In the Classroom
Simple Dynamic Models for Hydrogen Bonding Using Velcro-Polarized Molecular Models
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Emeric Schultz Department of Chemistry, Bloomsburg University, Bloomsburg, PA 17815;
[email protected] A significant student misconception involves distinguishing between the properties and characteristics of bonds and attractions (weak chemical forces). One manifestation of this misconception is the incorrect explanation for the evaporation of water. The majority of secondary students believe that the bubbles seen in boiling water consist of hydrogen and oxygen (1). This misconception extends deep into post high school chemical education. In a study by Mulford and Robinson (2) only 40% of 928 freshman enrolled in the first semester of science majors chemistry, identified the bubbles as water vapor in the pretest at the beginning of the course; in a post-test at the end of the course this increased to 47%. In contrast 43% of students believed the bubbles consist of a mixture of hydrogen and oxygen in the pre-test and 39% still believed this in the post-test. In a study of students completing organic chemistry, and thus having had two full years of chemistry, there were still a significant number of students who could not correctly apply one or more phenomena connected to hydrogen bonding (3). Even at the graduate level in chemistry 25% of students gave the wrong explanation for the phenomenon of water boiling (4). These misconceptions are compounded by the unfortunate historical naming of hydrogen bonds, which has been commented upon frequently in the chemical education community. A contributor to this Journal even suggested that the term “H-FON bonding” replace hydrogen bonding and opined that “soon (this would) become the accepted name for this concept” (5). Models and demonstrations are a standard part of chemistry teaching. The primary goal is to make the content more accessible and understandable to the student. There have been several models and demonstrations describing hydrogen bonding. One set of models used magnets, either in simple overhead demonstrations (6, 7) or as a more elaborate threedimensional model with tetrahedral characteristics (8). Hill described an easily made hook (appropriate H atom) and eye (electronegative O, N, or F) model (9). Hill also described using Velcro and people in an interactive exercise in which individuals with Velcro hook patches connect and disconnect from people wearing Velcro loop patches (10). Hill gives two analogies: (i) the breaking of an H-bond at the end of a network of students simulating vaporization and (ii) a hexagonal array of connected students representing the structure of ice. In this article I describe an extension of Hill’s idea of using Velcro but in this case affixing Velcro onto molecular models. Secondly, a dimension of quantification is added by directly relating the magnitude of the interaction to the quantities of hook and loop Velcro. Lastly, the models are used in a way that more correctly portrays the way H-bonding networks form both geometrically and statistically without being contrived.
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Materials and Construction of Models The notion of using Velcro to model weak attractions was quickly followed by the recognition that the most commonly used ball and stick and other model kits typically used in introductory courses would not work. Other types of materials unrelated to chemistry models were tried but found to have flaws. Space filling models emerged as the best option and models using three different types of material were constructed. Models of water and other simple substances (ammonia, HF, methane, methanol) can be made from wood or Styrofoam. Alternatively Corey–Pauling–Koltun (CPK) space filling models, available through Harvard Supply,1 can be used. For the CPK model of water a tetrahedral carbon with two hydrogens is used. The 109.5⬚ angle is close enough to the actual 105⬚ value and allows uniformity when other structures are made. For wooden ball structures, ball knobs (one flat surface and dowel hole) with a diameter of 0.75 in. are used for H; similar 1 in. ball knobs are used for C, N, O, or F. For Styrofoam, balls of 1 in. and 1.5 in. are used for H and for C, N, O, and F, respectively (Figure 1). The actual atomic radii for the five atoms are: H (0.37 Å), C (0.77 Å), N (0.75 Å), O (0.73 Å) and F (0.71 Å) (11). The ratio of the size of H to the size of other atoms in the models is slightly larger than the actual atomic ratios; this minor shortcoming of the model is justified by the convenience of using common materials. Styrofoam and wooden balls are available in the craft sections of discount stores as well as hobby and craft stores.
Figure 1. Styrofoam balls (1.5 in. and 1 in.) and wooden ball knobs (0.75 in. and 1 in.).
