A recent innovation in automobile safety is the airbag. In the event of a collision, an airbag in front of the driver inflates in about 0.02 seconds. As the driver's body comes to a rapid stop, the airbag provides a soft cushion, softer than the steering wheel in any case.
The essential mechanism of the airbag consists of a sealed combustion chamber containing iron(III) oxide and sodium azide, which react to form the nitrogen gas which fills the airbag:
In the same chamber is an igniter, consisting of an electric coil, zirconium and potassium hyperchlorate, which react somewhat as follows:
The main active ingredient in the airbag is the sodium azide. This ingredient is prepared using the reaction of sodium amide and nitrous oxide:
During a collision, an electric coil starts reaction (2). This provides the heat which ignites reaction (1). After 1–5 ms, the seal of the combustion chamber bursts and releases the products (see figure). A filter screen removes the solid products of reaction (1) and other trace reaction products (some possibly noxious) while passing the nitrogen gas.
The nitrogen gas fills the bag in about 20 ms, and the whole reaction (1) continues for about 50 ms. The bag is porous, so as the driver presses against it, the bag deflates. Thus, it is almost completely deflated by the time the accident is over. After that, it is hoped that the driver can walk away from the collision with no injuries worse than abrasions from the inflating bag.
What is the best possible purpose for having the bag deflate as the driver presses into it?