edited by
tested demonstrations
GEORGE L. GILBERT Denison University Granville, OH 43023
The CoC12 Thermosiphon Submined by
Justina L. Brown
Northmont High School Union, OH 45322 Rubin 6attino1 Wright State University Dayton, OH 45435 Checked by
Paul F. Krause
University of Central Arkansas Conway, AK 72032 Cobalt(I1) complexes provide eye-arresting demonstrations for the color shifts that appear upon addition of appropriate reagents and also for changes in temperature. The paper by Zeltmann, et al. (I)reports on the NMR shifts for oxygen-17 and chlorine-35 for aqueous HCI solutions of Co(I1). Ophardt(2) gives some briefinstructions for using this system for demonstrations. Shakhashiri (3) gives more detailed directions for four demonstrations involving the chloro and thiocyanato complexes of Co(II),as well as a discussion of the chemistry involved and comments on safety. In this brief paper we will describe a novel way to present the temperature dependence of the equilibrium shift. The reaction that is responsible for the color change can be represented as C O C ~ ( H ~ O +) ~Cl-(aq) + ( ~ ~=)C O C I ~ ( H ~ O + )3Hz0 ~(~~) octahedral tetrahedral pink blue cold hot There is a shiR from the octahedral structure that is pink and primarily exists at sub-ambient temperatures (cold)to the tetrahedral structure that is blue at super-ambient temperatures (hot). Reference (1)gives details of the shifts and the relevant equilibrium constants. We have used two recipes for the preparation of the Co(I1) solutions used. Slowlv (and carefullv!) add 200 mL ofcorkentrated HCI with s t i k n Kto200 m i of 0.1 M CoCh (1 1.0.1 M CoCI? is 23.8 eCoC19-H,O dissolved in sufficient distilled water ipto 1 ~ 7The . ;olition color should he purole. If it is blue. slowlv add di~tilledwater until it becomes purple. The second & r pie involves adding 25 mL of 2-pmpan01 to 1g CoClp6H20.This solution is blue. Slowly stir in distilled water (-4-5 mL) until you get a light purple color. The proportions can, of course, be scaled up or down. We prefer handling the 2-propanol solutions. CAUTION:The HC1 solution is corrosive and the 2-propan01flammable. We obtained the spectra of the HC1 solutions at 5 'C intervals from 10 to 70 'C on a Hewlett Packard model 8452A diode array spectrophotometer. Figure 1gives the absorbance at three wavelengths as a function of temperature. 'Author to whom correspondence should be addressed.
Figure 1. Absorbance versus temperature forCo(ll)solutions, The absorbance at the pink end very slowly increases with temperature, while that at the blue end rapidly increases. At 40 'C the blue to pink absorbance ratio is about 3:l and at 50 'C about 41. So, the major detectable color change occurs in this range. The color chances for Co(I1) solutions can be simply shown using test tubes of the solutions at room temperature, ice water, and a bath of about 50 'C. Amore interesting way to show this is to use the thermosiphon where density changes due to differing temperatures cause the fluid to flow. We constructed an inexpensive version out of plastic and glass tubing and an elegant version solely out of glass tubing. The two versions will be described separately, but the basic design can be seen in Figure 2 that shows the glass version. The inexpensive version is made out of 12 mm wide-bore transparent plastic tubing, two 20-cm lengths of 12-mm 0.d. Pyrex tubing, and two tees (either plastic or glass) for the top and bottom of the loop. Sealed concentrically about 3 cm from the end of one the class tubes with silicone bathtub caulking (or RTV)is a 32Toz expanded polystyrene cup. All elassl~lastictubine connections are sealed with hose clamps. Lpowder f u m k is connected to the top tee for fdline and a stoocock or screw clam^ on a ~ i e c of e ~lastictubing is connected to the bottom tee for se*&ng an2 draining. The entire assembly is mounted on a large stable ring stand with appropriate clamps. The loop is filled to 1-2 cm Volume 70 Number 2 February 1993
