edited by
JAMESO.SCHRECK University of Nonhern Colorado Greeley, CO 80639
filtrates e residues The Ropes: A Molecular Polarity Activity Thomas H. Bindel and Timothy C. Smiley Pomona High School ,8101 West Pomona Drive, Arvada, CO 80005 The concept of molecular dipole moments can confuse a great many high school chemistry students. Students often encounter this abstract concept early in the year (our aproach bepins bv develodng atomic theorv) and manv of them needa concrete examge of bow elec&onegativitj relates to chemical bonding. Prior to the activity, students must understand general electronegativity trends in the periodic table, hut need not memorize values. Our approach emphasizes the ability to analyze what will happen once the values are known. The students also are experienced in dealing with Lewis electron dot structures, VSEPR theory, molecular geometries, and three-dimensional drawings of molecular arrangements. This activity makes the transition from 3-D reality to drawing in two dimensions easier. Ultimately, the students should have a feeling for the molecular
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Materials Four ropes, 1 meter in length. Use large-diameter, nonNylon ropes to prevent rope burns. One cart. Casters must be able to pivot in all directions. Check cart for safety in operation before using. The surface area must be large enough for one human occupant on top. (Our cart has surface dimensions of61 cm x 47 cm.) Procedure Concepts are demonstrated through a lecturelactivity with the following molecules:' hydrogen chloride (HCI), beryllium chloride (BeCI2), beryllium chloride fluoride (BeCIF), boron triflnoride (BFB),water (HzO), and a s a n option, carbon tetrachloride (CCld. Compounds a r e treated as discrete molecules. First, students are asked to draw a Lewis electron dot structure for hydrogen chloride. Next, the instructor develops the concepts of a bond dipole moment and a molecular dioole moment (net dioole). This is done hv asking the &estion, will the molecule have a preferrei oriengtion between two oooositelv chareed - ~.l a t e s If? i t does. then the molecule has a net dipole moment and if not, there is no net dipole. In order to make it easier for beginning students to visualize the net dipole, we set up a "human molecule". For hydrogen chloride, the instructor poses a s the hydrogen atbm and sits on the cart (see the f;gure). A student is then selected to be the chlorine atom. Arope is tied
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'In adtilon, we prefer to wnsider ionic molecules, such as, sodium chloride (NaCI)and magnesium chloride (MgCI,). We acknowledgethat most freshman chemist^ textbooks chose not to deal with these ionic molecules. However. the idea of ~~. t h e chemical literature does su~wrt onc molec~lesano s concerned w lh tneir georne~nesano polar l!es (1-3). Beca~seI n s paper s a mo ec, ar po arify a n vry, we tee mar one s h o ~ onot eave 04 tne onc moec~les.10come, n more aovanced discussions, most chemical bonds can be viewed as having some combination of ionic and covalent character (4.) ~~~~~~
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onto the wrist of hydrogen atom (the instructor) and the chlorine atom grabs hold of the other end of the rope. The instructor holds out the arm that is connected to the rope, in order to simulate the linear geometry, and each tries to pull. Of course the hydrogen atom is pulled toward the chlorine atom, because the chlorine atom has the greater electronegativity. The net dipole is in the direction of the motion of the cart. At this point one must emphasize that in realitv. ". molecules are not beine oulled around like the can. T h ~ method s allows students to visualize the electron shift that results in a molrcular dioolr moment. The terms polar and nonpolar molecules are then introduced. The class is next asked to consider beryllium chloride. They draw a Lewis electron dot structure, do a VSEPR (valence shell electron air repulsion model) analvsis, draw the actual shape of the molehe, draw in the bond dipoles, and ~ r e d i cwhether t there is a molecular dipole moment or not. - h e r the instructor goes through the analysis on the board, except for the net dipole determination, the "human molecule" demonstration is set up again. The beryllium chloride molecule is such that there is no net dipole moment and the cart does not move.
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Results from Human Molecule Demonstration Molecule
Molecular Geometry Direction Cart Moves
HCI
Linear
Toward Chlorine
BeCln
Linear
Does not move
BeClF
Linear
Toward Fluorine
BF3
Trigonal Planar
Does not move
Angular (109.5")~
Moves along the HOH angle bisecto?
CC14 Tetrahedral Does not move a The nonbonded and bonded pairs are not treated differently. The nonbonded pairs were ignored in the determination of the molecular dipole moment. The lecture continues in order through the molecules listed in the table. The boron trifluoride simulation is shown in the figure. I n the case of BeClF, students are instructed to pull with different forces so as to simulate the differences in bond dipoles. The water molecule is different from the other molecules in that the bond dipoles are pointing toward the central atom. I n this situation, the cart person holds out the arms a t approximately 109 degrees apart and the two students push on the extended arms toward the cart person. The last molecule, carbon tetrachloride, is optional. Its geometry is somewhat challenging! Volume 71 Number 11 November 1994
945
tunity to work up a number of drawings i n the follow-up worksheet that i s presented a t the conclusion of the period. (A sample can be obtained by writing the authors.) Typically, time is spent a t home and in a subsequent class period discussing the examples provided on the worksheet. The test results over the past six years indicate that average students achieve a solid comprehension of the material t h a t h a s been covered. Additionally, t h e activity is mentioned frequently as being one of the most memorable in the school year by students who have returned to visit u s following their high school graduation.
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Boron trifluoride simulation. Discussion either di. The key benefit to students who rectly or indirectly in this activity is the development of the ability to link up what is seen i n three dimensions to creating a two-dimensional drawing. Students have oppor-
946
Journal of Chemical Education
Acknowledgment The authors would like to express gratitude to Charles Mueldener for willingness to participate a s the cart man on a regular basis and to Larry Winegar for assistance with the photography. Literature Cited 1. Buchler, A,; Stauffeer,J.L.:Lemperer, W. J. Am. Chem Soe. 1884.86.4544.
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