Instructor Information
JCE Classroom Activity: #70
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The Nature of Hydrogen Bonding
That students find it difficult to distinguish between bonds and noncovalent attractions is well documented. Fewer than half of high school and college chemistry students correctly described in molecular terms the evaporation of water (1). Commercially available models that demonstrate dynamic hydrogen bonding use imbedded magnets (2). The models here use K’Nex toy components to illustrate hydrogen bonding and are a simplified form of other models that use Velcro as the “H-bonding component” (3). A description of how to modify the K’Nex models for a closer approximation of the actual bond angle in water is in this issue of JCE Online.W Understanding the dynamic nature of hydrogen bonding is crucial to understanding water solubility, protein folding, and DNA structure and dynamics. These models present a “freeze frame” or static view, but the analogy can be extended to imagine a continuous moderate level of shaking (ever present molecular motion) where connections between molecules are continuously broken and made.
Integrating the Activity into Your Curriculum This Activity complements curricular coverage on changes of state such as the melting and boiling of water. The Activity could introduce the importance of intermolecular forces in determining the states of matter. It could also be used after units on electronegativity and bond and molecular polarity. Labeling the water molecule models using different colored central connectors could also be used to demonstrate the nature of equilibrium.
About the Activity K’Nex does not make kits for the models described in this Activity. The needed K’Nex components could be ordered individually from K’Nex; part numbers for the needed components are below (4). Kits of K’Nex to build 100 models (enough for ~15 groups of students) are available from the Bloomsburg University Mathematics and Science Learning Center for a nominal cost plus shipping; contact the author at the email address above. Velcro with adhesive backing is available in fabric or craft stores. Whole black loop circles (~1.5 cm diameter) can be used for electron pairs on oxygen. Strips of white hook Velcro can be cut up for the partial positive charge on the hydrogens. The hook Velcro should be trimmed to just cover the ends of the hydrogen K’Nex connector to minimize non hydrogen-bonding connections. The most common unwanted event is the connection of two hydrogens to one oxygen electron pair. Over time, there may be degradation of the Velcro and adhesive backing, resulting in the Velcro not connecting correctly or the Velcro falling off the models. “Super glue” can be used in these cases.
perforated
Answers to Questions 1. The bond angle in the model is 135⬚; the bond angle in water is 105⬚. 2. A rod represents the two electrons in a covalent bond between hydrogen and oxygen. A black loop Velcro patch represents an electron pair (and a lot of negative charge). A white hook Velcro patch represents a slight excess of positive charge due to the polarization of the electrons in the covalent O–H bonds toward the central oxygen. 3. Hydrogen bonds are represented by the spots where the hook and loop Velcro patches connect to each other. 4. Gentle swirling models condensation of gas phase water molecules. Vigorous shaking represents the application of outside energy. The physical change is vaporization or evaporation of water. 5. One would need to separate the model’s hydrogen and oxygen components by removing the connecting rod. Breaking a covalent bond requires much more energy than breaking hydrogen bonds. 6. It is unlikely that exactly the same array of hydrogen bonds will occur since the connections between individual water models tend to be different each time. In equilibrium, the balance of products and reactants is the same but individual chemical species making up this balance is different. 7. Step 10 models ice.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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photo by J. J. Jacobsen & E. K. Jacobsen
Emeric Schultz Department of Chemistry, Bloomsburg University, Bloomsburg, PA 17815;
[email protected] In this Activity, students build models of polarized water molecules using K’Nex toy components and adhesive Velcro. Students investigate hydrogen bonding by shaking the models in various ways. They observe the resulting interactions and relate their observations to physical states of water and the difference between strong bonds and weak attractions.