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In the Classroom
Figure 2. Molecular model components: (left) Styrofoam tetrahedral atom (center) Styrofoam hydrogen atom (right) Tetrahedral wooden atom
Figure 4. Polarized molecular models of water: (left) CPK model (center) Wooden model (right) Styrofoam model
To make space filling models with correct bond angles and lengths, a bit of sanding is required (except for the H wood ball knobs that can be used as is). This is best done using a drill press with a drum sander attachment and a wooden jig. The wooden jig is made using an equilateral triangle base with the dihedral angle of a tetrahedron (directions for construction and use are available from the author upon request). Wooden and Styrofoam balls with variablesized flat surfaces that are the sides of a tetrahedron can be easily made by rotating the balls after one plane has been established (the 1 in. wooden ball already has this plane established) (Figure 2). Various models can be made by combining appropriate balls. Additional holes can be drilled into wooden balls and the balls can be connected with wood glue and dowels. For Styrofoam models hot glue works exceptionally well in “welding” component atoms. Atoms in the models can be colored using magic markers. With care in sanding of planes, the bond lengths in the models are almost exactly proportional to the actual lengths in the molecules. The bond angles (and angles to electron pairs) for the molecules described in this article are all 109.5⬚. The addition of Velcro patches completes the models, henceforth called polarized molecular models (PMM). For ease of construction and consistency between structures, standardization of Velcro size and type versus actual partial charge on various atoms is required. Partial charges can be obtained from texts or by molecular modeling. We use these models
in the context of having completed model building using Hyperchem and having determined partial charges on various atoms in certain small structures. By convention, hook Velcro is defined as positive and loop Velcro is defined as negative. The convenience of small circular Velcro patches (SCVP) with peel off adhesive that are just the right size (1.5 cm diameter) warrants these patches being used as a “standard charge unit” (Figure 3). The term SCVP will be used for standard sized unit whereas VP will be used for any sized unit. With other applications in mind, the hook SCVP is given a charge of +1兾6 (+0.167) and the loop SCVP is given a charge of ᎑1兾6 (᎑0.167). Fortuitously, the partial charge on each H in water is calculated as +0.171, or one hook SCVP. Likewise the partial charge on the O in water is ᎑0.342 or two loop SCVP. The last convention in constructing polarized molecular models is placement of partial charges. Given the small size of the H atom, whatever partial positive charge is determined by molecular modeling, is placed as one contiguous Velcro patch on the H atom. Partial negative charges with atoms that have electron pairs are divided between the electron pairs available and appropriate quantities of loop Velcro are placed at each electron pair position on the atom (i.e., one SCVP for each electron pair at open tetrahedral positions of O in water). Positive and negative charges calculated for atoms without electrons pairs (usually C) are divided between the number of positions that are bonded to other atoms and appropriate quantities of VPs are placed on the atoms (these are not dealt with in this article). There are some technical issues that need to be addressed at this point. For the CPK models VPs can be placed directly on the H atoms and on the vacant tetrahedral positions (after the connector hole is either covered or filled). The adhesive backing on the SCVP has variable effectiveness. For Styrofoam models in particular the integrity of the model is compromised if the SCVPs or VPs are directly pasted onto the models. It has been found that VPs placed on a planar surface (including a planar area on H atoms) using hot glue have remarkable stability without in any way compromising the effectiveness of the model. In fact pasting VPs with hot glue on all models (despite the adhesive backing) is probably a good idea for long term use. Several complete polarized molecular models of water are shown in Figure 4.
Figure 3. Small circular Velcro with adhesive.
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In the Classroom
Figure 5. Collection of six polarized molecular models of water in plastic container.
Figure 6. Collection of six polarized molecular models of water in plastic container after gentle shaking.
Demonstrations Using Polarized Molecular Models
on (the intensity of the air flow can be varied). The device is shown in Figure 7 (directions for construction of device will be sent upon request). When air is directed into a collection of eight unconnected Styrofoam PMMs of water in the “reaction” container (Figure 8), the waters begin to tumble in the air space above the air jet (Figure 9). Individual water models collide with each other until appropriate attractions take hold; in less than a minute only an extended H-bonding network remains (Figure 10). Although the wooden and CPK models will work in this demonstration, the tumbling is rather sluggish and less impressive. In addition, as noted earlier, CPK models tend to fracture, at least the ones I used, which were a bit old and brittle.
The context in which we use this set of demonstrations involve lecture and lab settings related to the concept areas of intermolecular forces.
