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Concept
Sodium azide
Summary
Sodium azide is an inorganic compound with the formula . This colorless salt is the gas-forming component in some car airbag systems. It is used for the preparation of other azide compounds. It is an ionic substance, is highly soluble in water, and is very acutely poisonous.
Sodium azide is an ionic solid. Two crystalline forms are known, rhombohedral and hexagonal. Both adopt layered structures. The azide anion is very similar in each form, being centrosymmetric with N–N distances of 1.18 Å. The ion has an octahedral geometry. Each azide is linked to six centers, with three Na–N bonds to each terminal nitrogen center.
The common synthesis method is the "Wislicenus process", which proceeds in two steps in liquid ammonia. In the first step, ammonia is converted to sodium amide by metallic sodium:
It is a redox reaction in which metallic sodium gives an electron to a proton of ammonia which is reduced in hydrogen gas. Sodium easily dissolves in liquid ammonia to produce hydrated electrons responsible for the blue color of the resulting liquid. The and ions are produced by this reaction.
The sodium amide is subsequently combined with nitrous oxide:
These reactions are the basis of the industrial route, which produced about 250 tons per year in 2004, with production increasing due to the increased use of airbags.
Curtius and Thiele developed another production process, where a nitrite ester is converted to sodium azide using hydrazine. This method is suited for laboratory preparation of sodium azide:
Alternatively the salt can be obtained by the reaction of sodium nitrate with sodium amide.
Treatment of sodium azide with strong acids gives gaseous hydrazoic acid (hydrogen azide; HN3), which is also extremely toxic:
Aqueous solutions contain minute amounts of hydrazoic acid , the formation of which is described by the following equilibrium:
K = 10−4.6
Sodium azide can be destroyed by treatment with in situ prepared nitrous acid (HNO2; not HNO3). In situ preparation is necessary as HNO2 is unstable and decomposes rapidly in aqueous solutions.
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