In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Ignition of Hydrogen Balloons by Model-Rocket-Engine Igniters submitted by:
Nicholas T. Hartman Department of Chemistry, Pennsylvania State University, University Park, PA 16802;
[email protected] checked by:
Ed Vitz Department of Chemistry, Kutztown University, Kutztown, PA 19530
Exploding hydrogen balloons are perhaps the most widely used lecture demonstration of all time. Variations of this demonstration have been described in numerous demonstration books (1–3). Traditionally a wick, placed on the end of a long pole, is used to initiate the explosion. This paper describes an alternative method for igniting the balloons via an “electronic match”. Model-rocket-engine igniters are inexpensive and readily available at most hobby stores or online hobby vendors (4). The author used Tiger Tail I igniters (Part No. 7010 from Quest Aerospace). When placed in a properly constructed electrical circuit, these devices will burn momentarily with the approximate intensity of a “traditional” match. The setup for this demonstration is illustrated in Figure 1.
An alternative method has been suggested (5) in which the model-rocket-engine igniter is replaced with several fine wires from a steel wool pad. Three to five, 4-cm strands of steel wool from a generic cleaning pad are attached to the ends of the previously described thin gauged, magnet wires with transparent tape such that a 2–3-cm fuse of steel wool wires separates the wires attached to the battery. The assembly is then attached to the side of a hydrogen balloon mak-
Materials and Methods Relatively light wires are required for the balloon to still float with the igniter attached. Thin, 26–30 gauge enamelcoated-magnet wire works ideally and is easily obtainable from most electronics shops (Part No. 278-1345 from Radio Shack). Although heavier wires can work, the balloon will no longer float. The igniter packaging provides instructions for using alligator clips to attach the wire leads to the igniter. However in order to have the igniter setup light enough to keep the balloon floating, an alternative method of connection can be used. First, the metal lead of the igniter (originally 5-cm long) is cut down to about 1 cm in length to eliminate excess weight. Next, the thin insulation is burned off the thin gauge, magnet wire using a soldering gun, and the wire leads are carefully soldered to opposite sides of the igniter, as shown in Figure 2. The integrity of the connection can be tested by measuring the resistance through the circuit. A correctly connected assembly should have a resistance of approximately 7.5 Ω. The igniter should be attached to a “standard sized” (less than 20-cm diameter) hydrogen-filled balloon with two pieces (about 5-cm each) of transparent tape. The first piece of tape should be used to secure the wires to the balloon while the second piece of tape is used to position the igniter head about 0.5 cm from the balloon’s surface. Two 9-V batteries in series can be used as the power source, eliminating the need for special battery-packs or transformers. To initiate the explosion, complete the circuit by touching the ends of the wires to the poles on the battery. The igniter should immediately flare up and cause the hydrogen balloon to explode. 774
Figure 1. Schematic for constructing the electronic ignition system.
Figure 2. Schematic of a correctly wired igniter assembly. The igniter (2-mm x 50-mm) consists of two copper strips separated by a thin insulating layer with a tip of ignition mixture. When a current is passed through the device, the resistive heating in the tip is sufficient to cause ignition.
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
In the Classroom
ing sure that the iron fuse touches balloon’s surface. Upon completing the circuit with two 9-V batteries in series, the burning steel wool wires immediately ignite the balloon. The overall effect is nearly identical to the previously described model-rocket-engine igniter system. With practice, the setup will look just like a regular balloon floating on the end of a string provided it is viewed from a distance. Additionally, a simple switch incorporated into the previously described circuit provides for fast and easy triggering of the demonstration. For a louder bang, the igniters also work for balloons filled with two-parts hydrogen and one-part oxygen, although they are often not buoyant enough to float with the igniter attached.
tect their ears for this demonstration. Only “standard” size balloons should be used in order to keep the explosion on a level that can be safely performed indoors. When attaching the igniter to a balloon, make sure that the batteries are totally disconnected to avoid accidental ignition.
Discussion The chemistry of “electronic matches” is nearly identical to the chemistry of traditional matches. While the exact composition of the pyrotechnic head varies with different manufacturers, it typically contains a strong oxidizer (potassium perchlorate), a fuel (finely ground magnesium or titanium), and a binding agent, which often contains nitrocellulose (6, 7). The tips of “safety matches” often contain potassium chlorate while the box’s striking surface is coated with red phosphorus (8). The heat produced by the friction between the box and match tip causes small amounts of the red phosphorus on the striking surface to convert into white phosphorus, which almost immediately reacts with the surrounding air. The resulting reaction releases enough energy to ignite the oxidizer–fuel mixture in the match tip and ultimately ignites the matchstick itself. Ignition in an electronic match is achieved by passing an electric current through the resistive head. The heat released as a result of the electrical resistance of the igniter provides the energy to initiate the reaction. In the simpler steel wool setup, the relatively high resistivity of the steel wool causes the fuse to rapidly heat up and immediately react with ambient oxygen. The resulting rapid reaction releases enough energy to ignite the hydrogen balloon. After performing this demonstration, an explanation of the chemistry behind both “traditional” and “electronic” matches provides a perfect lead into a discussion on the chemistry of fireworks.
Editor’s Note
Hazards Wear eye protection when performing this experiment. Individuals with sensitive hearing should be advised to pro-
Acknowledgments The author would like to thank R. Kreuter for providing input on wiring options and E. Vitz, R. D. Minard, J. Bortiatynski, and J. T. Keiser for their comments on the design of this demonstration.
Readers who perform demonstrations with hydrogenfilled balloons should read the description of an accident with such balloons, found on page 743 of this issue. Literature Cited 1. Summerlin, L. R.; Ealy, J. L. Chemical Demonstrations: A Sourcebook for Teachers Vol. 1, 2nd ed.; American Chemical Society: Washington, DC, 1988; p 27. 2. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1983; pp 106–112. 3. Classic Chemistry Demonstrations; O’Driscoll, C., Reed, N. Eds.; The Royal Society of Chemistry: London, 1995; pp 88– 89. 4. If the igniters are not available locally they can be easily obtained from online vendors. The following sites are recommended: http://www.ehobbies.com (accessed Apr 2003), http:// www.discountrocketry.com (accessed Apr 2003), http:// www.questrockets.com (accessed Apr 2003). 5. Vitz, E. Kutztown University, Kutztown, PA. Personal communication, August 2002. 6. Dyben, J. F. Pyrogen Compound Kit for an Electrical Model Rocket Ignitor. U.S. Patent No. 5,780,765, July 14, 1998. 7. Hans, P. C.; Meyer, D. H.; Rosenfield, G. C. Rocket Ignition Assembly and Means and Methods for Making and Using Same. U.S. Patent No. 5,123,355, June 23, 1992. 8. Safety Match Chemistry: Red Phosphorus and Potassium Chlorate. http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/ MAIN/MATCHES/PAGE1.HTM (accessed Apr 2003).
JChemEd.chem.wisc.edu • Vol. 80 No. 7 July 2003 • Journal of Chemical Education
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