In the Classroom
Tested Demonstrations
An Alcohol Rocket Car— A Variation on the “Whoosh Bottle” Theme submitted by:
Dean J. Campbell Department of Chemistry, Bradley University, Peoria, IL 61625-0208;
[email protected] checked by:
Stacy DeWees Department of Chemistry, Michigan State University, East Lansing, MI 48824
The “whoosh bottle” demonstration has been around for a long time and seems to be well established. This demonstration of thermodynamic principles and gas laws involves the ignition of alcohol vapors in some form of open-mouthed bottle. As a flame burns down into alcohol vapors in the bottle in an exothermic reaction, the heated gaseous by-products of combustion and other gases expand past the neck of the bottle (Charles’s law) with a “whooshing” noise and with sufficient thrust to move a lightweight bottle. The following describes one way of converting the whoosh bottle into a simple and rather spectacular rocket car. Figure 1 illustrates the construction of the rocket car. Cut a plastic soda straw into two tubes 3 cm long and fasten the tubes to the edge of the jug with duct tape, one at each end, as shown. The wheel axles run though the plastic tubes. One can use all sorts of designs for the wheels, but they should spin freely and have small mass. For example, I use round wooden applicator sticks (Catalog No. 01-340 from Fisher Scientific) as axles and plastic film canister lids as wheels. To place the wheels on the axles small slits are cut in the center of each canister lid with a razor blade and the lid simply slides onto the end of the wooden stick. Pour about 0.75 mL of methanol into the dry jug. It is imperative that the jug be dry inside for proper combustion of the alcohol vapors. I have used varying quantities of both methanol and ethanol in my rocket cars. Methanol is superior because it is more volatile; in addition, its use as a racing car fuel can be mentioned in class. Combustion of more than 0.75 mL of methanol can provide too much thrust and send the car violently tumbling. There is no need for the dangerous practice of charging the jug with oxygen for extra thrust. Place the cap on the end of the jug, slosh the alcohol back and forth briefly, and let the car sit on its wheels for about 15–20 minutes. The 0.75 mL of methanol should evaporate completely. Larger quantities will also evaporate completely, but will provide excessive thrust. If excess liquid alcohol is present, remove the cap and drain any excess alcohol from the jug into a sink or suitable storage container (the excess alcohol may be reused). Remove excess alcohol from the area (run water in the sink or move the container of alcohol). Place the car wheels-down on a smooth, flat, nonflammable surface. Give the car at least four meters of the space to move forward without running into combustible objects or people. Make sure no puddles of alcohol have formed under the car and that no alcohol is on your hands. Turn out the lights and hold a lit butane grill lighter (or another flame source
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Figure 1. Construction of the rocket car. A: Dry one-gallon plastic milk jug. B: Soda straw. C: Duct tape. D: Round wooden stir sticks. E: Plastic film canister lids.
that would give your hand a little distance from the flame) near the open end of the milk jug. In about one second a blue flame will appear at the open end as the alcohol vapors ignite. With a whooshing noise, the car will move forward with a blue flame coming out of the opening. Occasionally the car will roll onto its side, but this has not ever been found to be a problem. Immediately and carefully pick up the car when it comes to a stop. There may occasionally be a small blue flame burning at the end of or within the jug, especially if too much alcohol is used. Immediately and thoroughly rinse the inside and the outside of the car with water to extinguish any possible flames and to cool the jug. On one occasion when too much alcohol was used in a prototype of the car, the burning alcohol was not sufficiently rinsed away, setting the milk jug on fire and melting its plastic wheels. Rinsing the jug by filling it with water and then draining it also helps to remove combustion by-products from the inside of the jug (1), although the demonstration is most effective if the jug is allowed to dry completely before reuse. It may be useful to have more than
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
In the Classroom
one rocket car setup available for multiple demonstrations, since it does take some time for the milk jug to dry.
Acknowledgments Special thanks to Wayne Bosma at Bradley University for introducing me to the static “whoosh” bottle experiment and to Sarah Rupe for assistance with the photography.
Hazards Wear eye protection when doing this experiment. Refrain from getting alcohol on your hands (gloves are recommended). Keep fire-extinguishing materials in the vicinity. John Fortman of Wright State University has written extensively on safety precautions involving alcohol combustion in a jug (1).
Literature Cited 1. Fortman, J. J.; Rush, A. C.; Stamper, J. E. J. Chem. Educ. 1999, 76, 1092.
Note from Ed Vitz, Tested Demonstrations Feature Editor The “whoosh bottle” demonstration has been the subject of recent controversy (1) and the cause of several serious accidents (2, 3), which may arise when detonation occurs rather than smooth combustion. While the explosive limits of methanol in air are 6% to 36.5% by volume, the stoichiometric ratio is near the low end of this range. The vapor pressure of methanol varies from 88 to 146 torr between 20 and 30 °C, and it is easy to create methanol–air mixtures significantly above the 6% lower explosive limit in a warm room (4 ). These “fuel-rich” mixtures may promote smooth combustion. Safe whoosh bottle demonstrations have been performed when an excess of liquid alcohol is allowed to evaporate long enough to establish equilibrium vapor pressure in the bottle, when the temperature is high enough so that a rich mixture is created, and when the excess alcohol is decanted and removed to a fireproof container at a safe distance. It is imperative that large glass carboys not be used. Why do these demonstrations at all? Bill Deese’s “ring of fire” demonstration (5) forces the viewer to think about the kinetics of flame propagation because in it the flame front is visible. Whoosh bottle demonstrations are interesting because (done right) they don’t explode (6 ); rather, the zone of rapid reaction apparently progresses smoothly and predictably through the mixture. A process like this is required in the propellant used for normal firing of large naval guns, and rapid detonation may have been the cause of the tragic gun turret explosion on the USS Iowa in 1989. “Pinging” in automobile
engines is symptomatic of fuel detonation that occurs much faster than the piston can move, and it is related to lean combustion mixtures. It may interest students to find that it is impossible to get too much methanol vapor in a whoosh bottle by allowing the liquid to evaporate for too long (counterintuitively, the longer the better may be the rule). Here’s an opportunity to talk about vapor pressures, partial pressures, stoichiometry, redox reactions, kinetics, and the commonalities between mistuned engines and improperly prepared cannon propellants. References and Notes 1. See, for example, the discussion and accident report on the Chemed-L list available in the archives at http://www.optc.com/ chemed-l-thread/Thread6290.html (accessed Mar 2001). 2. For example, a 7-gallon glass jug containing methanol vapor exploded on November 3, 2000, in Cache County, Utah, hospitalizing two students injured by flying glass. 3. Lee, C. J. Chem. Educ. 1998, 75, 543. 4. For example, methanol at 146 torr would coexist with about (760 – 146) × 0.2 = 123 torr of oxygen at 760 torr, a ratio near 1.2, compared to the stoichiometric ratio of 0.5. At 20 °C and 88 torr methanol, the ratio is 0.65, while the volume percent of methanol is greater than 10%. 5. Deese, W. C. CHEM 13 News, May 1998, p 3. 6. Fortman, J. J.; Rush, A. C.; Stamper, J. E. J. Chem. Educ. 1999, 76, 1092.
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