Ruth A. Walker Hunter College of the City University of New York New York
Teaching Basic Chemistry Concepts with the Overhead Projector
The effectiveness of an overhead projector in bringing classroom demonstrations closer to the student has been proven by a number of investigators (1) including particularly H. N. Alyea (8) and his co-workers who have developed a comprehensive series of general chemistry experiments for such projection. The present work aims to show that the overhead projector is equally effective in that area of general chemistry where basic concepts are taught with the aid of charts, graphs, and diagrams. As yet there has been no positive discussion of the maximum utilization of the unique advantages of the overhead for such presentations. These advantages arise from the flat accessible stage which permits the use of both masks and overlays. Such an arrangement makes possible the presentat,ion of material by steps; the final image is constructed gradually so that the students anticipate each successive step, and therefore are actively involved in the presentation.'
image can present a formalization of the Law and, if desired, additional diagrammatic material to initiate a discussion of the kinetic molecular principles behind the Law. Other L'laws"which have been treated in a similar manner include those of Charles, Avogadro, Dalton, and Faraday as well as those concerning t,he consematon of mass, constant composition, and multiple proportion.
Student Participation
The area of general chemistry dealing with fundamental laws such as those concerned with the states of matter provides much material that can profit by being organized and presented in steps. The nature of the "scientific method" can be brought out by a graphic depiction of experimental data followed by a request t,hat the student draw his own conclusionsand formulate a general law from the information given. The visual on Boyle's Law illustrates how a series of pictures can guide the thought process toward this end. The initial image (Fig. la) represents two cylinders a t the same t,emperature but a t different pressures and volumes. With t.his in view, the students can be asked to predict the effect of again doubling the pressure. When the ensuing discussion has produced an appropriate answer, it can be confirmed by positioning an overlay wit,h a third cylinder as shown in Figure l b . The students can then be directed to summarize all the observations in one unified form. The final
Figure lo. The original diagram, which can be used as the basis for the question "What would be the effect of agoin doubling the pressure?
Figvre I b. An overley supplier the onrwer m d provider the opportunity for the student to draw a general conclurion from specific data.
Based on a paper presented to the Division of Chemical Educatian a t the 146t,h Meeting of the ACS, New York, September, l "C7 *S".,. 1 The set of general chemistry visuals from which the examples are taken consists of 59 different projeetuals which may he purchased from the Tecnifax Corporation, Holyoke, Maas. The complete set costs $300 but individual items may be purchased. Depending upon the number of overlays, the price range is from $1.50 to $11.50. A catalogue describing each visual iis available on request. A Teacher's Guide, "Basic Information for General Chemistlly," has been prepared with detailed instructions for each visual, including enough discussion to clarify the underlying principles hoth as to ehemicd content and techniques of presentation.
BOYLE'S LAW Figvre lc. The third ond fourth overloyr present a rtotement of Boyle'r Law and introduce the kinetic molecvlor theory.
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Superimposable Images
The transparency of film which makes it possible to superimpose one image upon another and to still see both of them, greatly enhances the value of overlays. Presentation of the topics of atomic orbitals and covalent bonding can profit from proper use of this technique. The basic concepts can be shown by a visual composed of a series of overlays which build up the electronic structure of the Period 2 elements. The static (Fig. 2) contains a red sphere with two electrons representing the 1s orbital. Surrounding this (both are clearly visible because of the transparency of the film) is an orange 2s orbital with one electron. I n one corner is the Lewis symbol for the represented element, lithium. The first overlay shows the addition of another electron to the 2s orbital with the forn~ationof beryllium. The second overlay adds a blne 2p, orbital and electron without blocking what is already present. Succeeding overlays add p, and p, orbitals and the additional electrons necessary to form each element through neon. With carefully chosen colors, it is possible to see the images of each overlay piled one on t,op of the other and to thereby est.ablish the idea that all of these are present in the final structure. The structure of a covalent bond can also be presentltcd in a manner which depends upon the ability to see images one upon the other. This has been effectively carried out for the compounds CH4, NHB,HzO, and HF, starting with diagrams of the nnhybridized orbitals and using shades of blne to differentiate between empty orbitals and orbitals containing one or two electrons. Compound formation has been represented by posilioning an overlay wherein red spheres r e p resent hydrogen atoms and are so located as to overlap those orbitals in the original image which contain only unpaired electrons, thuz showing bond formation by orbital overlap. A subsequent image indicates the effects of hybridization in all four instances, (8) and produces CH4 instead of CH1.
