Hydrazoic acid

From Sciencemadness Wiki
Jump to: navigation, search
Hydrazoic acid
IUPAC name
Hydrogen azide
Other names
Hydrogen azide
Jmol-3D images Image
Molar mass 43.03 g/mol
Appearance Colorless liquid
Density 1.09 g/cm3
Melting point −80 °C (−112 °F; 193 K)
Boiling point 37 °C (99 °F; 310 K)
Solubility soluble in alcohol, alkali, ether
Acidity (pKa) 4.6 [1]
300.25 kJ/mol
Safety data sheet Guidechem
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Hydrogen azide structure.png

Hydrazoic acid, also known as hydrogen azide or azoimide, is a colorless, volatile, and explosive liquid at room temperature and pressure, with the chemical formula HN3.



Hydrazoic acid is a mid-strength acid, it will dissolve many metals, such as iron, copper, aluminium, magnesium, zinc. Antimony dissolves slowly in hydrogen azide, as does silver at high concentrations. When mixed with hydrochloric acid, it can attack gold and platinum. Platinum black will decompose hydrogen azide to ammonia and nitrogen.[2]

Dissolution in the strongest acids produces explosive salts containing the H2N=N=N+ ion, for example:

HN=N=N + HSbCl6 → [H2N=N=N+] + [SbCl6]

Hydrazoic acid decomposes to nitrogen and hydrogen:

2HN3 → H2 + 3N2


Hydrazoic acid is a colorless liquid, with a strong and very unpleasant smell. It is very soluble in water, as well as alcohol and ether. Its density is 1.09 g/cm3. Anhydrous hydrozoic acid melts at −80 °C and boils at 37 °C. It is a mid-strength acid (pKa = 4.75), of approximately the same strength as formic acid.


Due to its long list of hazards, hydrazoic acid is not sold, and has to be prepared in the laboratory.


Hydrazoic acid can be prepared by reacting a strong acid, such as sulfuric acid, with an azide, such as sodium azide or barium azide. The latter is preferred as the insoluble barium sulfate produced can simply be filtered out, giving a clean solution of hydrogen azide. It's recommended to produce diluted acid, as pure hydrazoic acid is prone to detonation.

Hydrazoic acid was originally prepared by the reaction of hydrazine with nitrous acid:

N2H4+ HNO2 → HN3 + 2 H2O

As nitrous acid is already diluted, this reaction is safer. It can similarly be accomplished using nitric acid or concentrated hydrogen peroxide.


  • Synthesis of 2-Furonitrile
  • Silver, lead and mercury azide preparation
  • Preparation of azides that hydrolyze in aqueous solutions, such as magnesium or antimony azide (should only be performed in aprotic solvents that do not react with hydrazoic acid, or with extremely small amounts of pure cold acid and metal or none at all)



Hydrazoic acid is volatile and highly toxic, similar in toxicity with cyanides, however it has no known antidote. It has a pungent smell and its vapors can cause strong headaches. In pure form it is prone to detonation, so it's best to work with diluted solutions. Azide salts carry the same dangers, and are prone to hydrolysis in aqueous solution.

In case of hydrazoic acid poisoning, the patient should immediately leave the contaminated room (or be carried from there) and given access to outside air. In case of severe poisoning, the patient should immediately receive medical attention. If no medical attention is available, the patient should be given pure oxygen to breathe.


Hydrazoic acid should only be stored as diluted solutions, is closed bottles, in well ventilated areas or in a special storage cabinet. Should not be stored for long periods of time.


Hydrazoic acid can be neutralized with an excess of sodium hydroxide. However, azide salts such as the sodium azide produced in this way are extremely toxic, and they should NEVER be disposed of as is, especially not down the drain. Sodium azide produced this way should either be destroyed with nitrous acid, crystallized out by evaporation and carefully heated to decomposition outside, or precipitated as a transition metal azide. Many transition metal azides are explosive or will hydrolyze, and these too must be very carefully disposed of.


  1. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. Robert Matyáš, Jiří Pachman, Primary Explosives, 2013, Chapter 4 Azides

Relevant Sciencemadness threads