In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Three-Dimensional Model for Water submitted by:
J. L. H. Johnson* and S. H. Yalkowsky Department of Pharmaceutical Practice and Science, University of Arizona, Tucson, AZ 85721;
[email protected] checked by:
Ed Vitz Department of Chemistry, Kutztown University, Kutztown, PA 19530
Because of its ubiquity, water is often thought to be a mundane and uninteresting compound. In reality, water is an extremely complex substance, capable of myriad special behaviors. These behaviors can be attributed to the bent structure and the symmetry of the water molecule. The position of the two hydrogen atoms in relation to the position of the two lone electron pairs on the oxygen atom allows the water molecules to readily “attach” themselves to one another. Lone electron pairs of one molecule repel those of other molecules but are attracted to the hydrogen atoms of other molecules. The attraction is intensified because the oxygen atom, being strongly electronegative, “pulls” electron density toward itself. This “pull” leaves the hydrogen atoms relatively electron-poor and more attractive to other electronegative oxygen atoms. This attraction results in a network of hydrogen bonds, which is particularly evident in the structure of solid water (i.e., ice). Traditional two-dimensional models for water help students to visualize its structure in a single plane but may not effectively convey the depth of the actual three-dimensional structure. The proposed self-assembling models (SAMs) will facilitate the understanding of water and its structural geometry.
with the surface of the sphere such that two north poles faced out and two south poles faced out. Note that whether the magnets slightly recess or slightly protrude within the holes is not important to the models’ ability to demonstrate their intended functions. In order to improve visualization, the two exposed south poles were painted black and the two exposed north poles were painted white. For the sake of consistency, a simple pocket compass can be used to distinguish a magnet’s north pole surface from its south pole surface. The north pole of each magnet was identified as the side which attracted the compass needle. When the acrylic paint was dry, one coat of clear nail polish was applied over top of each surface containing a painted magnet. This prevented the colored paint from chipping and restored the sphere’s original shine. The blue spheres contained no magnets. A less precise version of the models may also be created by hand in the following way: Wooden spheres, 1.25-in. diameter, and 0.25-in. cylindrical dipole magnets can be purchased from a craft store. Each sphere can be clamped into a drill press, and the first 0.25-in. diameter hole can be drilled at any location. Using a simple geometric compass with pencil, a circle with a radius of 1.02 in. describes all possible
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Approximately 20 red, Molymod, 4-hole, 23-mm spheres (part #60402E); 20 blue, Molymod, 4-hole, 23-mm spheres (part #60401E); and 80 neodymium disk magnets (5.0-mm diameter × 5.0-mm length; part #33513A) were donated.1 Quick-setting glue, acrylic craft paint in carbonblack and titanium-white colors, and clear nail polish were purchased locally. One magnet was glued into each of the four holes of each red Molymod sphere resulting in four equal sized magnets per sphere. As is shown in Figure 1, the four magnets were oriented with their surfaces essentially flush
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Figure 1. Front (left) and rear (right) view of the same water molecule model. The white areas are the exposed north poles, which represent hydrogen atoms, and the dark areas are the exposed south poles, which represent electron lone pairs.
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Figure 2. A depiction of flickering water molecule clusters similar to the one proposed by Frank and Wen (3). The lines surrounding the clusters change shape as individual molecules constantly move from cluster to cluster.
Journal of Chemical Education • Vol. 79 No. 9 September 2002 • JChemEd.chem.wisc.edu
In the Classroom
points 109.5° from the initial hole. Next, any point on that circle can be chosen for the location of the second hole drilled. A second 1.02-in. radius circle, describing all points 109.5° from the second hole, will reveal two intersections. These two intersections are the locations for the third and forth holes to be drilled. Insertion of the magnets can proceed as above.
which molecules quickly change membership when orientation and distance makes attraction to another cluster strong enough. A physical simulation of this phenomenon can be observed using a transparent container to hold a number of SAMs. Significant manual kneading of the models will facilitate the movement of the models such that a student can watch individual molecular models change cluster membership.
