overhead projector
edited by DORIS KOLB Bradley University Peoria. IL 61625
Overhead Projection of Graham's Law of Gaseous Diffusion Dlanne N. Epp East High School 1000 South 70th Lincoln. NE 68510 Many high school laboratory manuals contain an experiment that shows the rate of diffusion of two gases, at constant temnerature and pressure, t o be inversely proportional to the sq;are roots o f i h e molar masses of thr two gases.[ These are based on the work of Thomas Graham' and others'.' who examined the diffusion of gases in nonuniform mixtures. In current experiments the visible reaction used to observe easeous diffusion is the formation of solid NHKI -~~~ . hv direct combination O ~ N Hand ~ HCI.
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along, thin "smoke"line will form as the gases meet. This appears as a very visible black line on the overhead screen. The position of the line is marked on the transparency sheet, the cover is removed, and the chemicals are blotted dry with a paper towel. The distance traveled by each gas may now he measured with reference to the ruled line. and calculationscan be made concernine the relative ratpsofdlffushn of theammonia molecules and the HC~' molcrules. It is readily apparent that the NH, molecules (17.0 g/ mol) diffuse more rapidly than do the heawr HCI (36.5 glmol, molecules, as the formation of the reaction line is noticeably nearer to the origin of the HC1. Measurement of the distances traveled, a measure of the velocity, consistently yields a ratio of 1.4 (*0.1), which agrees well with the theoreticalrelative ratio of the velocity of NH3 to HC1 of 1.47 derived from the Graham relationship.
-
~
NH,
+ HCI-
NH,C1
Concentrated ammonia and hydrochloric acid solutions are inserted at opposite ends of a glass tubing, and the formation of a white "smoke ring" is a measure of the distance traveled by each gas. This reaction may be enlarged to lecture hall size using a large glass tube, and microscale versions using capillary tubes have also been tried. Over several vears I have found that students consistenth have some trouble with this experiment. If the glass tubing is not scrupulously clean, the smoke ring will not adhere to the glass, and, if the liquids are not inserted simultaneously, serious experimental error results. It is also very important t o mark the first appearance of the ring as i t appears to "travel" toward the HC1 as the experiment proceeds. As a result of the studv of a microscale diffusion demonstration wmtten for thisrolumn5, Idecided toadapt asimilar technique to this classical experiment. I have found that use of the overhead projector makes this experiment much more visttde, and the quantitative results have been excellent. Procedure A clear acetate sheet is prepared with an ll-cm ruled line. The markings should be drawn on the reverse side of the acetate sheet to prevent interaction with the chemicals. The cover of a 96-well plastic tissue culture plate acts as a container for the reaction. Place the cover on the overhead just above the mled line so that you can move it into place immediately after putting the chemicals onto the acetate sheet. Using a l-mL micropipet that has a very fine tip (or, alternatively, a 3-mL micropipet with an enlongated tip) simultaneously place a drop of concentrated HCI at the O-cm mark and a drop of aqueous concentrated NHs at the ll-cm mark. Cover the reaction immediately with the plastic cover. Within several seconds
' Wilbraham, A. C.; Staley, D. D.; Simpson, C. J.; ~
a k M., S. Chemishy, Laboratory Manual; Addison-Wesley: Menlo Park, CA, 1987. >(a)Graham. T . Phil. Mag. 11833, 2, 175-351: (Reprinted in Graham, T. Chemical and Physical Researches: Edinburgh University: 1876: pp 44-70) . Mason, E. A.; Kronstadt, B. J. Chem. ~ d u c1967,44,740. 'Kirk. A. D. J. Chem. Educ. 1976.44.745. Epp, D. N.: Lyons, E. J.; Brooks. D. W. J. Chem EEduc. 1989, 66. 436.
A Visible Spectrometer James D. Martin Indiana University Blwmington. IN 47405 T h e emission and absorption of discrete wavelengths of light is fundamental to the undergraduate chemistry student's basic understanding of electronic structure and hondine. Such discussions are freauentlv accomnanied hv a demo n k a t i o n of the emission s p e c t r k of Garinus elements using excited gas tubes and diffraction gratings. T o demonstrate the complementary absorption of light and its relation to the observed color, I have prepared the following apparatus. A diffraction grating with 13400lines per inch is placed in a poster-board mount in front of the projection lens of a n overhead projector as shown in Figure 1. (The orientation of the diffraction grating will determine whether the projected visihle spectrum is horizontal or vertical on the screen.) A second niece of noster hoard is cut to mask all hut a hole in ;he center of the projection platform of the overhead nroiector (see Fie. 1). Two 1- X %in. strips of noster board . provide a h i d e & the color strip.
WLOR SmW
PROJECTON HOLE
Figure 1. Diagram of the overhead projector spectrophotometer. The diffraction grating Ismourned overtheprojection lens, and Ihs color stripsand mount cover the projection platform. Volume 67
Number 12 December 1990
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a
0
red
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violet
Figure 2. A block diagram representation d lhe projected specha: (a)Me white light spechum. (b) the red absorption specrmm,and (c) the red absorption spectrum conwasted with the white ligM spectrum.
A color strip is prepared by cutting a 1-in.-wide strip from an overhead transparency. One-inch square blocks of color are prepared using a variety of felt pens. I t is best to leave approximately '/z in. in between each block of color. With no color block over the 1-in. projection hole, asquare of white light flanked by two images of the visible spectrum is projected on the screen as shown in Figure 2a. A colored block is then slid over the 1-in. hole. The absorbed wavelengths of light are now absent from the projected spectrum (Fig. 2b). T o strengthen the comparison with the white light spectrum, slide thecolor block backso that now seven-eights of the projection hole is masked by the color block, and one-
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eighth of the hole transmits the white liebt 2c). This .. (Fiz. . clearly demonstrates which wavelengths are abiorbid and which are transmitted by the dye of the ink. The success of this detnonstratioo is largely in the choice of colored pens used to make the colored blocks on the color strip. Several felt pen inks absorb all wavelengths of light except the said color. These will not as readily demonstrate the absorption of select wavelengths of light. If necessary, root through your colleagues' desks for some different pens. Dyes appropriately chosen will demonstrate that the observed color is a result of the absorption of its complementary color. For example, when the red block of color covers the projection hole a red block of color flanked by the visible spectrum lacking the green wavelengths will be projected onto the screen. Conversely the red wavelengths should be absent from the spectrum of the green color block. Similar results are observed for orangelblue and yellow/violet color combinations. This demonstration provides a useful visual image when discussing electronic transitions of the valence electrons between various energy levels. Furthermore, such a demonstration preceding a spectrophotometry laboratory experiment alleviates some student confusion when they find that a red solution of CoC12absorbs green wavelengths of light, or a violet solution of KMnOa absorbs yellow wavelengths of light.
