relationships between structure and boiling points. © XXXX American Chemical Society and Division of Chemical Education, Inc.
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LABORATORY EXPERIMENT The first component of this experience is for the students to measure the boiling point of several compounds. In my implementations, students measured the boiling points for water, Received: September 11, 2016 Revised: April 30, 2017
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DOI: 10.1021/acs.jchemed.6b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX
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time they have used a pure liquid other than water. The process may also be a light bulb moment regarding instrument error depending on the quality of thermometers or temperature probes being used because each group of students could have different measurements. Before beginning with the measurements, it is important the students understand that if a liquid at its boiling point were to get any hotter, it would become a gas, and therefore the boiling point is measured once the temperature of the liquid stops increasing. A description of how to achieve this understanding with the students is included in the Supporting Information. After measurement of the boiling points, the second component is the categorization of the molecules based on structure and boiling point. Students are provided with the condensed structural formulas and boiling points for additional compounds and are asked to draw Lewis structures for all of the molecules to visualize them. All of the compounds are listed in Table 1, though some of the boiling points are measured by the students. To not influence the grouping, the molecules’ names are not provided, and the list is randomized. Student hand-outs are available in the Supporting Information.
Table 1. Molecules in Order of Increasing Boiling Point
a
Molecule
Condensed Structural Formula
Boiling Point (°C)a
methane ethane propane methoxymethane butane methoxyethane ethoxyethane acetone methanol ethanol isopropanol diethyl sulfide 1-butanthiol water butan-1-ol glycerol
CH4 C2H6 C3H8 CH3OCH3 C4H10 CH3OCH2CH3 CH3CH2OCH2CH3 CH3COCH3 CH3OH CH3CH2OH CH3CHOHCH3 CH3CH2SCH2CH3 C4H9SH H2O C4H9OH HOCH2CHOHCH2OH
−161.4 −88 −42.1 −24.82 −0.50 10.8 34.6 56.5 64.7 78.5 82.5 92 98.4 100 117 290.0
Data are from ref 10.
ethanol, methanol, and isopropanol because they were readily available and inexpensive. (Sources, pricing, and safety information are available in the instructor materials in the Supporting Information. Caution should be taken while boiling alcohols due to the flammability.) Measuring the boiling point is often an enlightening experience for students as it may be the first
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STUDENT ANALYSIS AND THE ROLE OF THE TEACHER Once students have gathered their own boiling point data for the compounds the teacher has chosen for measurement, it is important to discuss how those data should be combined with the provided data. As students inevitably share their findings around the room, a discussion of instrument error comes up. If there is a systematic error with the measured boiling points, the measured points can be adjusted to accepted values for comparison, vice versa, or because the compounds in Table 1 have boiling points which are far enough from each other, the measured boiling points can be used as collected without affecting the categorization with the understanding that the students are not comparing values taken under the same conditions. If the experiment is carried out far from sea level, a discussion of boiling point in terms of vapor pressure and atmospheric pressure would also be warranted.
Box 1. Category Rules Students Are Expected To Develop (1) Within a group, molecules with more atoms/mass/ electrons have a higher boiling point. (2) Molecules with only carbon and hydrogen have the lowest boiling point when compared with molecules of similar size. (3) Molecules with atoms in the chain other than carbon have higher boiling points than those with only carbon. (4) Molecules with an −OH on the end have the highest boiling points. (5) The more −OH groups on a molecule, the higher the boiling point. Table 2. Refinement of Students’ Categorizations Student Categorization/Reasoning
Molecules Which Can Help Refine
Teacher Intervention
Molecules with negative boiling points, molecules with “medium” boiling points, and molecules with “high” boiling points as groups.
Alkanes, ethers, alcohols
Students often first group solely by arbitrary boiling point cut-offs and ignore structural characteristics. To introduce the importance of structural features in their groups, start with the negative boiling points and ask if methoxymethane looks more like the alkanes with negative boiling points, or more like the methoxyethane. A similar question can be posed for an ether compared with an alcohol if needed.
