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edited by GEORGE L. GILBERT Denison University Granville. Ohio 43023
Rossini, William Tell and the Iodine Clock Reaction: A Lecture Demonstration
Solution: So1n.A
Checked by:
L. K. Brice Virginia Polylerhnic Institute a n d State University Blacksburg, VA 24061 Daniel Babb Huntington, WV25701
The iodine clock reaction has been widely used as a lecture demonstration to illustrate the dependence of chemical reaction ratc on c,mrentration and teml~erntt~re.' The purpose ofthis article is to d t x r ~ b ae livelv variation on this demonstration that is guaranteed to stimulate interest in chemical rate processes among even the most disinterested students. Ten pairs of solutions containing the iodine clock reaction components are prepared as described below and placed on a long tahle in front of the class. The concentrations of the solutions are adjusted so that when the ten pairs are mixed simultaneously, the hlue color appears after successively longer time intervals starting from one end of the tahle. The tenth solution turns blue after exactly 50 sec. A pre-recorded cassette tape is made of three portions of the William Tell Overture: ( I ) the horn call that introduces the last section of the overture (about 14 sec); (2) about 29 sec of the "galloping theme" as it is repeated for the last time, starting about 110 sec from the end; (3) the coda or ending (the last 23 sec). This gives a total playing time of a little over 1 min. To assist in performing the demonstration, ten "volunteers" are called forward to stand behind the tahle facine the class. each behind one-of the pairs of solutions. The cassette player is turned on after the volunteers have been instructed to mix the solutionson me. Thecue isgiwn after rheend d t h e htrrn call, < I that the last wlt~tirmwtll chance color iwt n i the mucir ends. (The response to this bit of sho\manship is always enthusiastic.) The ten pairs of solutions may he prepared the day before if they are tightly stoppered. (Corks in 250 ml Erlenmeyer flasks are adequate.) The tenth pair of solutions should be "finely tuned" within an hour before the demonstration to adiust the time of aooearance of the hlue color so that it centrations of the othkr solutions are not critical hut should he adjusted if necessary to produce roughly 5-sec intervals between the appearances of the blue color. (The final concentration adjustments can he made by mixing 2 ml aliquots of the solutions described below and then adding water or stock solution as needed.) The solutions are prepared as follows. Stock Solution A: Dissolve 2.0 g KIO:i in one liter of water. Stock Solution B: Dissolve 2.0g soluble starch in 500 ml boiling water, cool, add 0.40 g NaHSO:!, 5.0 m16 M H'SOd and dilute to 1I with water. Place the following solutions in 20 labeled 250-ml Erlenmeyer flasks and stopper tightly. 152 1 Journal of Chemical Education
A2
A3
A4
A5
A6
A7
A8
A9 A10
50
48
45
42
39
36
34
32
29
27
0
2
5
8
11
14
16
18
21
23
Volumes (ml) Water
ibmitted by:
A1
Similarly for 8 1 . 8 2 , etc.
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Alyea, H. N., "Tested Demonstrations in Chemistry," 6th Ed., J. CHEM. EDUC. Easton, Pa., 1965, p. 19. This demonstration was presented by J. W. Viers (Virginia Polytechnic Institute and State University) at the Fifth Biennial Conference on Chemical Education, Colorado State University, Fort Collins, Colorado, July 24-28, 1978.
Crystallization of Sodium Acetate Gene A. Hiegel California s t a t e Uniuersity, Fullerton Fullerton, CA 92634 One of the most fascinating demonstrations I encountered as a student dealt with crystallization. The demonstration has now hecome a favorite of my students. Sodium acetate is extremely soluble in hot water and much less so a t room temperature. Careful preparation and cooling
about 5 ml of water, and heat gently over a low flame from a Meker hurner using a wire gauze between the burner and the flask. Wear gloves and use tongs to hold and swirl the flask until the crystals dissolve. If any crystals remain on the sides in the flask, quickly wash down the sides with a small amount of water. Remove the flask from the heat, cover it tightly with a clean piece of foil, and allow it to cool. The solution and flask must be free of all traces of crystals. If too much water is added, a watery matrix will result, hut the excess water can he boiled off. To crystallize, place a few sodium acetate crystals on the bench and slowly and steadily pour the solution on them. Crystallization will begin immediately and continued pouring will give a tall column of crystals.' If a seed crystal is added to the solution remaining in the flask, crystals will grow rapidly in all directions from the seed. The sample can be dissolved and used again, hut a hot filtration of a somewhat diluted solution may he necessary to remove debris that promotes premature crystallization. Excess water should he hoiled off.
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F w other crvstallization demonstrations see Bacon. E. K.. d. (!HEM. E D U C . , Z . (1948),Barelli, ~,~~~ N. and Leite, C. R.,J. C H ~ M . HDVC., S3, 508 (1976), and Engels, C. J., A m r r J. Phys., d l , 744 1147:41~ ,.. . ~ , .
A cmsiderable amount of heat is released during thecrystallization. This can he felt hy touching the crystals.