Forming and Breaking H-bonds A collection of six unconnected wood PMM water molecules in a large plastic container is shown in Figure 5. When this collection is gently shaken (with the top on or off ), PMMs connected by hydrogen bonds are formed. An array after gentle shaking is shown in Figure 6. The connections are invariably between O and H atoms except on rare occasions when an H bridges two O’s or an O bridges two H’s. If one now puts on the top and shakes the container vigorously, most of the H-bonds come apart and the original array (Figure 5) or a close equivalent is restored. Gentle shaking gives back a hydrogen bonding network; vigorous shaking disrupts it. One can do this over and over again. Since the unconnected arrays and the “network connections” are invariably different, the geometric and statistical characteristics of Hbonding are a natural topic for discussion. Note that the extent of disruption depends on the vigor of the shaking. This demonstration can be done with either the wooden models or the CPK models, although on vigorous shaking some of the CPK models have fractured. For Styrofoam models it is quite difficult to get vigorous enough shaking to disrupt the H-bonding network once it has formed. The light weight apparently does not allow enough energy to be generated to overcome the attractive force (a point for an interesting discussion) There is the obvious connection between the quantity of thermal energy (strength of shaking) and the arrays that are obtained. This is a very nice model for the evaporation and condensation of water as well as the formation and melting of ice. Dynamic Formation of a Hydrogen-Bonding Network The author has constructed an apparatus in which a strong jet of air from a leaf blower is directed through a clear plastic container of variable size. The top of the container is held on gently to allow air to escape as the blower is turned www.JCE.DivCHED.org
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Dynamic Formation of Heteronuclear Hydrogen-Bonds A Styrofoam model of ammonia can be constructed according to conventions given above (partial positive charge on each H is +0.53 SCVP; partial negative charge on N is ᎑1.58 SCVP). When this ammonia model is placed in the chamber described above and allowed to tumble with eight molecules of water, correct H-bonds are always formed. Three panels of before, during, and after the start of the demon-
Figure 7. Home-built blower for experiments with Styrofoam molecular models.
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Figure 8. Collection of eight polarized molecular models of water in blower reaction chamber prior to turning on blower.
Figure 9. Collection of eight polarized molecular models of water in blower reaction chamber shortly after blower was turned on.
Figure 10. Collection of eight polarized molecular models of water in blower reaction chamber after the blower has been turned off.
stration are shown in Figure 11. The connections between the N on ammonia and a H on water are more frequently formed than the connections between a H on ammonia and an O on water. Digital video of both these demonstrations has been recorded and can be replayed either in slow motion or frame by frame. Experiments to determine ideal conditions for the most effective demonstrations for video presentation are currently underway: the factors include vessel size and shape, air flow, and number of models. Overview By attaching “positive” and “negative” Velcro quantities related to partial charges directly onto space-filling molecular models, dynamic demonstrations of the various aspects of hydrogen bonding are possible. The manner in which the demonstrations are done clearly conveys the difference be-
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tween covalent bonding and intermolecular forces, an area that presents difficulty for many students. Additionally, the demonstrations clearly model molecular behavior in terms of how units interact, especially the importance of the orientation of the units and how they collide. Lastly, the critical connection between energy and the state of matter is clearly modeled. The author is currently working on the further refinement of this idea to model other types of molecular attractions (or the lack of these). In addition to the use of the Velcro models to demonstrate various types of molecular interactions, it should immediately be obvious that the use of Velcro on molecular models and in the “reaction chamber” can be extended in several ways to model various aspects of kinetics and thermodynamics as well as other phenomena. The author is currently developing demonstrations that model these phenomena.
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In the Classroom
Figure 11. Collection of eight polarized molecular models of water with polarized ammonia model: (top) Prior to turning on blower (center) Shortly after blower was turned on (bottom) After the blower has been turned off
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Supplemental Material
Digital videos of the demonstrations are available in this issue of JCE Online. Acknowledgment This article is dedicated to the memory of Wayne P. Anderson, colleague, mentor, and friend. Note 1. Harvard Apparatus, Inc. 84 October Hill Road, Holliston, MA 01746, 800-272-2775, http://www.harvardapparatus.com (accessed Dec 2004).
Literature Cited 1. Osborne, R.; Cosgrove, M. J. Res. Sci. Teach. 1983, 20, 825– 838.
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2. Mulford, D. R.; Robinson, W. R. J. Chem. Educ. 2002, 79, 739–744. 3. Henderleiter, J.; Smart, R.; Anderson, J.; Elian, O. J. Chem Educ. 2001, 78, 1126–1130. 4. Bodner, B. M. J. Chem. Educ. 1991, 68, 385–388. 5. Hill, J. W. J. Chem. Educ. 1986, 63, 960. 6. Davies, W. G. J. Chem. Educ. 1991, 68, 245. 7. Pravia, K.; Maynard, D. E. J. Chem. Educ. 1996, 73, 497. 8. Johnson, J. L. H.; Yalkowsky, S. H. J. Chem. Educ. 2002, 79, 1088–1091. 9. Hill, J. W. J. Chem. Educ. 1986, 63, 503. 10. Hill, J. W. J. Chem. Educ. 1990, 67, 223. 11. Brown, H. E.; LeMay, H. E.; Bursten, B. E. Chemistry, The Central Science, 7th ed.; Prentice-Hall: Upper Saddle River, NJ, 1997; p 263.
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