Results and Discussion
Formation of Ice Lattice Yalkowsky (4) described a two-dimensional model illustrating the hydrogen bonding patterns of water and ice. The oxygen is represented by the center of a circle with the two hydrogen atoms, being depicted as solid lines, emanating at 90° angles from each other. Hydrogen bonds are indicated as dashed lines. Figure 3 shows these two-dimensional schematic views of water at three temperatures. As temperature decreases from room temperature (Figure 3A) toward 4 °C (Figure 3B), the energy of water molecules decreases and they become more densely packed. It is at this temperature that rigid hydrogen bonding starts to occur more rapidly. Eventually, the expanded structure of ice forms as the temperature drops below 0 °C (Figure 3C). In order to visualize the simulation of water freezing, a number of SAMs were placed in a transparent container and agitated. The models spontaneously self-assembled into the known less-dense formations of ice. Figure 4 shows that each nonsurface molecule is attached—hydrogen bonded—to four other molecules. The clusters in Figure 4 are composed of a random array of the regular ice structures illustrated in Figure 5.
Each sphere is used to represent a water molecule with the oxygen atom at its center (1); the two white north poles represent the hydrogen atoms and the two black south poles represent the electron pairs of the oxygen. These simple models enable the physical demonstration of a number of properties and behaviors that are unique to water. Although the hydrogen atoms of water are known to be close to 104.5° from one another (2), the use of uniform angles of 109.5° in the proposed model allows for simplicity of design and does not compromise the behaviors of water meant to be simulated by the model. SAMs can be used to illustrate the flickering clusters described by Frank and Wen (3) as well as the structures of several ice lattice forms and ice’s ability to melt under pressure. Furthermore, the prevention of dissolution of nonpolar solutes in water can be simulated with the proposed model.
Flickering Clusters A version of flickering water clusters, first proposed by Frank and Wen in 1957 is shown in Figure 2. This figure conveys water’s tendency to form loose molecular clusters between
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Figure 4. Various simulated ice formations spontaneously assembled using SAMs.
C Figure 3. Two-dimensional models of water proposed by Yalkowsky (4). A: Water at room temperature (25 °C). B: Water at 4 °C. C: Water at 0 °C. (ice). Note the increase in molecular density from A to B and the decrease from B to C. These density changes correspond to water’s increasing ability to hydrogen bond as vibrational and rotational freedom is reduced. Dotted lines represent hydrogen bonds.
Figure 5. Simulated ice formations. Left: Five-member-ring ice dodecahedron. Center: Six-member-ring ice structure. Right: Mixed five- and six-member-ring structure.
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In the Classroom
Figure 6. Ice melting under pressure. Left: Ice simulation before pressure application. Right: The application of pressure compacts the ice formation and molecule models become more densely packed representing liquid water.
the exclusion of the oil molecules. By hydrogen bonding, water will effectively restrict the entrance of nonpolar oil molecules into the polar phase. This results in a visible separation of phases driven by the hydrogen-bond forces. The separation of phases can also be characterized as the result of the hydrophobic effect described by Tanford (5). The models simulate the outcome of the hydrogen bonding of water; hence, they portray the prevention of hydrocarbon dissolution. Although SAMs do not simulate distinct enthalpic and entropic factors, they do realistically demonstrate the end result, which is phase separation. Whether nonpolar molecules float or sink in relation to water depends upon their density relative to water. In our demonstration, the blue Molymod spheres sink as would chloroform in water. By comparison, if polystyrene spheres were used they would float like hexane in water. Conclusion
Figure 7. Simulation of squeezing out of nonpolar molecules: lightcolored spheres without magnets represent nonpolar molecules (such as chloroform) and dark-colored spheres with magnets represent water molecules. Left: Prior to manual agitation. Right: After manual agitation.
