Using Computer-Based Visualization Strategies to Improve Students

Research: Science and Education edited by

Chemical Education Research

Diane M. Bunce The Catholic University of America Washington, D.C. 20064

Using Computer-Based Visualization Strategies to Improve Students’ Understanding of Molecular Polarity and Miscibility

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Michael J. Sanger* and Steven M. Badger II Department of Chemistry, University of Northern Iowa, Cedar Falls, IA 50614-0423; *[email protected]

One of the main goals of chemical education researchers in recent years has been to improve students’ conceptual understanding of chemistry (1). Efforts include identifying common student misconceptions in chemistry (2–4), identifying students’ difficulties with problems that require them to think at the molecular level (5–7), creating methods of assessment that measure conceptual understanding (5–13), and creating and implementing chemical instruction that improves conceptual understanding (6, 7, 13–16 ). Strategies that help students visualize atoms, molecules, and ions are among the most successful ways to improve their conceptual understanding of chemistry at the molecular level. Visualization strategies include the use of physical models (16–20), static drawings (6, 12, 21, 22), and dynamic computer simulations and animations (7, 15, 23–26 ). Shusterman and Shusterman (27) described how three-dimensional electron density plots of simple molecules generated using Spartan1 (28, 29) can be used as visualization tools in introductory and organic chemistry classrooms. We investigated how the use of visualization strategies associated with dynamic computer animations and electron density plots affects students’ conceptual understanding of molecular polarity and miscibility. The purpose of this study was not to suggest that computer animations or electrostatic potential maps are somehow inherently better methods of conveying conceptual information. Instead, our purpose was to determine whether using these methods along with traditional methods (such as model kits and demonstrations) would improve conceptual understanding. It is an important distinction to make because we were not trying to determine whether animations and electrostatic potential maps should replace the other methods, but rather whether they should be used to supplement them. To determine the effects of instruction including the use of these visualization strategies on students’ conceptual under-

standing, we compared responses from two sets of students. The control group (N = 36) received instruction on molecular polarities and miscibilities using static drawings, wooden models, and physical demonstrations. The experimental group (N = 36) received similar instruction with the additional use of computer animations and electron density plots. The groups came from the same population—traditional students (roughly half male and half female) majoring in natural sciences at a noncommuter Midwestern university—and were enrolled in two sections of a second-semester introductory chemistry course that were taught during the same hour. Although the groups were taught by different instructors, comparison of previous students’ scores on ACS standardized tests (California Chemistry Diagnostic Test–1993, General Chemistry–1995 Exam, General Chemistry (1st term)–1997 Special Exam) suggests that students enter these instructors’ classes with similar chemistry knowledge and leave with similar chemistry knowledge. As part of their final examination, the two groups involved in this study took the General Chemistry (2nd term)–1997 Special Exam and the results (Table 1) suggest that the groups were equivalent: t65 = 0.07, p = .94. Molecular Polarity The lessons describing how to determine whether a molecule is polar or nonpolar were planned to ensure that the students in the two groups received similar instruction. Instruction included the use of wooden molecular model kits and focused on the polarities of individual bonds within the molecules and the overall shape of the molecules. Students in the experimental group also viewed electron density plots of the molecules that had the molecules’ electrostatic potentials mapped onto them (27 ). These “elpot” maps displayed negatively charged atoms as red and orange, neutral atoms as yellow

Table 1. Comparison of the Performance of the Control and Experimental Groups Test

Average Score Control

Treatment

Statistic

p

t 65 = 0.07

.94

2.7/4

3.5/4

F (1,52) = 10.57

.0020*

1.8/2 0.9/2

1.9/2 1.6/2

F (3,52) = 10.57

.025*

Immiscibility of water and nonpolar compounds

1.1/3

2.2/3

F (1,67) = 45.20

.0001*

Intermolecular attractions between salt and water

1.8/4

3.1/4

t 70 = 6.80

.0001*

Intermolecular attractions between soap, grease, and water

2.4/4

3.3/4

t 66 = 2.67

.0048*

1997 ACS Special Exam (2nd term) Identifying Polarity of Molecules Polar molecules Nonpolar molecules

22.6/40 22.5/40

*A p value ≤ .05 indicates a significant difference between groups.

