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GEORGEL. GILBERT Denisan University Granville. Ohio 43023
A Multlpwpose, Dramatic Combustion Effect
SUBMITTEO BY Arlel E. Fenstel
Davld N. Harpp and Marcel Dore McGlll Unlverslty Montreal, Quebec. Canada H3A 2K6
Joseph A. Schwarcz vanlei College 5160 Deearle Blvd. Montreal. Quebec. Canada
C ~ C K E ev D
Luther K. Brice Vlrglnla Polylechnic lnstltute Blacksburg. VA 24061
We wish to report a powerful demonstration that we have used for some time both in and out of the classroom. I t involves the controlled combustion of a mixture of metals with black and smokeless oowder in a small Erlenmever flask. The effect is exceptidnal, resulting in a jet of flame, some smoke and soarks aenerallv risine to a heieht of 8-12 feet. The reactioncan quite easiiy andkffectiveiy be scaled up or down. This demonstration can serve a variety of purposes rangingfrom an illustration of rocketry principles to the gas laws; it serves as a simple fireworks display or an example of controlled combustion. Since the reaction is initiated with benzoyl peroxide and aniline, the exothermicity of this process is also highlighted. Renzoyl peroxide can be explosive when heated. so care should he exercised in its handline. The mixt&e is concocted as follows: in a 250-ml kenmever flask are laced 1e fine (3F) or sunerfine (4F) black 8 g of smokelesspowder (both ot'which are usually available from soortine. - -aoods stores).. . and about 0.3 e each of magnesium, aluminum, and iron powders. T o th; gently blended mixture is placed 0.6 g of benzoyl peroxide in a small
pile in the center. The flask is then placed in a 1000-ml beaker (for safety purposes) and positioned in the lecture room with a cleared 6-10-foot radius. The reaction is initiated by carefully adding 3-4 drops of aniline directly to the peroxide. There is a reliable delay of 12 to 20 s before the full effect takes hold. The sparks from the metal oxidation are visually attractive and mostly extinguish in mid-air. A few fall to the floor not more than 1-2 feet from the flask and immediately die out. A fire extinguisher should be handy and the reaction preferahly positioned so that resulting vapors can be prop~rlyvented The reaction does not generate a great deal ufsmoke so an exhaust system is not absolutely critical. We have performed this demonstration over 200 times under a wide variety of conditions, and the safety record is perfect except for two occasions when the flask cracked (the beaker ~rovidesa m d e ~rotection)and in one situation when a smokk detector in the room was set off. In addition to providing an effective illustration of the principles mentioned earlier, it is an excellent means dramatically to begin or end a course, special lecture, or open house.
SUBMI-D
BY
Howard P. Wllllams and J. Emory Howell Udverslly 01 Southern Misslsslppl BOX5043 Southern Station Hattlasburg, MS 39406 CHECKED BY
Davld A. Franz LycornlngCollege Wllllarnrport, PA 17701
Quite often chemical demonstrations have hazardous or noxious odors associated with them. A device has been constructed that alleviates the problem. This device precludes the breathine of hazardous vaoors durine class demonstrations, and i t i s convenient to &ansport to classes in lecture halls that do not have sinks. The apparatus is constructed of glass from two 2.5-cm X 15-cm test tubes. A 2-mm stopcock is sealed about 2.5 cm
Volume 63
Number 4
April 1986
349
air. Next open both of the stopcocks, and observe that uniform distribution of bromine is obtained on both sides. (A discussion of gaseous diffusion may follow.) Since bromine reacts rather quickly with rubber, you should use cork stoppers or cover the rubber stopper with a polyethylene fiim before inserting the stoppers. Another diffusion demonstration can be carried out by filling one side with about 2-3 cm of concentrated hydrochloric acid and the other side with about the same amount of concentrated aqueous ammonia. Only the top stopcock should be opened for this demonstration. Careful observation will show that the cloud formed is in the chamber containing the acid. This effect is due to the fact that the effusion rate of ammonia is much greater than that of hydrogen chloride. Damonstrallng Oxldatlon-Reduction Reactions
Ti-m demonsbation apparatus
from the mouth and 1.0 cm from the bottom as shown in the figure. The stopcocks used in this construction were made of Teflon, but glass stopcocks would work for most demonstrations. The toos of the test tubes are sealed usine rubber or cork stoppers. The followine demonstrations have nroven to be odor free with the use of the device described: There are countless other demonstrations that can be used, and you should have no difficulty adapting it to your favorite reaction/demonstration. Solublllty Demonstration
Usually students think of the sulfides, carbonates, and hydroxides of metal ions when solubility is discussed. An old demonstration utilizine concentrated hvdrochloric acid and a saturated yolution of sodium chloride gives an impressive disdav of the common ion effect on the soluhilitv ot'a famili a r s u h a n c e . The stopcocks are closed and one of the test tubes is filled with HCI and the other is half filled with saturated KaCI. Upon opening the top and then the bottom sropcocks an appreciable precipitate of solid NaCl results. Demonstrallng Gaseous Dlffuslon
Place a drop of bromine in one side (caution) with both stopcocks closed. After the drop has vaporized, remove and quickly replace the stopper on the side containing the bromine in order to equalize pressure with the side containing
350
Journal of Chemical Education
A number of oxidation-reduction reactions that involve color changes can he demonstrated. For example, a drop or two of bromine can be placed on one side and allowed to vaporize. On the other side, add 2-3 cm of 6 M sodium hydroxide solution. To carry out the reaction open both stopcocks and cautiously tip the device so some of the sodium hydroxide solution runs into the side containing the bromine. Observe the disappearance of color from the bromine side. The reaction by which the color is removed is interesting because it represents a disproportionation that is reasonably rapid a t room temperature.
Vollalc Cell Prlnclples
Fill one side with a colored solution with both of the stopcocks closed. Draw marks a t regular intervals on the test tubes to show more clearly the liquid height changes. The difference in the height of the liquid a t any given time represents the cell potential difference. Next open the top stopcock, and note that no liquid is transferred to the other chamber. This is analogous to connecting two half cells with a wire without a salt bridge or ion path. Close the top stopcock, and open the bottom stopcock. This is analogous to a cell with a salt bridge but no external cell connection. Note that very little or no liquid flows. Next slowly open the top stopcock, and allow a small quantity of liquid (current) to flow. A small flow of liquid illustrates a high resistance external circuit. Placing a small cork in the empty side illustrates that work is done only when current (liquid) flows. As the liquid level reaches the same level in both sides, it is clear that equilibrium is reached and no more work can be done. This cell can be "recharged" by use of a pressure bulb or by tilting the cell. Very few chemical cells are reversible; the lead storage cell and nicad cell are examples of the reversible type.
