edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
Chemistry of Air Bags Wllllam I. Bell Red Rocks Communlly College 12600 West Sixth Avenue Golden. CO 80401 Automobile air bags, designed to inflate rapidly in the event of a crash, thus cushioning the occupants against imDact. a ~ n e a r e don several models of American-made cars in the 1989model year ( I ) . This event offers an opportunity for students to e x ~ l o r an e a ~ d i c a t i o nof chemistry. The exercise I have used with my cl&ies asks students tostudy some of the requirements for an air bag. First, a few basics: One air bag will be mounted in the steering column; some systems will have a second air bag in the dashboard to protect the front-seat passenger. Both will be triggered by an impact and inflate very rapidly (-40 ms), then deflate so as to provide a steady, negative acceleration. The physics of air bags have been reviewed in an article in The Physics Teacher (2). Air bag systems employ one or more sensors consisting of a ball held in a tube by a stiff spring. At a pre-set negative acceleration along the axis of the tube, the ball is moved far enough to close a n electrical switch, putting current through a detonator cap that initiates the gas-producing reaction. Since the air bags are built into the steering wheel or dashboard, the installation is not particularly apparent until the bags inflate. Here are the instructions I give my students: 1. What volume of gas would be required to fill a pair of air bags? Make measurements of any passenger car, and express the an-
swer in liters. 2. The current system calls for using the reaction of sodium azide to
produce the gas, according to this equation:
a. Balance the equation. b. What type of reaction is this (combination, decomposition, replacement, double replacement)? 3. How many moles of Npare required? Use the volume calculated in question 1,and assume a pressure of 1atm and a temperature of 20 O C . 4. What weight of sodium azide would be required? 5. Write an electron-dot formula for the azide ion, N3-. There are several related points that may be pursued here. Teachers and students interested in the interaction of science, technology, and public policy may wish to explore the development of "passive restraint" systems: air bags and automatic seat belts. The history has been long and contro-
versial (3-11). On a more chemical note, the toxicity of sodium azide (and its reaction products) and itseventual disposal have recently been atop& of concern and the subject i f an interesting exchange of letters in Chemical & Engineering News (12-15). T h e following questions will lead students to discover that sodium azide, in addition to being explosive, is roughly as toxic as sodium cyanide. Hydrazoic acid, produced on acidification of sodium azide, is a volatile liquid that is both toxic and explosive.
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6. Dws sodium azide have anv hazardous orooerties? Trv some .
reference hooks in the library, such as: a. Hmordvua Chemical 1)ara Hook; C.. Weisa. Ed.; Noyes Data Corp.: 1980. b. Dangerous PIoperties of Zndustriol Materials, 6th ed.; Sax, N. I., Ed.; Van Nostrand: New York, 1984. Under "Toxicity by Ingestion" you will find a value quoted for the LDW;this indicates a lethal dose for 50%of the test animals, usually mice or rats. The units used are milligrams of the toxic substance per kilogram hody weight of the test animal. Assuming the amount of sodium azide that constitutes a lethal dose is directly proportional to hody weight, calculate an approximate lethal dose for an 80 kg person. 7. Sodium azide can react with acids to form hydrazoic acid, HNB. a. Write a balanced equation for the reaction of sodium azide with sulfuricacid to form hydrazoic acid and sodium sulfate. h. What type of reaction is this (combination, decomposition, replacement, douhle replarementi? r. Suppose that 60 g of aodium azide reacts with 100 ml. of G M sulfurrc acid: rslculalp the wrieht - of hvdrazoic acid that could he produced: d. Does hydrazoic acid have any hazardous properties? Students have been enthusiastic about this exercise, and the response has been encouraging. Literature CHed
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6. Consumer Reports 1987, (April), 202.
7. Sherman. D. Car ondDriuer 1986, (April), 7. 8. Dinkel,J.Rood&T m d 1986. (Feb.1.33. 9. High Techno1 Business 1987. (Dec.l.11. 1L New Republic 1985. (June 81.4. 11. Mayer. R. N.:Zick, C. 0. J . C o ~ v m e Allaim r 1986, ISummerl, 1. 12. Environment 1984. (Dec.l.202. 13. a. Szmant,H.H.Cham.Enp. News 1987, (Jan. 20.55. b. Jonw.1.S. Chrm.Eng.Nelva 1987, (May 41.2. c. Kamlet, M. J. Chom. E n g Nsms 1967, (July 131.36. 14. Mintzer. J. G. Autornoliue lndust~y1985.165 Web.). 26. I 5 Tho motallicsodium oroduced in the decomoosition ofsodium arideis alsoaootential
Volume 67
Number 1
January 1990
61