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Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu

Research: Science and Education

and green, and positively charged atoms as light and dark blue. Elpot maps were used for molecules that do not contain any polar bonds (CH4, CH2S), polar molecules (HF, H2O, PCl3, SF2), and molecules with polar bonds that are nonpolar owing to symmetry (BF3, PCl5, SF6). Figure 1 shows elpot maps for CH2S, PCl3, and SF6. Spartan also calculates the dipole moments of these molecules (28), and the students in the experimental group were shown that all symmetrical molecules have zero dipoles regardless of the polarities of the individual bonds. The students in both groups were asked on an hour exam to determine the polarity of three molecules that they had not previously seen (CF4, CH2O, and HOCl). They were also asked to predict whether Br2 or Fe(NO3)3 would be more likely to dissolve in liquid CO2 and to explain their answer. Results of a two-factor repeated measures ANOVA suggest that students who viewed the elpot maps were better at determining the polarity of these molecules [F (1,52) = 10.57, p = .0020]. Both sets of students were more successful at identifying polar molecules than nonpolar symmetric molecules (1.8/2 versus 1.1/2; F (3,52) = 23.16, p < .0001). Although the two groups were equally successful at identifying the polar molecules (1.9/2 for the experimental group, 1.8/2 for the control group), the students who viewed the elpot maps were better at classifying symmetric molecules with polar bonds as nonpolar (1.6/2 for the experimental group, 0.9/2 for the control group) [F (1,52) = 3.21, p = .025]. Instruction for both groups of students focused on the idea that polar molecules have a net separation of positive and negative charge; in other words, they have a positive end and a negative end. Both instructors used wooden models to show that symmetrical molecules with polar bonds are nonpolar because they have no positive or negative end. Students in the experimental group also saw, using elpot maps of symmetrical molecules, that even though these molecules have “blue” (positive) and “red” (negative) regions they do not have a net positive or negative side, and the dipole moments for these molecules were calculated by Spartan to be zero. It appears that these visual images helped students recognize the importance of a molecule’s shape in determining its polarity. Miscibility of Compounds with Water Both sets of students received similar instruction about the miscibility of polar, nonpolar, and ionic compounds. This instruction focused on the intermolecular forces in pure samples of each component and the relative strengths of these forces, as an explanation of the “like dissolves like” rule; it stressed the enthalpic contributions to miscibility and the hydrophobic effect and ignored entropic contributions (30). (Both instructors agreed to deal only with enthalpic contributions primarily because neither had introduced the concept of entropy at the time of the study.) Both groups of students viewed chemical demonstrations of the miscibility of several mixtures (water/ acetone, water/salt, water/pentane, acetone/pentane, and water/ pentane/acetone). The experimental group also viewed computer animations of water/acetone, water/pentane, acetone/pentane, and water/pentane/acetone mixtures and elpot maps of acetone, ethanol, pentane, toluene, and water molecules. Each computer animation began with a homogeneous mixture of the molecules and showed the London forces, dipole–dipole forces, and hydrogen bonds present in each

Figure 1. Elpot maps of, left to right, CH2S, PCl3, and SF6.

Figure 2. Elpot maps showing the intermolecular attractions present in a pentane/water mixture.

mixture. After these forces were displayed, each animation (except the water/pentane/acetone animation) showed the motions of the molecules as a result of these forces. The water/ pentane animation showed the water molecules migrating to the bottom half of the screen and the pentane molecules migrating to the top half of the screen; the water/acetone and acetone/ pentane animations showed that the random motions of the molecules result in solutions that remain homogeneous. The water/pentane/acetone animation showed the “before” picture of randomly oriented molecules and the “after” picture of micelles (pentane molecules surrounded by acetone molecules with the methyl groups pointed inward, surrounded by water molecules pointed toward the carbonyl groups of the acetone molecules), but were not animated. Each animation also showed the intermolecular forces present after the particles had moved, and the instructor discussed how the molecular movements maximized these forces. The elpot maps of the acetone, ethanol, pentane, toluene, and water molecules were also used to explain the miscibility of these molecules. Students were reminded that all intermolecular forces are based on the attraction of positively and negatively charged atoms—in terms of the elpot maps, “blue” atoms are attracted to “red” ones. The miscibility of water with itself, acetone, and ethanol were all explained in terms of the attraction of “blue” to “red” atoms. The immiscibility of pentane and water (Fig. 2) was explained by the fact that water molecules are more attracted to each other (“blue” to “red”) than they are to the pentane molecules (“blue/red” to “green”). The miscibility of water and pentane with the addition of enough acetone was explained using elpot maps that showed the “red/blue” attraction of the water molecules to each other and to the carbonyl groups of acetone and the relatively weak attraction of the “green” pentane molecules and the methyl groups of acetone. To measure their understanding of why nonpolar compounds do not dissolve in water, students were asked

