Ammonia (NH3): Preparation, Structure, Properties and Uses

Ammonia, a colourless gas with a distinct odour, is a chemical building block and a significant component in producing many everyday items. It is found naturally in the air, soil, water, and in plants and animals, including people. 

Nitrogen is the first member of group \(15\) of the periodic table. Daniel Rutherford discovered nitrogen in \(1772\) Several compounds are made of nitrogen like nitric acid, nitrogen dioxide, ammonia, ammonium hydroxide, etc. The article will learn everything about ammonia, its structure, physical and chemical properties, preparation, manufacture by Haber’s process and uses.

What is Ammonia?

Ammonia is an important compound of nitrogen and hydrogen. It contains one nitrogen atom and three hydrogen atoms. Its chemical formula is \({\text{N}}{{\text{H}}_3}.\) Ammonia is produced by the natural decomposition of animal and plant bodies because, with the death and decay of plants and animals, the nitrogen compounds present in them decomposed to give ammonia. Ammonia also occurs in the soil in the form of ammonium salt.

Example: Formation of ammonia by the decomposition of nitrogenous organic matter urea \(\left({{\text{N}}{{\text{H}}_2}{\text{CON}}{{\text{H}}_2}}\right).\)

\({\text{N}}{{\text{H}}_2}{\text{CON}}{{\text{H}}_2} + 2{{\text{H}}_2}{\text{O}} \to {\left( {{\text{N}}{{\text{H}}_4}} \right)_2}{\text{C}}{{\text{O}}_3} \rightleftharpoons 2{\text{N}}{{\text{H}}_3} + {\text{C}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}\)

Structure of Ammonia

The ammonia molecule is formed due to the overlap of three \({\text{s}}{{\text{p}}^3}\) hybrid orbitals of nitrogen and \({\text{s}}\) orbitals of three hydrogens. The \({{\text{4}}^{{\text{th}}}}\,{\text{s}}{{\text{p}}^3}\) hybrid orbital of nitrogen is occupied by a lone pair of electrons. This gives a trigonal pyramidal geometry to the ammonia molecule.

The \({\text{H-N-H}}\) angle is \({107.8^ \circ }\) with \({\text{N-H}}\) bond length \(101.7\,{\text{pm}}.\) The \({\text{H-N-H}}\) bond angle is slightly less than the tetrahedral angle of \({109^ \circ }28’\) due to the lone pair-bond pair repulsion, which tends to push the \({\text{N-H}}\) bond slightly inwards. In liquid and solid states, ammonia is associated due to hydrogen bonding.

Physical Properties of Ammonia

1. Ammonia is a colourless gas with a characteristic pungent smell called ammoniacal smell.
2. It is lighter than air and, therefore, collected by downward displacement of air.
3. It brings tears into the eyes when suddenly inhaled.
4. It can be easily liquefied at room temperature by passing a pressure of about \(8\) to \(10\) atmospheres.
5. Liquid ammonia boils at \( – 33.5\,^\circ {\rm{C}}\left( {239.6\,{\mkern 1mu} {\rm{K}}} \right)\) under one atmosphere pressure. It has a high enthalpy of vaporization \(\left({1370\,{\text{J}}/{\text{g}}} \right)\) and is, therefore, used in refrigeration plants of ice making machines.
6. The liquid ammoniazes at \( – 77.8\;^\circ {\rm{C}}\left( {195.3{\mkern 1mu} \,{\rm{K}}} \right)\) to give a white crystalline solid.
7. It is extremely water soluble. About \(1300\) ammonia gas volumes are dissolved by one volume of water. As Ammonia Gas can not be collected on water due to its high water solubility.

Preparation of Ammonia

1. By heating ammonium salts with a strong base: On a small scale, ammonia is prepared by heating ammonium salts with a strong base.
\({\left({{\text{N}}{{\text{H}}_4}} \right)_2}{\text{S}}{{\text{O}}_4} + 2{\text{NaOH}} + {\text{Heat}} \to 2{\text{N}}{{\text{H}}_3} + 2{{\text{H}}_2}{\text{O}} + {\text{N}}{{\text{a}}_2}{\text{SO}}4\)
\({\text{N}}{{\text{H}}_4}{\text{Cl}} + {\text{KOH}} + {\text{Heat}} \to {\text{N}}{{\text{H}}_3} + {{\text{H}}_2}{\text{O}} + {\text{KCl}}\)
In the laboratory, ammonia is prepared by heating a mixture of slaked lime and ammonium chloride.
\(2{\text{N}}{{\text{H}}_4}{\text{Cl}} + {\text{Ca}}{\left({{\text{OH}}} \right)_2} + {\text{Heat}} \to 2{\text{N}}{{\text{H}}_3} + 2{{\text{H}}_2}{\text{O}} + {\text{CaC}}{{\text{l}}_2}\)
2. By the hydrolysis of metal nitrides: Ammonia gas can also be produced by hydrolysis of metal nitrides like magnesium and aluminium nitride, with water or alkalies.
\({\text{M}}{{\text{g}}_3}~{{\text{N}}_2} + 6{{\text{H}}_2}{\text{O}} \to 2{\text{N}}{{\text{H}}_3} + 3{\text{Mg}}{\left({{\text{OH}}} \right)_2}\)
\({\text{AlN}} + 3{{\text{H}}_2}{\text{O}} \to {\text{N}}{{\text{H}}_3} + {\text{Al}}{\left({{\text{OH}}} \right)_3}\)

Ammonia gas is dried by passing over quicklime \(\left({{\text{CaO}}} \right).\) Ammonia being a basic gas, cannot be dried by passing through concentrated sulphuric acid or phosphorus pentoxide, because it reacts with them to form ammonium sulphate or ammonium phosphate, respectively. Calcium chloride also cannot be used for drying ammonia gas as it forms ammoniates with calcium chloride.

