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Chemistry That Makes Air Bags Work.

  
 

Inside the airbag is a gas generator containing a mixture of NaN3, KNO3, and SiO2. When the car undergoes a head-on collision, a series of three chemical reactions inside the gas generator produce gas (N2) to fill the airbag and convert NaN3, which is highly toxic (The maximum concentration of NaN3 allowed in the workplace is 0.2 mg/m3 air.), to harmless glass (Table 1). Sodium azide (NaN3) can decompose at 300oC to produce sodium metal (Na) and nitrogen gas (N2). The signal from the deceleration sensor ignites the gas-generator mixture by an electrical impulse, creating the high-temperature condition necessary for NaN3 to decompose.

 
 


The nitrogen gas that is generated then fills the airbag. The purpose of the KNO3 and SiO2 is to remove the sodium metal (which is highly reactive and potentially explosive, as you recall from the Periodic Properties Experiment) by converting it to a harmless material. First, the sodium reacts with potassium nitrate (KNO3) to produce potassium oxide (K2O), sodium oxide (Na2O), and additional N2 gas. The N2 generated in this second reaction also fills the airbag, and the metal oxides react with silicon dioxide (SiO2) in a final reaction to produce silicate glass, which is harmlessand stable. (First-period metal oxides, such as Na2O and K2O, are highly reactive, so it would be unsafe to allow them to be the end product of the airbag detonation.)

  
     
 

Gas-Generator Reaction

Reactants

Products
Initial Reaction Triggered by Sensor. NaN3 Na
N2 (g)
Second Reaction. Na
KNO3		 K2O
Na2O
N2 (g)
Final Reaction. K2O
Na2O
SiO2		 alkaline silicate
(glass)

Table 1

This table summarizes the species involved in the chemical reactions in the gas generator of an airbag.

Note: Stoichiometric quantities are not shown.