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above the upper tee. Acrystallizing dish of about 10cm dia is suspended by wire midway down the second glass tube. An inexpensive 1500-W hair dryer is clamped so that it blows into the crystallizing dish that serves to concentrate the hot air around the glass tube and also permit viewing. If the hair dryer is mounted too close, its safety switch will turn it off. For enhanced visibility, aquarium lamps in appropriate safe mountings may be clamped behind and above the '?lotnglass tube and behind and below the "cold* glass tube. In operation, ice and ice water are placed in the expanded polystyrene cup and the hair dryer is turned on. ARer a short time you can see the pink solution descending from the wld side and the blue solution ascending from the hot side. Close examination will show color streaming. The thermosiphon is visible in modest size classrooms and is a dynamic demonstration of the shifting equilibria involved. The more expensive version of the thermosiphon is made wmpletely of glass tubing and requires a skilled glass blower. Dimensions are given in Figure 2. Acknowledgment JLB was supported by the NIH Minority High School Research Apprenticeship Program. RB first saw a primitive version of the thermosiphon a t a science fair in New Zealand and thanks the originator for the idea. Figure 2. The all-glass thermosiphon. All dimensions are in millimeters Literature Cited 1. Zeltmann, A H.;Mahnyaff, N.A: M e ,L. 0. J Phys. C h m . 1968,72, 121. 2. Ophardt,C. E. J C h .Edue. 1980.67,463. 3. Shakhsahiri, B. 2. Che& &mnsfrations, Vol. 1, 280, Universitj af Wiecrmsin Press,Msdi~on,WI. 1983.
Bleaching with Chlorine: Another Tomato Juice Demonstration Thomas M. Nemeb and David W. Ball' Cleveland State University Cleveland. OH 44115
Chemicals 'calcium hypochlorite 12 M hydrochloric acid tomato juice
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~ e b o r a hWhalen and David Blackman Un vers fy of tne Distr ct of Col~mbia washington, DC 20008
Procedure
When saturated bromine water is added carefully to tomato juice, . . an array of colors is produced (I).This demonstration can be seen on the cover of this Journal's December 1986 issue. The visual appeal of this demonstration inspired us to explore brominkwater reactions with other carotenoid-containing substances;' regretfully, none supplied as vibrant a visual array. The lessons of the periodic table suggested to us that we trv a different halorren. We have found that. bv bubbline cfiorine gas througK a cylinder of tomato juice, dramat; bleaching of the pigments is produced. The similarities and differences between the bromine and chlorine reactions act as a catalvst for the discnssion of the relative reactivities of the hal"ogens, as well as an introduction to the bleaching effects of chlorine.
a
'Author to whom correspondence should be addressed 'Among the carotenoid sources tried were V-8 juice, carrots, peaches, sweet potatoes, egg yolks, red and green peppers, and beets. Only the V-8 juice gave a similar reaction with the bromine water. Curiously, the carrots showed linle change in color. Journal of Chemical Education
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Assemble the apparatus according to the figure.
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Apparatus 100-mLgraduated cylinder 50-mL filter flask ring stand -3 ft of rubberlplastic hose disposable pipet hose clamps metal scaop rubber gloves apparatus clamps
CAUTION:Chlorine gas is toxic. This demonstration should be carried out in a hood. Rubher gloves should be worn when performing this experiment. Set up the apparatus as depicted. The graduated cylinder should be filled only about half way with tomato juice (to keep it from bubbling over). The mbber hose should be connected using hose clamps to ensure that the chlorine gas does not leak through the connections. To generate chlorine gas, add about 15-20 mL of concentrated hydrochloric acid to the fdter flask. When ready to perform the demonstration, add a large swopful of calcium hypochlorite to the filter flask and immediately close offthe flask securely with a rubber stopper. Chlorine gas is generated by the following aqueous reaction:
(Alternately, chlorine gas can be dispensed from a leeture bottle directly into the mhher hose.) As the chlorine