until such a time as these areas come under discussion. This technique works best if the image is clear or lightly colored and the background is black. Rutherford's experiment on the scattering of alpha particles (4) haa been handled in this manner (Fig. 3). The basic static contains a diagram of the apparatus and of his explanation (6). One overlay covers up what happens after the alpha particles collide with the gold foil and another overlay covers up the explanation. Thus it is possible to get the class to speculate on both the results and the explanation before actually revealing either. It has also been found that calculations such as those typified by the Born-Haber cycle lend themselves to this technique. For example: calculations of the lattice energy of KCl, given the heat of formation, has been worked out on a static base and can he presented step-wise. The general outline of the changes involved can be introduced without any numbers because overlay one covers up the values for cation formation and overlay two covers up those for anion formation. Each set of values can be uncovered in turn, while overlay three keeps the final calculation from view until the appropriate time. Color has been used to differentiate the exothermic from the endothermic changes.
Overlays as Masks
Instead of organizing the overlays so that they contain parts of the image to be added, they can be used merely as inasks to cover up specific areas of the image Graphs PERIOD 2 ELEMENTS
Be:
Li' I$, 2s'
152,
8:
22
Is',
2s'. 2pl
'C: Is2, 2r7, 2p2
z
Figure 2. Continuoui'build-up overlays leads to the structure of neon showing the 2p.. 2p.. and Zp, orbitolsover the original a orbitals.
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Graphs are particularly well-suited to this medium and there are several approaches that have proved effective. The coordinates and the grid can be projected and then the curves can be plotted before the class, either with previously prepared overlays or by writing on the filmwith a wax pencil. For example, problems involving the crystallization of salts can be worked directly on the solubility graph and then readily wiped off. Alternately, the cuwe itself can be projected and overlays employed to color the specific area or l i e under discussion. Phase diagrams have been successfully presented in this way, as has the Boltzman distribution curve showing the effect of temperature change on reaction rate (Fig. 4). The original image gives the cuwe a t T I indicating the energy of activation. An overlay then colors the area which represents those molecules with sufficient energy
Figure 4. Overlay produces curve for second temperature, wherein the ore. of interest is yellow. Overlap of the two colors emphasizes the difference in rizeof thetwo oreas.
to react.. Another overlay produces the curve for T2 and still another covers the area of this graph to the right of the energy of activation and below the curve. The colors have been chosen so that it readily becomes apparent from the overlap that there are many more reactive molecules a t T2. The effect of a catalyst on reaction rate has been similarly treated. Graphs can be made more meaningful by including diagrams along with the curves. Figure 5 shows a diagram (left) useful in the explanation of vapor pressure. An overlay then depicts the effect of increased temperature by adding more vapor molecules. The next overlay (right) indicates that this change can be graphed and the last overlay serves to introduce a discussion of boiling point. Charts and Masks
Charts can be made very clear by the strategic use of masks. Sometimes it becomes necessary to present and discuss a series of data. If all of the figures are given a t once in a chart there is no way to assure that the student is considering the point of the moment. He may be wondering why the last value in the thud column is out of lime, instead of listening to a discussion of the relationships within the first column. On the other hand if the chart is projected with an overhead,
Figure 5.
Third overlay Introducer the concept of boiling polnt.
the use of a mask can focus the student's attention on a particular column or row of data by covering up everythmg else. These masks are constructed of material that is opaque to the projector but sufficiently translucent that the instructor can see the entire image and be guided by it to his next point. Such masks make it possible to show a given visual one column a t a time on one occasion and one row a t a time on another, depending upon the emphasis needed. A number of charts have been prepared covering much of the data fundamental to general chemistry. Literature Cited (1) SLABAUGH, W. H., J. CHEM.EDUC.,28,575 (1951); KEENAN, C. W., J. CHEM.EDUC.,35,36 (1958); REBER,P., J. CHEM. K. S., ET AL., EDUc., 38, A23 (1961); AND SPIEGLER, J. CHEM.EDUC.,39, 86 (1962). (2) ALYEA,H. N., "TOPS," J. CHEM.EDUC.,39, 1%15 (1962), continued monthly to the present. R. B., AND ROBINSON, P. S., "Inorgank Chemistry," (3) HESLOP, Elsevier Publiahing Co. (D. van Nostrrtnd Co., Inc.), Princeton, New Jersey, 1960, pp. 1024. Phil. Mag., 21, 669 (1911). (4) RUTHERPORD, SIRERNEST, (5) P I ~ N T E G. L ,C., editor, "Chemistry-An Experimental Science," prepared by the Chemical Education Materials Study Group under a. grant from the NSF. 2nd ed., 1961, chap. 14, p. 23.
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