Ice Melting under Pressure Melting under pressure is demonstrated by the application of manual pressure to the previously mentioned simulated ice structure. When pressure is applied, the formation’s relatively open structure is compacted. The open spaces between water molecules are lost as an external force crushes the lattice and breaks the hydrogen bonds. The resulting structure of the “crushed” water molecules, shown in Figure 6, again takes on the more dense form of liquid water with most of the molecules not being connected to their neighbors. When the pressure is released the ice structure reassembles spontaneously. This melting principle is demonstrated in ice-skating where the pressure exerted by the blade on the ice produces a layer of liquid water upon which the blade glides. The Prevention of Dissolution of a Nonpolar Solute If nonmagnetized Molymod sphere models are intermixed with magnetized sphere models and agitated, the two entities separate into clusters of like kind (Figure 7). Using the example of oil and water, when a mixture of the two is attempted, the polar nature of the water molecules causes the water molecules to cluster around one another resulting in
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The unique behaviors of water discussed above are only a sampling of the many behaviors that can be displayed by SAMs. However, it should be noted that this type of magnetized sphere proposed as a molecular model for water cannot demonstrate the behavior of water in certain situations. Because the bond angles used in the simulation are not flexible, the actual changes in bond angles (increase in the bond angle between the two hydrogen atoms and decrease in bond angle between the lone pairs) that occur as water undergoes its transformation from liquid to solid cannot be demonstrated. Despite the structural limitation, these molecular models can be a useful tool in an educational setting. Not only do they provide a “hands-on” means to teach the general chemistry of water and enable students to visualize how water molecules interact with one another in three-dimensional space, they are also fun to play with. Acknowledgments We would like to thank the editor, Ed Vitz, for his helpful suggestions. Notes 1. Indigo Instruments. 169 Lexington Court, Unit 1, Waterloo, ON. N2J 4R9, Canada. 877/746-4764, http://www.indigo.com (accessed June 2002).
Literature Cited 1. Nezbeda, I. Fluid Phase Equil. 2000, 170, 13–22. 2. Laing, M. J. Chem. Educ. 1987, 64, 124–128. 3. Frank, H. S.; Wen, W. Y. Discuss. Faraday Soc. 1957, 24, 133– 140. 4. Yalkowsky, Samuel H. J. Chem. Educ. 1993, 70, 614–615. 5. Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973.
Journal of Chemical Education • Vol. 79 No. 9 September 2002 • JChemEd.chem.wisc.edu
In the Classroom
Editor’s Note: The three dimensional models (SAMs) in this demonstration do a great job of illustrating the hydrophobic effect, where nonpolar substances are exsolved from water to form two distinct layers. The authors use a “squeezing out” model, and focus on hydrogen bond formation in water to explain the exclusion of nonpolar molecules. The SAMs are valuable in explaining alternative models for the hydrophobic effect which are more consistent with other data. For example, if exsolution were caused by water hydrogen bonding to itself, as in the “squeezing out” model, the process would be exothermic. Surprisingly, it is endothermic (1, 2). If formation of two layers is endothermic, it must be accompanied by a positive entropy change, so that the two (nearly) pure layers that result must be seen as “more disordered” than the solution. Most current theories explain the exsolution in terms of an increase in entropy (3, 4). This situation is a good example of a case where increases in what we intuitively call “order” is difficult to correlate with decrease in entropy (5). Disorder or order is everywhere, and the question is how to recognize the disorder that entropy measures. It might be better not to try (6). Separation into pure phases (incorrectly) appears to involve a decrease in entropy, because pure phases appear to be more “orderly” than a mixture. While the paper above focuses on hydrogen-bond formation in bulk water as the driving force for the separation, the accepted entropic explanation has been used in at least one general chemistry text (7), and the SAMs in the present demonstration might be used as a segue to discussions of its weaknesses as well as its strengths. The SAMs cannot show the increased “order” of the water molecules around dissolved hydrophobic species, possibly caused