Molecules with more carbons have higher boiling points (within a group).
Glycerol, diethylsulfide, water
Point to any/all molecules with three and four carbons and then point out glycerol, which also has three carbons, and water. Comparing ethoxyethane and diethylsulfide can also help students get to the concept of the number of electrons playing a role.
Molecules with more hydrogens have higher boiling points
Ethane, methoxymethane, ethanol
Each of these molecules has six hydrogens, yet they have significantly different boiling points. This will help students in forming more specific groups based on structural characteristics.
Molecules with oxygen have higher boiling points
Diethylsulfide
Point out ethoxyethane and diethylsulfide.
Molecules with sulfur have boiling points higher than those with oxygen.
Butanthiol, butan-1-ol
This conception can result from addressing the previous one; pointing out butanthiol and butan-1-ol can alleviate the issue.
Molecules with an oxygen on the end have boiling points higher than those with the oxygen in the middle (not addressing specialty of hydroxyl group).
Diethylsulfide, butanethiol
Have the students build models of diethylsulfide, butanethiol, diethyl ether, and butanol to develop a more precise grouping statement which explains why butanol has a boiling point higher than that of butanethiol but diethyl ether has a boiling point lower than that of diethylsulfide. It sometimes helps to pull in water and glycerol as well. B
DOI: 10.1021/acs.jchemed.6b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX
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appropriate conclusions through questioning.) Some students start sorting by whether or not the molecule contains an oxygen atom but then realize that sorting by size within the group that has oxygen atoms proves difficult. At this point, students sort by molecules having oxygen in the middle or at the end. To avoid having students come to the conclusion that oxygen is necessarily special, diethylsulfide is included, which follows the general trend within the ethers. To then help the most advanced groups of students determine that the hydroxyl group is in fact special, 1-butanthiol is included for comparison with butan-1-ol. Also included for further delineation are glycerol and water (for increased number of hydroxyl groups) and acetone (for a polar molecule with an oxygen not in the main chain), which only the most advanced students are able to place. Table 2 lists common student statements which need further refinement to meet the classification guidelines described above and which set of molecules can help students further refine their groupings and reasonings. Some example student responses are shown in Figure 1. Some weaker groups of students will end up placing all of the outliers in one group and stating their reasoning as, “they did not fit with the other ones”. Depending on how long it has taken these students to get to the three main groups, it might be worthwhile to try to get them to see that the number of hydroxyl groups is important; otherwise, the teacher could just circle back to them after going over everything as a class to make sure they understand how each unique molecule fits into their scheme.
As students start looking at the data, some are completely overwhelmed, while others jump in right away. For this reason, it is useful to group students by ability level so that more guidance can be provided to the weaker students, enabling them to make progress instead of just following along with stronger group members. Groups of two or three work best so abilities can be matched. The molecules were chosen in such a way as to help the students sort by symmetry and size, so for groups that cannot find a place to start, they can sort the molecules by boiling point and then try and find structural categories. If students draw the molecules on index cards and include their boiling points, they can easily move molecules between categories and see the groups more spatially than if listed on paper. When the students are finished, the goal is for them to have developed something similar to the sets of rules found in Box 1. To help students develop the last two ideas, 1,4-butandiol (boiling point 235 °C11) could be added to the molecule list because it is less of an outlier structurally than glycerol and water. It is critical to help the students identify when oxygen is important as opposed to any element other than carbon so that molecules which can form dipole−dipole forces can be distinguished from those with hydrogen bonding capabilities. It is crucial that the teacher continuously question each group of students on their progress and reasoning throughout this portion of the experience because the students generally make conclusions along the way that are inconsistent with some of the other molecules that are included. (The teacher is not providing instruction but rather is guiding students to reach
Figure 1. Two sample student responses on how the molecules were categorized. Student 1 is missing some molecules but provides a typical response. Student 1 organizes the molecules within each category by boiling point but does not explain why the boiling points increase within a category. Student 2 categorizes only the molecules. When probed, students are able to explain why the boiling points differ for the molecules within a category as well as between categories. C
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Figure 2. Examples of three student drawings and explanations of the IMFs present within a sample of methane. The students clearly indicated instantaneous, temporary shifts in electron clouds and the resulting electrostatic interactions. Students 2 and 3 also indicate the presence of bond dipoles with student 2 explicitly indicating that the bond dipoles cancel making the molecule nonpolar.
