overhead projector dernon~tration~ Magnetic Models of Ions and Water Molecules tor Overhead Projection Wllllam G. Davles Emporia State University Emporia. KS 66801
This paper describes a set of lecture-demonstration models that are easily made from ceramic magnets like those found in many hobby shops and discount stores. These models can be placed on the flat smooth glass surface of an overhead projector where they can easily slide and rotate under the influence of magnetic attraction and repulsion. The magnetic forces simulate the electrical forces acting between ions and polar molecules. Models of ions are made from 1%-in.-diameter round magnets with a central hole (e.g., Radio Shack Cat. 64-1885). When placed on a flat surface, the top side of one of these magnets is of opposite polarity to the bottom. Anion models are constructed by gluing one of these magnets to a sheet of blue transparent acetate plastic, taking care that the north side is being glued. Cyanoacrylate adhesive (crazy glue) is recommended for this purpose. Once the adhesive has set, excess acetate sheet can be trimmed off with scissors or a razor blade. Cation models are made in a similar manner, except that the south side of the magnet is glued to a red transparent acetate sheet. These ion models are placed on the projector surface with the acetate sheet at the bottom. They then slide very readily along the top of the glass. When projected, the two different kinds of ion are readily distinguished by the blue and red color of their centers. Models of unlike color attract each other while those of like color repel. Figure 1shows how these models of ions can he arranged on the projector surface to illustrate the sodium chloride lattice. This is not the only two-dimensional lattice that can he built from these models. A more open hexagonal arrangement is also possible, but this is inherently unstable and readily collapses to the structure illustrated when disturbed. A visual demonstration of this occurring shows much better than words can convey how the lattice that ions adopt is the one that corresponds to the lowest potential energy.
Figure 1. Use of models to illustrate an ionic crystal
edited by DORISKOLB Bradley Univer~ity Peoria, IL 61625
If the lattice made from these ion models is large enough, a 4 X 4 square containing 16 ions for example, it is very easy to show the property of cleavage. One hand is used to hold one side of the structure firmly to the projector surface. The other hand is now moved abruptly across the projector surface in a direction parallel to one of the lattice planes until it strikes the lattice shar~lv. . . Immediatelv the model cleaves into two fragments along that plane. These magnetic models can also be used to illustrate rhe concept of lattice enthalpy. A lattice is first constructed on the glass projector surface, and the ions are senarated from each other one by one against their mutual attraction. The effect is enhanced (and some amusement eneendered) if the instructor pretends to be exerting considerable energy during this operation. A second useful type of model that can he constructed from magnets is the water molecule. Examples of water molecule models are shown in Figure 2 along with models of ions. These can be constructed from two %-in.-diameter round mamets reoresentine.. bvdroeen atoms and one %-in. . round magnet representing the oxygen atom. Ceramw magnets with rhese dimensions are readilv ava~lable.The hvdrogen magnets are placed north side up, and the oxygen atoms south side up. The three atoms can easily be glued together with cyanoacrylate glue, but the resulting model is not very robust. A much better method is to glue all three magnets onto a sheet of Plexiglas 0.03 in. thick. The glue should be liberally applied, and care must he taken to have the mag~~
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Flgwe 2. Use 01 tha mcdels to Illustrate an ionic solmlon.
Volume 68
Number 3 March 1991
245
nets touching each other. When the glue has hardened somewhat, more can be applied to the space between the magnets in order to bond them to each other as well as to the plastic base. Because of the liberal avvlication of due. the model should be left overnight todry:~xcessplastirsheet can then he trimmed off with a utilitv knife or a file and then sanded. The finished models slide readily on their Plexiglas bottoms. These water models can he used to illustrate manv of the properties of polar molecules. Their obviously strong attraction for each other explains why water has a high boiling point and a large enthalpy of vaporization. The immiscihility of polar and nonpolar liquids can he illustrated using pennies to represent the nonpolar molecules. Water models are placed on the projector surface widely separated from each other while pennies are placed between them. When the assembly is stirred by hand to simulate thermal agitation, the polar water models quickly coalesce leaving the nonpolar pennies "out in the cold". It is also easy to illustrate the hydration of ions in aqueous solution. When a cation model is placed on the projector plate, anywater model close by is immediately drawn toward i t until the oxygen atom makes contact. Similarly, when an anion is used. the water molecules arranee " themselves with hydrogen atoms touching the ion. P e r h a ~ the s most telling demonstration with these models shows why ionic soiids tend to he soluble in water, but not in nonpolar solvents. First, equal numbers of anions and cations are placed on the projector plate dispersed sufficiently widely so as not to coalesce. Then pennies are interspersed hetween the ions. When thisassembly is stirred by hand, the ions immediately roalesce into a crystal lattice, leaving behind the nonpolar molecules represented by the The experiment is now repeated with water models replac-
248
Journal of Chemlcal Education
ing the pennies. We then have a situation like that illustrated in Figure 2. On stirring, there is now no tendency for the ions to crystallize as they did in the first part of the demonstration. Magnetic models can also he used to illustrate protontransfer reactions. A single 'I2-in. magnet serves as a model for the proton. An hydroxide ion is also made by gluing a3/ain. maenet to a %-in. maenet in a manner similar to that emploqed for the water mudels. An anion model (X-, and a proton model (H7Jare allowed to come together under their mutual attraction so as to form a single entity representing the strong acid HX. A water molecule is now moved past this H X molecule so that the oxygen atom in the water molecule brushes past the proton. The proton readily detaches itself from the anion model and attaches itself to the water model to form a model of the HsOf ion. In turn, when a model of an OH- ion is brushed past this new entity, the magnet representing the proton again transfers, this time from the hydronium ion to the hvdioxide ion. formine two water mol&ules in the process. he relative strengths-of the acids involved (HX > H3Of > H20) and also of the bases involved (OH- > Hz0 > X-) can thus be illustrated, along with the idea that the stronger an acid. the weaker its coniueate base. A warning is in Gder here. he relative stren2huof the three types of magnets em~lovedin these models varies somewhat with source of s u p p & h particular, the a/4-in. magnets used by the author show a variability among themselves. Thus care must he taken during assembly to select magnets of the correct relative strengths. The author has used these models for several years now. Student reaction has always been positive. The models are so flexible that new applications for them keep on suggesting themselves.
