Ammonium Nitrate (NH4NO3)

What is Ammonium Nitrate?

Ammonium nitrate is an ionic salt made up of the ammonium cation (NH4)+ and the nitrate anion (NO3). The chemical formula of this compound is NH4NO3. Ammonium nitrate readily dissolves in water by dissociating into its constituent ions. This salt is acidic in nature since it is derived from a weak base (NH3) and a strong acid (HNO3).

NH4NO3 can be prepared from the acid-base reaction between nitric acid and ammonia, described by the following chemical equation:

NH3 + HNO3 → NH4NO3

This reaction is highly exothermic and proceeds in a violent fashion. Ammonium nitrate is also known for its oxidizing powers. It is widely used in many explosives used in the mining and construction sectors. NH4NO3 is also one of the key components of ANFO, one of the most popular choices of industrial explosives.

Structure of Ammonium Nitrate

This compound features an ionic bond between an ammonium ion and a nitrate ion. The structure of an NH4NO3 molecule is illustrated below.

Ammonium Nitrate Structure

The pi electrons in the nitrate ion are delocalized due to resonance. The net charge on this ion is -1 (since the nitrogen atom holds a charge of +1 and each oxygen atom holds a charge of -⅔). Therefore, only one NH4+ ion can form an ionic bond with one NO3 ion.

Ammonium Nitrate Properties

Chemical Data

Chemical Formula NH4NO3
Molar Mass/ Molecular Weight 80.043 grams per mole
Density 1.725 grams per cubic centimetre
Melting Point 442.8K (169.6oC)
Boiling Point Decomposes at 483K (210oC)

Physical Properties

  • Ammonium nitrate is a crystalline solid having a white/grey colour.
  • It has a trigonal crystal structure.
  • It is quite soluble in water; its solubility at 20oC is 150g/100ml. The solubility increases to 1024g/100ml when the temperature is raised to 100o
  • The dissolution of NH4NO3 in H2O is highly endothermic.
  • This compound has very low shock and friction sensitivities.

Chemical Properties

  • When NH4NO3 is reacted with the hydroxides of alkali metals, the nitrates of alkali metals are formed along with ammonia.
  • Upon heating, this compound decomposes to form nitrous oxide (N2O) and water.
  • When exploded, this compound yields N2, O2, and water as the byproducts.
  • Despite being a component of many explosives, ammonium nitrate is not an explosive by itself. It must be mixed with a primary explosive such as an azide to form an explosive mixture.

Uses of Ammonium nitrate 

Some of the important applications of this ionic salt are listed below.

  • NH4NO3 is a key component of many fertilizers since it is quite rich in nitrogen (34%).
  • Another advantage of this compound is that it doesn’t lose its nitrogen to the atmosphere, as is the case with urea.
  • When mixed with fuel oil, it forms an explosive known as ANFO, which is one of the most popular industrial explosives.
  • The dissolution of this compound in water is quite endothermic, making it an ideal choice of material in instant cold packs.
  • It is also used in the mining and construction industry as a component of explosives.

Despite its numerous applications, the use of this compound is being slowly phased out by the governments of many countries due to its scope for misuse.