References, Additional Related Activities, and Demonstrations 1. Mulford, Douglas R.; Robinson, William R. An Inventory for Alternate Conceptions among First-Semester General Chemistry Students. J. Chem. Educ. 2002, 79, 739–744. 2. 3-D Molecular Designs. http://www.3dmoleculardesigns.com/news2.php (accessed Jan 2005) 3. Schultz, Emeric. A Simple Dynamic Model for Hydrogen Bonding. J. Chem. Educ. 2005, 82, 401–405. 4. K’Nex Parts. http://www.knex.com/customer/home.php?cat=260 (accessed Jan 2005). Part numbers: 3/4-in. rod = 90950; large green connector (O atom) = 91905 or 90905; gray (H) connector = 90901. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 82 No. 3 March 2005 •
Journal of Chemical Education
400A
JCE Classroom Activity: #70
Student Activity
The Nature of Hydrogen Bonding Matter is held together by both strong and weak attractions. One strong attraction is the covalent bond. Covalent bonding involves sharing of electrons between nuclei of two atoms, which can be of the same or different elements. Breaking strong attractions requires quite a bit of energy. Weak attractions occur between positive and negative regions of chemical species, usually molecules. One of the most common weak attractions is the hydrogen bond. The name here is unfortunate because it is not really a bond. Hydrogen bonds are responsible for water being a liquid at room temperature and also hold together the two strands of DNA. These attractions can come and go fairly easily— you have probably heard of the evaporation of water and the replication of DNA! In this Activity, you will discover the fundamental difference between strong covalent bonds and weak hydrogen bonds. You will make models from K’Nex toy components and Velcro and then use the models in your investigation. All models are only representations of real items, so they are not identical. Keep this limitation in mind as you work through the Activity. You will need: K’Nex pieces: 14 3/4-in. rods, 7 spoked green connectors, 14 dark gray horseshoe-shaped connectors; 14 adhesive patches of black circular Velcro loop; 14 adhesive patches of white Velcro hook; large plastic container with lid; protractor. __1. Draw a Lewis structure of water. __2. Make a K’Nex model of your step 1 structure: take a spoked green connector and add a rod to each of the two outermost connect points. Connect a dark gray horseshoe-shaped connector to each of the rods. What does each of these pieces represent? __3. At this point, your water-molecule model is incomplete because it does not show all of the electrons in a water molecule nor the polarity in the water molecule caused by the electronegativity difference between oxygen and hydrogen. Attach two circular adhesive patches of black loop (softer) Velcro to the model in the places where you would find the two pairs of unshared electrons on the oxygen atom in water. __4. Add two small adhesive patches of white hook Velcro to the hydrogen ends of the water molecule. It is easier to remove the backing and then cut the pieces to fit over the ends. Trim the pieces so they just cover the ends. You have now made a model of water that indicates its approximate polarity and shape. __5. Repeat steps 2–4 to make six more water-molecule models. __6. Place the seven water-molecule models in a large plastic container such as an empty five-quart ice cream bucket. Make sure the models are not touching each other. Gently swirl and shake the container so the models come into contact with each other. __7. Describe what you observe. Pick up the connected array of water-molecule models and spread it out on a flat surface without breaking the Velcro connections. Are there any connections that shouldn’t occur? If so, how can they be explained in terms of limitations of the model and how it was built? __8. Return the connected array of water-molecule models to the plastic container. Place the lid on the container. Shake the container vigorously for several seconds. Open the container and describe what you observe. __9. Repeat steps 6 though 8 two or three more times. _10. Connect the water-molecule models in a fixed pattern so that each of the Velcro patches on a central watermolecule model is connected to a complementary Velcro patch on another water-molecule model.
Questions 1. 2. 3. 4.
Using a protractor, measure the bond angle in the model. How does it compare to the actual bond angle in water? What do these model parts represent: rods; black loop Velcro patches; white hook Velcro patches? Between what two parts of this model are the model’s representative hydrogen bonds formed? What physical change is modeled when individual waters are allowed to connect to each other by gentle stirring? What does vigorously shaking the container represent and what physical change of water is being modeled? 5. What would have to change in the model for water to have had a chemical change? How would you do this? Could this change occur if you shook really hard? What does this suggest about the quantity of energy needed to break a hydrogen bond versus the energy needed to break a covalent bond? 6. Are the connections the same after each round of shaking and bringing the water molecules back together again? How does this relate to the idea of equilibrium? 7. What state of water is modeled in step 10?
Information from the World Wide Web (accessed Jan 2005) The Velcro Molecular Models Page. http://facstaff.bloomu.edu/eschultz/Velcro%20Models%20Page.htm Quia–Chemical Bonds Quiz. http://www.quia.com/jq/19617.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
400B
Journal of Chemical Education
• Vol. 82 No. 3 March 2005 •
www.JCE.DivCHED.org
photos by E. Schultz
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