by stronger than average hydrogen bonding, that is proposed by other authors to explain both the negative enthalpy and entropy of dissolution (8). Nor can the SAMs mimic the self-assembly of the hydrophobic solute molecules proposed by source (9). These papers suggest that entropy as “order” has been useful to researchers, who, when confronted with the paradoxical negative entropy of dissolution in the “hydrophobic effect” described above, looked for the creation of order as hydrophobic substances dissolve (exothermically). Models are never perfect (10), and we believe that even their inaccuracies, if they are made explicit, can be valuable teaching tools. Thinking about how a model differs from its referent can lead to insights about how the real system behaves. The magnetic SAMs require another important caveat: the size of the hydrogen atoms is not well represented. This deficiency can be partially mitigated by painting circles of a contrasting color on the surface of the spheres around the magnets that represent the hydrogen atoms. Hydrogen atoms can also be represented by using longer painted magnets that protrude 0.5 cm from the sphere. We experimented with two 1/4-in. diameter × 1/2-in. length neodymium magnets,1 which press-fit into the holes of 1 1/4-in. wooden models available from many suppliers,2 and two of the 5-mm diameter × 5-mm length magnets described in the demonstration, glued with silicon seal or hot-melt glue into the remaining holes. Models may also be constructed with four 1/8-in. diameter × 3/8-in. length rod magnets3 pressed into 1/8-in. holes drilled in the wooden spheres. In the latter case, the
holes were drilled concentrically with the existing 1/4-in. holes, entirely through the sphere. This gave the proper orientation, and also allowed the magnets to be pushed out with a steel pick from the opposite side if changes were desired. For this reason, it may be advisable to drill 1/8-in. holes through the spheres before magnets are inserted in all designs. Large spheres with four south pole magnets representing oxygen and smaller spheres with four north pole magnets for hydrogen can roughly model proton exchange and other effects, but this is an expensive approach—each sphere will cost $2–$4 and 40 may be required—whose benefits would have to be weighed carefully. Smaller (3/4-in.) woodenball models4 are available, predrilled with 1/8-in. holes, that can accommodate the 1/8-in. diameter magnets.3 If visibility is not a problem, these can reduce the expense. Ferrite magnets are much weaker than neodymium ones and create a much less dramatic effect. Finally, excellent models of water and water clusters can be found on the Web (11), along with extensive descriptions of water structures (12). Notes 1. Forcefield. http://www.wondermagnet.com (accessed June 2002), 877/944-6247 or 970/484-7257, order #16, $1. 2. Science Kit. http://www.sciencekit.com (accessed June 2002), 800/828-7777, #47730-00, Advanced Molcular Model Set, $45.00; #62607-10, 1 pack of 12 carbon (black) balls, $7.95; SargentWelch/Cenco. http:// www.sargentwelch.com (accessed June 2002), 800/727-4368, #WLS-61815, set, $29.95, or WLS-61825F, 12 carbon atoms, $9.90; Carolina Biological Supply. http://carolina.com (accessed June 2002), 800/334-5551, #RG-84-0210, set, $30.15. 3. Forcefield. http://www.wondermagnet.com (accessed June 2002), 877/944-6247 or 970/484-7257, order #26, $0.25. 4. Science Kit. http://www.sciencekit.com (accessed June 2002), 800/828-7777, #62606-00, set of 100 atoms, $29.95 or #62606, 12 carbon atoms, $4.50; Carolina Biological Supply. http://carolina.com (accessed June 2002), 800/334-5551, #RG-84-0212, set, $30.15.
Literature Cited 1. Aerts, T.; Clauwaert, J. J. Chem. Educ. 1986, 63, 993. 2. Huque, E. M. J. Chem. Educ. 1989, 66, 581. 3. Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973. 4. Southall, N. T.; Dill, K. A.; Haymet, A. D. J. J. Phys. Chem. B 2002, 106 (3), 521–533. 5. Lambert, F. L. J. Chem. Educ. 1999, 76, 1385. 6. Lambert, F. L. J. Chem. Educ. 2002, 79, 187. 7. Chemistry: A Project of the American Chemical Society; Trial Version, W. H. Freeman & Co., 2002. 8. Silverstein, K. A. T.; Haymet, A. D. J.; Dill, K. A. J. Am. Chem. Soc. 1998, 120 (13), 3166–3175. 9. Marmur, A. J. Am. Chem. Soc. 2000, 122 (9), 2120–2121. 10. Bhushan, N.; Rosenfeld, S. J. Chem. Educ. 1995, 72, 578. 11. MathMol Library. http://www.nyu.edu/pages/mathmol/library/ (accessed June 2002). 12. South Bank University, School of Applied Sciences. http:// www.sbu.ac.uk/water/ (accessed June 2002). —Ed Vitz
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