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two questions. The first, on an hour exam, asked them to explain why oil and water do not mix. More than 80% of the students in the control group responded with the idea that water is polar and oil is nonpolar or that “like dissolves like”— a mere statement of the rule with no real explanation of why it is valid. In contrast, just over 30% of the students who viewed the animations and elpot maps provided the rule without an explanation (z = ᎑ 4.23, p < .0001), and a larger number of them stated that water molecules are more attracted to each other than to oil particles. It is interesting that similar numbers of students in each group suggested that there are no attractions between oil and water molecules (18%) or that oil and water molecules repel each other (7%). The second question (Fig. 3, left) asked students to show the intermolecular attractions between water molecules and hydrogen molecules, ignoring London forces because they weakly attract every molecule to every other. A correct response should show attractions between the H and O atoms in water but no attractions of the H atoms in hydrogen to the O atoms in water. This question was designed to determine whether students believed that all H and O atoms are attracted to each other or if they recognized that H and O atoms are attracted to each other only when they have opposite charges. It is similar to the oil and water question in that a correct response should state that the attractions of water molecules to each other are stronger than the attractions of water molecules to nonpolar molecules. Results of a two-factor repeated measures ANOVA performed on responses to these two questions (Table 1) suggest that students who viewed the elpot maps were more likely than students in the control group to recognize that water molecules are more attracted to each other than to nonpolar molecules [F (1,67) = 45.20, p < .0001]. Both sets of students were more successful at identifying the intermolecular forces in the visual picture than at explaining these attractions in words [64% versus 35%; F (2,67) = 15.18, p < .0001]. Responses to the second part of the question in Figure 3 (right) were analyzed to determine students’ understanding of the intermolecular attractions between water molecules, sodium cations, and chloride anions. Both groups of students received instruction including the use of chemical demonstrations, but the experimental group also viewed elpot maps of water and ammonium nitrate ions (Fig. 4), and the attraction of the ammonium ions to the oxygen atoms in water and the nitrate ions to the hydrogen atoms in water was explained in terms of the attractions of positive and negative atoms (“blue” attracted to “red”). A correct response to this visual question should show four kinds of attractions: (i) attraction of the H and O atoms between water molecules; (ii) attraction of Na+ ions to the O atoms in water; (iii) attraction of Cl᎑ ions to the H atoms in water; and (iv) attraction between Na+ and Cl᎑ ions. One point was given for each attraction mentioned in the response. Students who viewed the animations and the elpot maps provided more complete responses to this question (Table 1; t70 = 6.80, p < .0001). A majority of the responses from students in the control group recognized attractions between the water molecules and the ions in sodium chloride but ignored the attractions of the water molecules to each other and of the sodium ions to the chloride ions, whereas the experimental students tended to show attractions of the water molecules to each other and to the sodium chloride ions, ignoring only the sodium ion–chloride ion attractions. 1414

water and hydrogen gas

O

H

O

H

H

H

H

H

H

O

H O

H

O H

H

H

H

H

H

H

H O

H H

H

water and sodium chloride H

O H

H

H

H

O H

H H

O O

H

H

H

H

O H

O O

H

H

H

H O

H

H O

H

Figure 3. Conceptual question regarding the intermolecular attractions in (left) immiscible and (right) miscible mixtures.

Figure 4. Elpot maps showing the intermolecular attractions present in aqueous ammonium nitrate.

Figure 5. Elpot maps showing the intermolecular attractions present in a pentane/soap/water mixture.

Comparison of the responses from students who viewed the animations and elpot maps with those from students who did not suggests that the use of these visualization strategies improved students’ understanding of the intermolecular forces present in polar, nonpolar, and ionic compounds and of the relative strengths of these forces. Although the use of animations to improve students’ conceptual understanding of chemistry has been well documented (7, 15, 30–32), this is the first study to demonstrate that electron density plots can improve students’ conceptual understanding. We believe that the instructional effectiveness of elpot maps lies in their ability to help students visualize positive, neutral, and negative atoms in a molecule by means of color. Soaps and Surfactants Both instructors discussed soaps and surfactants as realworld applications of the miscibility of polar and nonpolar