Manufacture of Ammonia by Haber Process

On a commercial scale, ammonia is manufactured by Haber’s process.

\({{\text{N}}_2}\left({\text{g}} \right) + 3{{\text{H}}_2}\left({\text{g}} \right) \rightleftharpoons 2{\text{N}}{{\text{H}}_3}\left({\text{g}}\right);\,\Delta {{\text{H}}^ \circ } = – \,92.4\, {\text{kJ}}/{\text{mol}}\)

This reaction is reversible, exothermic and occurs with a decrease in volume. Therefore, according to Le Chatelier’s principle, the favourable conditions for the manufacture of ammonia are:

1. Low temperature: Since the forward reaction is exothermic, low temperature will favour the formation of ammonia. However, at low temperatures, the rate of reaction will be slow. The optimum temperature for the reaction has been found to be around \(700\,{\text{K}}.\)
2. High pressure: Since the forward reaction occurs with the decrease in volume, high pressure will favour the formation of ammonia. The reaction is usually carried out at a pressure of \(200 \times {10^5}\,{\text{Pa}}\) or \(200\) atmospheres.
3. Catalyst: The rate of reaction is fairly low, around \(700\,{\text{K}}.\) It is increased by using iron oxide as a catalyst with a small amount of \(\,{{\text{K}}_2}{\text{O}}\) and \({\text{A}}{{\text{l}}_2}{{\text{O}}_3}.\) The use of molybdenum as a promoter increases the catalyst’s efficiency.

In Haber’s process, a mixture of \({{\text{N}}_2}\) and \({{\text{H}}_2}\) in the molar ratio of \(1:3\) is compressed to about \(200\) atmosphere pressure. The compressed gases are then cooled and passed through the soda lime tower to them from moisture and carbon dioxide. Then these are fed into the catalyst chamber packed with iron oxide with a small amount of \({{\text{k}}_2}{\text{O}}\) and \({\text{A}}{{\text{l}}_2}{{\text{O}}_3}.\) The chamber is heated electrically to a temperature of \(700\,{\text{K}},\) when the two gases combine to form ammonia. The reaction is exothermic, the heat involved maintains the desired temperature, and further electrical heating is not required.

There is approximately \(15 – 20\) per cent ammonia, in the gases leaking from the chamber, and the remaining are nitrogen and hydrogen without reaction. They pass through the condensing pipe, which liquefies and collects ammonia from the receptor. The gases that are not reacted are pumped back to the compression pump where the fresh gas mixture is mixed.

