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Joseph S. Merola, a chemistry
professor and associate dean of the
College of Arts and Sciences at Virginia Tech, offers this explanation:
Although we don't usually associate automobiles with chemistry, a lot
of chemistry takes place in a working car--the burning of gasoline to
run the engine, for example, and chemical reactions in the battery to
Joseph S.
Merola
generate electricity. Another reaction--one that most drivers would
just as soon not experience firsthand--involves the air bag. Air bags
are not inflated from some compressed gas source but rather from the
products of a chemical reaction. The chemical at the heart of the air
bag reaction is called sodium azide, or NaN3 .
Under normal circumstances, this molecule is quite stable. If
heated, though, it will fall apart. The chemical equation 2 NaN3
--> 2 Na + 3 N 2 describes exactly how it falls apart. Notice that
the second product of the above reaction is N 2 , also known as
nitrogen gas. A handful (130 grams) of sodium azide will
produce 67 liters of nitrogen gas--which is enough to inflate a
normal air bag.
That's not the only chemistry involved. Notice
that the other chemical into which sodium azide
falls apart is Na, or sodium. Sodium is a very
reactive metal that will react rapidly with water
to form sodium hydroxide; as a result, it would
0.03
SECOND is all be quite harmful if it got into your eyes, nose or
it takes to
mouth. So to minimize the danger of exposure,
inflate an air
Image: NEW CAR
air bag manufacturers mix the sodium azide with ASSESSMENT
bag.
other chemicals that will react with the sodium and, in turn,
PROGRAM, CRASH
TEST AREA
make less toxic compounds.
http://www.scientificamerican.com/article.cfm?id=how-do-air-bags-work[8/29/2012 1:25:18 PM]
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How do air bags work? : Scientific American
CRASHES trip sensors
in cars that send an
electric signal to an
What prompts an air bag to inflate by way of this reaction?
ignitor. The heat
There are sensors in the front of the automobile that detect a generated causes
collision. These sensors send an electric signal to the canister sodium azide to
decompose into sodium
that contains the sodium azide and the electric signal detonates
metal and nitrogen gas,
a small amount of an igniter compound. The heat from this
which inflates the car's
ignition starts the decomposition of the sodium azide and the air bags.
generation of nitrogen gas to fill the air bag. What is particularly amazing is that
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from the time the sensor detects the collision to the time the air bag is fully inflated
is only 30 milliseconds, or 0.03 second. Some 50 milliseconds after an accident, the
car's occupant hits the air bag and its deflation absorbs the forward-moving energy
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