Figure 3. Student responses to why ethanol is a liquid at room temperature while iodine is a solid. Students show understanding of the relative strengths of different types of IMFs and are able to convey that London dispersion forces can be stronger than the other types for a molecule with a large enough electron cloud. Students are also able to relate boiling points to state of matter at a given temperature. It should be noted that Student 3 states “break it’s molecules up”, which may signal the misconception that the bonds within a molecule break during phase changes. However, in responses to other questions, Student 3 clearly drew IMFs between molecules breaking during phase changes, similar to those seen in Figure 4.
Formalizing and Applying the Knowledge
each category are listed by increasing boiling point as well, corresponding to rule 1. The extra molecules are then placed in categories. Here, the stronger students lead the discussion and, as some of the molecules are placed, weaker students are able to categorize some of the other molecules as a bigger scheme unfolds. Diethylsulfide, butanthiol, and acetone are placed in the same category as the ethers because they are asymmetric. Glycerol and water are included in the category with the
Once each group of students has finalized their molecule categories and reasonings, the class reconvenes to formalize knowledge together. First, three main categories of molecules are generated and drawn on the board in increasing order of boiling point: the alkanes, ethers, and alcohols corresponding to rules 2, 3, and 4 described above. The molecules within D
DOI: 10.1021/acs.jchemed.6b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 4. Student drawings and explanations of what happens when water boils. Students indicate in words and with drawings that phase changes involve breaking IMFs, not molecular bonds.
Figure 5. Student responses to why glucose is soluble in water but cyclohexane is not. Structures of both molecules were provided. Students had not previously discussed solubility or solutions in any way prior to this question which was posed on a unit exam. Students 1 and 3 include the idea that in order for one substance to be soluble with another, the attraction between the two must be stronger than the attraction between molecules within each substance. These three typical responses to the question show a deep understanding of interparticle attractions and ability to apply that understanding to a completely new situation. An extension to this activity could evaluate solubility and its relation to IMFs, possibly using the experiment described by Montes et al.14
alcohols. A schematic and notes are available in the instructors materials in the Supporting Information. After all molecules have been placed (or almost all in a weak class), the concept of IMFs is introduced, and the appropriate scientific terminology is provided. The introduction to IMFs is most easily started with dipole−dipole forces. Here, the concept of bond dipoles and dipole moments are brought in. Students quickly see the interaction between partial positive and partial negative charges on adjacent molecules. London dispersion forces are then explained as instantaneous dipole− dipole forces and hydrogen bonding as a special case of dipole− dipole which is stronger due to the large dipole moments.
