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That’s One Fast Reaction!

By Lori R. Stepan

Did you know that a really fast chemical reaction makes riding in a car safer? Most cars have airbags built into the dashboard and steering wheel that can blow up like super-fast balloons during a collision, and cushion riders from getting hurt. What makes the airbag blow up like a balloon? Chemistry! Instead of transporting compressed gas in the car to inflate the airbag, we take advantage of a very fast reaction that produces the needed gas.

Many car airbag inflators contain small amounts of a toxic molecule called sodium azide, or NaN3 (one sodium atom and three nitrogen atoms combined). Sodium azide breaks down very quickly when heated or physically shocked. This is called decomposition. The products of the decomposition are sodium atoms and nitrogen gas.

If the car is involved in a collision, sensors send an electric signal to the container where the sodium azide is located. The signal ignites a flammable compound, and the heat it creates starts the decomposition of the sodium azide.

A huge amount of nitrogen gas immediately rushes out with an explosion and fills the airbag. It is amazing that from the time the sensor detects the collision to the time the airbag is fully inflated is only 30 milliseconds, or three one-hundredths of a second! A normal blink of your eye is 100 milliseconds. Around 50 milliseconds after a collision, the person riding in the car hits the air bag, which absorbs their forward-moving energy and protects them from slamming into other parts of the car. Chemistry has saved the day!

A relatively small amount of sodium azide (4.6 ounces or 130 g) will produce a lot of nitrogen gas very quickly; it takes almost five party balloons’ worth of gas to fill a normal air bag! The sodium metal product can be a potential hazard, but in this case, other ingredients react with the sodium to form safe compounds.

You may be able to think of other situations where fast production of a gas is needed. For example, sodium azide is also used to inflate airplane escape chutes in case of an accident. If you were an inventor, how would you use this fast production of gas?

Lori R. Stepan, Ph.D. is an Associate Teaching Professor of Chemistry at Penn State University in State College, PA.