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QUESTION ONE
Car airbags
One type of car air bag uses sodium azide, NaN3, to produce nitrogen gas. The canister also
contains potassium nitrate and silica. The reactions initiated by an electric discharge are:
2NaN3(s)
2Na(s) + 3N2(g) (i)
The sodium formed reacts with potassium nitrate
2KNO3(s) + 10Na(s)
K2O(s) + 5Na2O(s) + N2(g)
(ii)
The alkali metals oxides then react with the silica to form harmless silicates
Data
Standard enthalpies of formation (kJ mol–1)
NaN3(s)
22
KNO3(s)
–495
K2O(s)
–363
Na2O(s)
–414
1
Molar masses (g mol–1)
Na
23.0
N
14.0
Write the thermochemical equation for the enthalpy of formation of sodium azide.
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2
Calculate rH for reaction (i).
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3
Calculate rH for reaction (ii).
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4
If an airbag contains 150 g of sodium azide calculate the total energy released by
reactions (i) and (ii) when an airbag is activated.
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5
Would it be better for the reaction between the metal oxides and silica to be
endothermic or exothermic. Explain your choice.
endothermic
exothermic
(Circle the correct answer.)
Explanation _________________________________________________________________
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