Airbags contain a mixture of sodium azide, potassium nitrate, and silicon dioxid

Question

Airbags contain a mixture of sodium azide, potassium nitrate, and silicon dioxide. A sensor detects a head on collision that cause the sodium azide to be ignited and to decompose forming sodium and nitrogen gas. This gas fills a nylon or polyamide bag such that the front face of the bag travels at a velocity of 150 to 250 miles per hour. This process, from the initial impact of the crash to full inflation of the airbag, takes only about 40 milliseconds. Sodium is very dangerous so the sodium reacts with potassium nitrate present to produce potassium oxide (K2O), sodium oxide (Na2O), and additional N2 gas. The N2 generated in this second reaction also fills the airbag. The metal oxides react with silicon dioxide (SiO2) in a final reaction to produce silicate(glass), which is harmless and stable.

a. Write a balanced chemical equation for the first reaction in the airbag gas generator (the decomposition of NaN3).

b. Write a balanced equation for the net gas-generating reactions (the combination of the first and second reactions).

c. If a car has an airbag that has a volume of 60.0L and the bag requires a pressure of   2.37 atm at 25.00C, calculate the grams of sodium azide needed to inflate the bag.

Explanation / Answer

a ) Decomposition of NaN3 :

              2NaN3(s) -------------> 2Na(s) + 3N2(g)

             This compound is used in airbags

b ) Net gas-generating reaction :

            Sodium azide and potasium nitrate are present inairbags . NaN3 decomposed and it produce Na & N2.The sodium reacts with KNO3 and produce K2O , Na2O & N2 .

              The third substance which is present in airbag reacts with K2O & Na2O to produce innert material . Since wet K2O & Na2O would produce caustic . The third substance is usually SiO2 . The SiO2 (glass) combines with other products to produce a substance that keeps them from reacting further.

     2NaN3 ----------> 2Na + 3N2

10Na + 2KNO3 ------> K2O + 5Na2O + N2

K2O + Na2O + SiO2 -----------> Glass mixture

c )    Volume, V = 60.0L , Pressure,P = 2.37atm , T = 25oC or 298K

     Needed NaNO3 = ? grams

     PV = nRT

       n = PV/RT = (2.37)(60) / (0.0821)(298)

       n   = 5.81mol

    We know that , mole(n) = wt / Mwt

                              Wt     = mole * Mwt = (5.81) * (65.01)

                               Wt     = 377.7 gr of NaNO3

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