Included in the set of molecules are instances where a molecule with a weaker type of IMF has a boiling point higher than that of a molecule with a stronger type of IMF. It is important to show the students that the relative strength of IMFs holds for molecules of similar sizes; otherwise, the strength of LDFs dominates.12 If the class of students overall had trouble placing all of the molecules, they can be categorized at the end of the discussion. The relationship between the electrostatic interactions and boiling point is easily identified by the students given their extensive training on the differences between liquids and gases in terms of particle spacing throughout their schooling. E
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(5) Peterson, R. F.; Treagust, D. F. Grade-12 Students’ Misconceptions of Covalent Bonding and Structure. J. Chem. Educ. 1989, 66 (6), 459−460. (6) Cooper, M. M.; Williams, L. C.; Underwood, S. M. Student Understanding of Intermolecular Forces: A Multimodal Study. J. Chem. Educ. 2015, 92, 1288−1298. (7) Williams, L. C.; Underwood, S. M.; Klymkowsky, M. W.; Cooper, M. M. Are Noncovalent Interactions an Achilles Heel in Chemistry Education? A Comparison of Instructional Approaches. J. Chem. Educ. 2015, 92, 1979−1987. (8) Glazier, S.; Marano, N.; Eisen, L. A Closer Look at Trends in Boiling Points of Hydrides: Using and Inquiry-Based Approach to Teach Intermolecular Forces of Attraction. J. Chem. Educ. 2010, 87 (12), 1336−1341. (9) Nichol, C. A.; Szymczyk, A. J.; Hutchinson, J. S. Data First: Building Scientific Reasoning in AP Chemistry via the Concept Development Study Approach. J. Chem. Educ. 2014, 91 (9), 1318− 1325. (10) The Merck Index; O’Neil, M. J., Heckelman, P. E., Koch, C. B., Roman, K. J., Eds; Merck & Co, Inc.: Whitehouse Station, NJ, 2006. (11) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press/Taylor & Francis: Boca Raton, FL, 2005. (12) Earles, T. T. Can London Dispersion Forces Be Stronger than Dipole-Dipole Forces, including Hydrogen Bonds? J. Chem. Educ. 1995, 72 (8), 727. (13) Becker, N.; Noyes, K.; Cooper, M. Characterizing Students’ Mechanistic Reasoning about London Dispersion Forces. J. Chem. Educ. 2016, 93 (10), 1713−1724. (14) Montes, I.; Lai, C.; Sanabria, D. Like Dissolves Like: A Classroom Demonstration and a Guided-Inquiry Experiment for Organic Chemistry. J. Chem. Educ. 2003, 80 (4), 447−449.
Having an understanding of the phase changes at a particulate level helps to avoid the misconception of molecules breaking apart into constituent atoms upon boiling. Examples of student drawings and responses showing their understanding of IMFs (Figure 2), their relative strengths (Figure 3), and relation to physical properties (Figures 4 and 5) can be seen in Figures 2−5. Although these representative responses show thorough understanding, it would be useful to further analyze them, as described by Becker et al.13
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SUMMARY In 4 years, this experience has been completed with 10 classes, totaling approximately 250 students. Students are easily able to explain the relationship in boiling or freezing point between any set of compounds. Students can also easily explain the states of matter under standard conditions for any set of compounds. The students’ understanding of IMFs and how they arise is thorough and consistent across all levels of students after this experience. The students were also able to apply their knowledge of IMFs to explain solubility of covalent compounds without guidance. Additionally, many of the same students were in 11th grade Advanced Placement chemistry the following year and were able to recall all of the details of IMFs without prompting and to accurately draw instances of IMFs of all types. Understanding the forces governing interparticle interactions is key to having a particulate level comprehension of a chemical system. With this experience, I found that students both understand IMFs and can apply their knowledge even a year later.
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ASSOCIATED CONTENT
S Supporting Information *
Student handout. (PDF) Student handout. (DOCX) The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00697. Instructor information (PDF; DOCX) Student handout (PDF; DOCX)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Melinda Ogden: 0000-0003-0916-6480 Notes
The author declares no competing financial interest.
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REFERENCES
(1) Luxford, C. J.; Bretz, S. L. Development of the Bonding Representations Inventory to Identify Student Misconceptions about Covalent and Ionic Bonding Representations. J. Chem. Educ. 2014, 91, 312−320. (2) Jasien, P. G. Helping Students Assess the Relative Importance of Different Intermolecular Interactions. J. Chem. Educ. 2008, 85 (9), 1222−1225. (3) Kind, V. Beyond Appearances: Students’ Misconceptions About Basic Chemical Ideas. http://www.rsc.org/learn-chemistry/resource/ res00002202/beyond-appearances?cmpid=CMP00007478 (accessed April 2017). (4) Nakhleh, M. B. Why Some Students Don’t Learn Chemistry. J. Chem. Educ. 1992, 69 (3), 191−196. F
DOI: 10.1021/acs.jchemed.6b00697 J. Chem. Educ. XXXX, XXX, XXX−XXX
