Reaction involved in the inflation of the airbag

Chemical Reaction involved in the inflation of the airbag:
The airbag contains a mixture of {\text{Na}}{{\text{N}}_3}, {\text{KN}}{{\text{O}}_3}, {\text{Si}}{{\text{O}}_2}NaN
3
​
,KNO
3
​
,SiO
2
​
.

A collision triggers the following reactions:

The deceleration sensor sends an electric impulse to the gas-generator mixture which creates high temperature conditions necessary for the decomposition of NaN₃. Sodium azide decomposes at 300℃ produce nitrogen gas and sodium metal. The nitrogen so produced fills the airbag.

\text{2}\ \text{Na}{{\text{N}}_{\text{3}}}\ \xrightarrow{\text{300}{{\ }^{\text{o}}}\text{C}}\ \text{3}\ {{\text{N}}_{\text{2}}}\ \text{+}\ \text{2}\ \text{Na}2 NaN
3
​

300
o
C
​
3 N
2
​
+ 2 Na

Sodium metal is highly reactive and is an explosive material. In order to remove this potential hazardous material produced, it is made to react with potassium nitrate and silica.

Sodium reacts with potassium nitrate (KNO3) to give Potassium oxide (K2O) and sodium oxide (Na2O). The nitrogen gas generated fills the airbag.

\text{Na}\ \text{+}\ \text{KN}{{\text{O}}_{\text{3}}}\ \xrightarrow{{}}\ {{\text{K}}_{\text{2}}}\text{O}\ \text{+}\ \text{N}{{\text{a}}_{\text{2}}}\text{O}\ \text{+}\ {{\text{N}}_{\text{2}}}Na + KNO
3
​

​
K
2
​
O + Na
2
​
O + N
2
​

These metal oxides are highly reactive and must not be allowed to stay in this form for long. So, upon reaction with silicon dioxide (SiO2), it forms sodium silicate and potassium silicate which are harmless and stable.

{{\text{K}}_{\text{2}}}\text{O}\ \text{+}\ \text{N}{{\text{a}}_{\text{2}}}\text{O}\ \text{+}\ \text{Si}{{\text{O}}_{\text{2}}}\ \xrightarrow{{}}\ \text{N}{{\text{a}}_{\text{2}}}\text{Si}{{\text{O}}_{\text{3}}}\ \text{+}\ {{\text{K}}_{\text{2}}}\text{Si}{{\text{O}}_{\text{3}}}K
2
​
O + Na
2
​
O + SiO
2
​

​
Na
2
​
SiO
3
​
+ K
2
​
SiO
3
​

Summarizing the above equations, it can be concluded that upon collision, an electric impulse generates enough heat to decompose the toxic sodium azide to elemental sodium metal and nitrogen gas. The elemental sodium metal which is very reactive is converted to sodium oxide by the action of potassium nitrate. The highly reactive oxides of sodium and potassium are converted to their respective silicates, which are very stable by the action of silicon dioxide or silica. Thus, from a highly toxic substance, non-hazardous compounds are generated along with nitrogen gas required to inflate the airbags.

The Physics behind working of Airbags
image

Nitrogen being inert in nature, its behavior can be approximated to that of an ideal gas. The ideal gas equation states that :

PV\ =\ nRTPV = nRT

\rm HereHere
P\ =\ \text{Pressure}P = Pressure
V\ =\ \text{Volume}V = Volume
n\ =\ \text{Number}\ \text{of}\ \text{moles}\ \text{of}\ \text{gas}n = Number of moles of gas
R\ =\ \text{Gas}\ \text{constant}R = Gas constant
T\ =\ \text{Temperature}T = Temperature

This law provides a relationship between volume, pressure and amount of gas required to inflate the airbag. If the pressure required to inflate the airbag within milliseconds is determined, the amount of N2 gas to be generated and thus the amount of sodium azide required can be calculated.

Pressure can be defined as force per unit area. The pressure excreted by the gas on the walls of the container, upon inflation, can be calculated using the formula P = F/AP=F/A
The pressure P used in the ideal gas equation, to calculate the amount of gas needed to fill the airbag is ‘absolute pressure’. The pressure P used to calculate the force the gas exerts on unit area is called ‘Gauge pressure’. Gauge pressure is required to push the airbag forward and act against atmospheric pressure. Therefore, we can say that

Absolute pressure = Gauge pressure + atmospheric pressure

Once all the chemicals in the airbag have reacted, Nitrogen gas generation stops. The gas molecules now begin to escape through the vents. This brings down the pressure within the bag, thereby giving a soft cushioning effect. Within 2 seconds after the impact, the pressure inside the bag becomes equal to atmospheric pressure.