AIRBAG STOICHIOMETRY
NAME: _____________________________
Information taken from http://www.chemistry.wustl.edu/~courses/genchem/Tutorials/Airbags/home.htm
Studies show that working airbags along with wearing a seatbelt can
dramatically increase your chance for survival in a car accident (up to 40%). For
most current airbag systems, the gas generant is a solid mixture of sodium azide,
NaN3, plus an oxidizer. The signal from the deceleration sensor ignites the gasgenerator mixture by an electrical impulse, creating the high-temperature
condition necessary for NaN3 to decompose. The gas that inflates the bag is
almost entirely nitrogen gas, N2, which is produced in the following decomposition
reaction:
2 NaN3(s)  2 Na(s) + 3 N2(g) (reaction 1)
However, this reaction alone cannot inflate the bag fast enough, and the sodium metal produced is
a dangerously reactive substance (it explodes in water!) Oxidizers such as potassium nitrate,
KNO3, are included in the gas generant so that they can immediately react with the sodium metal
to oxidize it to an oxide. This exothermic reaction also raises the temperature more than a
hundred degrees so that the gas fills the bag faster.
10 Na(s) + 2 KNO3(s)  K2O(s) + 5 Na2O(s) + N2(g) (reaction 2)
But even the oxides produced are unsafe because they are extremely basic. SiO2 is also available
in an airbag to react with the oxides to form alkaline silicate, which is glass, and therefore not toxic.
2 Na2O(s) + SiO2  Na4SiO4(s) (reaction 3a)
2 K2O(s) + SiO2  K4SiO4(s) (reaction 3b)
The volume of gas needed to fill an airbag of a certain volume depends on the amount of gas
available and the density of the gas. To calculate the amount of gas generant (sodium azide)
needed, airbag designers must know the stoichiometry of the reactions and account for energy
changes in the reaction, which may change the temperature, and thus the density of the gas. Be
thankful, chemistry can save your life!! 
After reading the article, answer these questions:
1. Overall, what are the three reactants? What are the products?
2. A standard airbag has 130.0 g of sodium azide. How many moles of nitrogen can be
produced from this amount from reaction 1?
3. How many moles of the dangerous metal, sodium, would be produced along with the
nitrogen in question 2?
4. Using your answer to question 3 and reaction 2, how much additional nitrogen (in moles) is
produced from reaction 2?
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
5. Add your answers to 2 and 4 together to get the total moles of N2 produced in an airbag.
You should get 3.199 moles. Based on this answer and using PV=nRT (which helps us
understand the relationship of variables of a gas), what volume of N2 gas is produced in an
airbag? (hint: P = Pressure = 1atm; n = # of moles, T = temperature = 298 K, and R = a
constant = 0.0821 L·atm/mol·K)
6. Your answer for 5 should be 78.3 L – that’s about 39 pop bottles full of N2 that save your
life! Based on your answer for #3, how many grams of sodium would be produced using
reaction 1?
7. Suppose an airbag producer put 30. g of KNO3 in an airbag. Also using your answer to 6,
compare those two numbers to find the limiting reactant in reaction 2. Did the supplier put
enough KNO3?
8. Based on your answer to #7, use the limiting reactant to figure out how many moles of
Na2O and moles of K2O are produced in reaction 2?
9. Based on your answer to #8, use reaction 3 to figure out how many total grams of glass
(Na4SiO4 + K4SiO4) are produced.
10. Use the limiting reactant in #7 to figure out how much sodium is used up (remember sodium
was not the LR). Then, based on your answers to 6, figure out how much dangerous
sodium is left over. Let’s hope your airbag manufacturer knows their chemistry and put at
least 40.46g of KNO3 in there!
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright