Ammonium Nitrate

What is Ammonium Nitrate?

Ammonium nitrate is a chemical compound which has a number of uses and is made up of atoms of nitrogen, hydrogen, oxygen. Ammonium nitrate has distinct chemical and physical properties which are used for a number of purposes. 

Ammonium nitrate is a chemical compound which is made up when an ammonium ion unites with a nitrate ion. The molecule of ammonium nitrate is formed by the union of 2 nitrogen atoms, 4 hydrogen atoms and 3 oxygen atoms. Ammonium has a chemical formula NH4+ and nitrate has the chemical formula NO3-. When these two unite they form Ammonium nitrate with a formula NH4NO3.  It is a white solid crystal which is readily soluble in water. It is largely used in the agricultural industry as a fertilizer. It is also used as an explosive. However, due to concern over its misuse, many countries are debating about stopping using ammonium nitrate.

What are the Physical Properties of Ammonium Nitrate?

Ammonium nitrate is a solid crystalline substance. It is crystalline in nature. Ammonium nitrate is either white or grey coloured in its pure form. The density of the substance is noted as 1.725 gm per cm³. 

What is the Molar Mass of Ammonium Nitrate?

The molar mass of Ammonium nitrate is 80.043 gm per mole.

How to Calculate the Molecular Weight of the Ammonium Nitrate?

Let us look at how to calculate the molecular weight of the given compound:

The molar mass of ammonium nitrate= sum of the molar mass of all its constituents in the correct proportion.

Molecular weight of nitrogen = 14.0067

Molecular weight of hydrogen= 1.00784

The molecular weight of oxygen= 15.9999

The molecular weight of ammonium nitrate= 2 * weight of nitrogen atom + 4 * weight of hydrogen atom + 3 * weight of an oxygen atom

= 28.0134 + 4.03136 + 47.9997

= 80.043.

Where Does Ammonium Nitrate Occur Naturally? 

In nature, ammonium nitrate is found as a natural mineral. It is found as gwihabaite which is called the Ammonium analogue of saltpetre. It is commonly found in the Atacama Desert in Chile. Only the driest regions of the desert are suitable for finding ammonium nitrate. In these regions, it is found as a crust on the ground. It is often found in conjunction with other compounds namely nitrates, iodates and other halide minerals.

Earlier, Ammonium nitrate used to be mined as a mineral from these regions. However, now most, if not all, of the ammonium nitrate is produced chemically.

How is Ammonium Nitrate Produced Industrially?

Most of the Ammonium nitrate that is produced is synthesized chemically. Industrial production of Ammonium nitrate is based on an acid-base reaction between ammonia and nitric acid. A molecule of Nitric acid (acidic) joins together with a molecule of ammonia (basic) to form a single molecule of ammonium nitrate. The reaction is summarized as:

HNO3 + NH3 → NH4NO3

For this process, ammonia is used in its gas form or anhydrous form. This gaseous ammonia is then added to concentrated nitric acid to form Ammonium nitrate. This reaction is highly exothermic. This means that it produces a large amount of heat energy during the process.

The resultant solution contains as much as 83% of Ammonium nitrate. The solution is then evaporated to remove excess water and obtain Ammonium nitrate content of 95-99% (depending upon the grade of the product).

This highly concentrated Ammonium nitrate is called Ammonium nitrate melt or AN melt. This AN is then converted into small beads in a spray tower. These beads are called prills. The AN melt is also converted to granules by spraying as well as tumbling inside a rotating drum. These granules or beads are then dried further and cooled. To prevent caking these prills or granules are coated. They are then prepared and sold off as commercial AN products.

This process requires ammonia which is is produced by the Haber process of combining hydrogen and nitrogen. The Ammonia produced by the Haber process is then oxidised to nitric acid through the Ostwald process.

Another method of Ammonium nitrate production that is in use at an industrial scale is the Odda process.

In this process, one molecule of calcium nitrate reacts with two molecules of Ammonia, one molecule of carbon dioxide and one molecule of water to produce calcium carbonate and Ammonium nitrate. The reaction is summarized as: 

Ca(NO3)2 + 2 NH3 + CO2 + H2O → 2 NH4NO3 + CaCO3

 The resultant products of this reaction are calcium carbonate and ammonium nitrate. These compounds can be separated and purified before being sold. Or they may be sold together as a mixture.

Another method used for Ammonium nitrate synthesis is the metathesis reactions.