compounds. The instruction included static drawings and chemical demonstrations and described the dual nature of soaps and surfactants (molecules with hydrophobic and hydrophilic ends), the formation of monolayers and micelles when soaps and surfactants are placed in water, and the role of soaps and surfactants in removing nonpolar stains such as grease or oil from clothes. The experimental group also viewed elpot maps of two surfactant molecules (C6H13CO2᎑ Na+ and C6H13OSO3᎑ Na+), water molecules, and pentane molecules (Fig. 5). The elpot maps demonstrated that surfactant molecules have both a hydrophilic end (represented by a region with “red” and “blue” atoms) and a hydrophobic end (represented by a region with “green” atoms). Elpot maps of water, pentane, and surfactant molecules showed that polar molecules such as water are attracted to the hydrophilic end of a surfactant, whereas nonpolar molecules such as pentane are attracted to the hydrophobic end—an application of the “like dissolves like” rule. Both sets of students were asked on an hour exam to explain how soap facilitates the removal of grease from an object. Both instructors agreed that a correct response should include four concepts: (i) soap is a special molecule in that it has a hydrophobic end and a hydrophilic end; (ii) soap is attracted to water by its hydrophilic end; (iii) soap is attracted to the grease by its hydrophobic end; and (iv) soap acts as a “molecular bridge”, working its hydrophobic tail into the grease and pointing its hydrophilic end toward the water, making the grease “soluble” in water so it can be washed down the drain. Student responses were evaluated using these criteria, one point being given for each of these ideas expressed. Students who viewed the elpot maps provided more complete responses to this question (Table 1; t66 = 2.67, p = .0048). More students in the experimental group than in the control group provided completely correct responses to the question (61% versus 39%, z = 1.89, p = .030).

are attracted to each other and to sodium and chloride ions but are not strongly attracted to hydrogen molecules. Considerable research on the instructional use of computer animations (7, 15, 31–33) suggests that animations are most effective when the instructional topic involves the attributes of visualization, motion, or trajectory (34). Computer animations are particularly effective at helping students to visualize dynamic chemical processes at the molecular level as they change over time (precipitation reactions, gas laws, equilibrium and acid– base reactions, electrochemistry, etc.). Shusterman and Shusterman (27) provided suggestions for using three-dimensional electron density plots of simple molecules as visualization tools in introductory and organic chemistry classrooms, but these were largely anecdotal and have not been corroborated by chemical education research until now. Our research demonstrates that electron density plots (mapped with the molecules’ electrostatic potentials) can be used as an effective visualization strategy to improve students’ conceptual understanding of chemistry topics. We believe that electron density plots have tremendous potential for improving instruction not only in organic, physical, and inorganic chemistry courses (where Spartan is already used at our university), but in firstyear introductory chemistry courses as well. The suggestions from Shusterman and Shusterman (27) and the results of our research suggest that electron density plots improve students’ conceptual understanding of topics involving electronic charges on individual molecules. These concepts include electronegativity of atoms, types of bonding (ionic, polar, and nonpolar), molecular polarity, intermolecular attractions, miscibility, and acid–base properties of compounds. Acknowledgment This work was supported by a Project Grant from the Graduate College of the University of Northern Iowa.

Conclusions W

For several years, chemical education researchers have stressed the importance of asking students to think about chemistry concepts at the particulate level (5–7 ), and their research suggests that when students receive instruction at the particulate level they are better able to answer conceptual questions about chemical processes (6, 7, 15, 26, 31–33). The effectiveness of visualization strategies associated with static particulate drawings and computer animations has been demonstrated; however, none of these researchers investigated whether classroom use of three-dimensional electron density plots of simple molecules as visualization tools would affect students’ conceptual understanding. Our research shows that students whose instruction includes the use of computer animations and electron density plots have a better conceptual understanding of molecular polarity and intermolecular forces. In particular, students who viewed electron density plots were more likely to identify symmetric molecules with polar bonds as being nonpolar, and they gave more complete descriptions of how soap molecules help to remove grease from an object. Students who viewed computer animations and electron density plots were also more likely to explain that the intermolecular attractions among water molecules are responsible at least in part for the immiscibility of oil and water, and were more likely to recognize that water molecules

Supplemental Material

In the online version of this article the figures are in color (see this issue of JCE Online). Note 1. Spartan is a product of Wavefunction, Inc., 18401 Von Karman Avenue, Suite 370, Irvine, CA 91711; [email protected].

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Using Computer-Based Visualization Strategies to Improve Students

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