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The mixture of chemicals which power the airbag is called the propellant.  It contains sodium azide, potassium nitrate and silicon dioxide (otherwise known as sand).

The sequence of reactions begins with the small amount of sodium azide in the initiator being ignited by an electrical impulse.  This develops a temperature of about 300 oc which causes the rest of the sodium azide to decompose.  The initial product of decomposition is molten sodium metal and nitrogen gas, the sodium metal then further reacts with the potassium nitrate so releasing more nitrogen and producing sodium and potassium oxides which then react with the silicon dioxide to form harmless glasses.

More information about sodium azide here.