In the first type of the reactions, one molecule of Ammonium sulphate reacts with one molecule of barium nitrate. The reaction produces two molecules of ammonium nitrate and one molecule of barium sulphate. 

(NH4)2SO4 + Ba(NO3)2 → 2 NH4NO3 + BaSO4

In the second reaction, one molecule of ammonium chloride reacts with one molecule of silver nitrate. The result is one molecule of Ammonium nitrate and one molecule of silver chloride.

NH4Cl + AgNO3 → NH4NO3 + AgCl

What are Some Important Reactions for Ammonium Nitrate?

Ammonium nitrate has a number of important reactions. Some of these are listed below:

  1. Reactions with Metal Hydroxides.

When metal hydroxides react with ammonium nitrate alkali metal nitrates are formed. Along with alkali metal nitrates, Ammonia and water are also released. The reaction is summarized below:

NH4NO3 + MOH → NH3 + H2O + MNO3 (M = Na, K)

  1. Ammonium Nitrate Releasing Nitrous Oxide

When Ammonium nitrate is subjected to decomposition it releases nitrous oxide and water. Both these results are in gaseous form. So on heating Ammonium nitrate decomposes but leaves no residues behind. The reaction is summarized below:

NH4NO3 → N2O + 2H2O

  1. Ammonium Nitrate Releasing Nitrogen

As mentioned earlier, heating Ammonium nitrate releases water vapour and nitrous oxide. However in case, the compound is subjected to rapid heating, the decomposition of the compound yields nitrogen, oxygen and water. This reaction also occurs when ammonium nitrate undergoes explosion. The reaction is summarized as:

2NH4NO3 → 2N2 + O2 + 4H2O.

What is the Chemical Structure of Ammonium Nitrate?

This compound is formed through an ionic bond uniting an ammonium ion and a nitrate ion.

The nitrate ions are resonance stabilized and their pi electrons are delocalized. The net charge on the ion is calculated below:

Charge on nitrogen atom = +1

Charge on oxygen = - ⅔

Thus net charge on Ammonium nitrate is -1.

This compound is acidic. This salt readily dissolves in water forming HNO3 which is a strong acid and NH3 which is a weak base.

FAQ (Frequently Asked Questions)

1) What are the Melting Point and Boiling Point of Ammonium Nitrate?

Ans: The melting point of a compound is the temperature at which the compound changes state from solid to liquid. The melting point of Ammonium nitrate is 442.8K (169.6°C).

The boiling point of a substance is the temperature at which the compound turns from liquid to gaseous state of matter. For ammonium nitrate, the boiling point is 483K (210°C).

2) What is the Density of Ammonium Nitrate?

Ans: Ammonium nitrate has a density of 1.725 gm per cm cube.

3) Is Ammonium Nitrate a Base or Acid?

Ans: Ammonium nitrate, when added to water, breaks into Nitric acid and ammonia. The nitric acid is a strong acid turns the solution acidic whereas Ammonia being a weak base doesn't contribute much to the pH. Therefore Ammonium nitrate is considered to be acidic.

4) What are the Uses of Ammonium Nitrate?

Ans: The uses of Ammonium nitrate are listed below:

a. Fertilizer: Ammonium nitrate is a widely used fertilizer. This is because it can provide nitrogen required for the growth of the plants. It is more stable than urea and doesn't give out nitrogen into the atmosphere.

b. Explosives: Ammonium nitrate alone is not an explosive. However, when combined with primary explosives like azides it readily forms explosive mixtures. The mixture has varying properties when it is combined with primary explosives. It can also be combined with fuels such as aluminium powder or fuel oil to bring out the explosive characteristics of Ammonium nitrate.

c. Used as ANFO: ANFO is made as a mixture of 94% ammonium nitrate ("AN") with 6% fuel oil ("FO"). It is in use widely as a bulk industrial explosive. Apart from this, ANFO is in high demands for applications or used in coal mining, quarrying, metal mining, as well as in civil construction. It is one of the most powerful explosives in use today.

d. Niche uses of ammonium nitrate:

Dissolving ammonium nitrate in water results in an endothermic reaction. Which means it absorbs thermal energy from the atmosphere or surrounding while cooling the surrounding media at the same time. This is why Ammonium nitrate is used in some instant cold packs. 

Another important application of ammonium nitrate is through the solution of ammonium nitrate with nitric acid. This solution is called Cavea-b. It showed great promise for use as an alternative to monopropellant hydrazine which is commonly used in spacecraft.