Chemical Properties of Ammonia

1. Basic nature: Ammonia is highly soluble in water. Its aqueous solution is weakly basic due to the formation of \({\text{O}}{{\text{H}}^ – }\) ions.
   \({\text{N}}{{\text{H}}_3}\left({\text{g}} \right) + {{\text{H}}_2}{\text{O}}\left({\text{I}}\right) \leftrightharpoons {\text{N}}{{\text{H}}_4}{\text{OH}}\left({{\text{aq}}} \right) \leftrightharpoons {\text{N}}{{\text{H}}_4}^ + \left({{\text{aq}}} \right) + {\text{O}}{{\text{H}}^ – }\left({{\text{aq}}} \right)\)
   Being basic, it turns moist red litmus blue and neutralises acids in the dry state as well as in aqueous solution forming their corresponding salts.
   \({\text{N}}{{\text{H}}_3} + {\text{HCl}} \to {\text{N}}{{\text{H}}_4}{\text{Cl}}\)
   \(2{\text{N}}{{\text{H}}_4}{\text{OH}} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\left({{\text{N}}{{\text{H}}_4}} \right)_2}{\text{S}}{{\text{O}}_4} + 2{{\text{H}}_2}{\text{O}}\)
2. As a Lewis Base: Due to the presence of a lone pair of electrons on the nitrogen atom, ammonia acts as a Lewis base. Consequently, it can easily donate its electron pair to form a coordinate bond with electron deficient molecules such as \({\rm{B}}{{\rm{F}}_{\rm{3}}}\) or transition metal cations having vacant \({\text{d}}\)-orbitals to form complexes. For example,
   \({\text{N}}{{\text{H}}_3} +{\text{B}}{{\text{F}}_3} \to {\text{B}}{{\text{F}}_3} \leftarrow {{\text{H}}_3}{\text{N}}\)
   \({\text{A}}{{\text{g}}^ + }\left({{\text{aq}}} \right) + 2{\text{N}}{{\text{H}}_3}\left( {{\text{aq}}} \right) \to{\left[{{\text{Ag}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_2}} \right]^ + }\left({{\text{aq}}}\right)\)
   \({\text{C}}{{\text{u}}^{2 + }}\left({{\text{aq}}}\right) + 4{\text{N}}{{\text{H}}_3}\left({{\text{aq}}} \right) \to {\left[{{\text{Cu}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_4}} \right]^{2 + }}\left({{\text{aq}}} \right)\)
   Thus, ammonia acts as a ligand.
3. Combustion: Ammonia is neither fuel nor a fuel supporter. However, it brennt to form dinitrogen and water in the presence of oxygen
   \(4{\text{N}}{{\text{H}}_3} + 3{{\text{O}}_2} \to 2~{{\text{N}}_2} + 6{{\text{H}}_2}{\text{O}}\)
4. Oxidation: When ammonia is passed through a solution of calcium hypochlorite (bleaching powder), bromine water or passed over heated copper oxide, it is oxidised to dinitrogen gas.
   \(4{\text{N}}{{\text{H}}_3} + 3{\text{Ca}}{\left({{\text{OCl}}} \right)_2} \to 2~{{\text{N}}_2} + 3{\text{CaC}}{{\text{l}}_2} + 6{{\text{H}}_2}{\text{O}}\)
   \(8{\text{N}}{{\text{H}}_3} + 3{\text{B}}{{\text{r}}_2} \to {{\text{N}}_2} + 6{\text{N}}{{\text{H}}_4}{\text{Br}}\)
   \(2{\text{N}}{{\text{H}}_3} + 3{\text{CuO}} + {\text{Heat}} \to 3{\text{Cu}} + {{\text{N}}_2} + 3{{\text{H}}_2}{\text{O}}\)
   When ammonia, mixed with an excess of air, is passed over \({\text{Pt}}/{\text{Rh}}\) gauze at \(500\,{\text{K}}\) under a pressure of \(9\) bar, it is oxidized to nitric oxide.
   This reaction forms the basis for the manufacture of nitric acid by Ostwald’s process.

Uses of Ammonia

The various applications of Ammonia are discussed below:

1. In Ostwald’s process of production of nitric acid.
2. Solvay process in the production of sodium carbonate.
3. In the manufacture of rayon and urea.
4. In the manufacture of fertilizers such as ammonium sulphate, ammonium nitrate, urea, diammonium phosphate, etc.
5. As a refrigerant in ice plants.
6. As a cleaning agent for furniture and glass surfaces.
7. As an important reagent and is used as a solvent in the laboratory.

Summary

Ammonia is one of the important compounds of nitrogen. In the article, Ammonia: Preparation, Structure, Properties and Uses, you have gained knowledge on what is ammonia? Its physical properties, structure, and different preparation methods with the emphasis on Haber’s process. Apart from this, chemical properties such as basic nature, Lewis base, combustion and oxidation and uses of ammonia is clear from this article.

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FAQs

Q.1. What are the properties of ammonia?
Ans: Ammonia is a colourless gas with a characteristic pungent ammoniacal smell. It is lighter than air and can be easily liquefied at room temperature by passing a pressure of about \(8\) to \(10\) atmospheres. Its boiling point is \( – {33.5^ \circ }{\text{C}}\left({239.6\,{\text{K}}} \right)\) under one atmosphere pressure. It acts as Lewis base.

Q.2. What is the method of preparation of ammonia?
Ans: On a commercial scale, ammonia is manufactured by Haber’s process.
\({{\text{N}}_2}\left({\text{g}} \right) + 3{{\text{H}}_2} \rightleftharpoons {\text{N}}{{\text{H}}_3}\left({\text{g}} \right);\Delta {{\text{H}}^ \circ } = – \,92.4\,{\text{kJ}}/{\text{mol}}\)
This reaction is carried out at a temperature of \(700\,{\text{K}}\) and \(200\, \times {10^5}\,{\text{Pa}}\) or \(200\,\) atmosphere pressure in the presence of iron oxide as a catalyst with a small amount of \({{\text{K}}_2}{\text{O}}\) and \({\text{A}}{{\text{l}}_2}{{\text{O}}_3}.\) Molybdenum is used as a promoter which increases the efficiency of the catalyst.

Q.3. What are the uses of ammonia synthesis?
Ans: Synthesised ammonia is mainly used for the manufacture of fertilizers such as ammonium sulphate, ammonium nitrate, urea, diammonium phosphate, etc. Apart from this, it is also used in the manufacture of nitric acid by Ostwald’s process, manufacture of sodium carbonate by Solvay’s process and manufacture of rayon and urea.

Q.4. What are the uses of ammonia?
Ans: Ammonia is used,
1. In the manufacture of nitric acid by Ostwald’s process, sodium carbonate by Solvay’s process and in the manufacture of rayon and urea.
2. In the manufacture of fertilizers such as ammonium sulphate, ammonium nitrate, urea, diammonium phosphate, etc.
3. As a refrigerant in ice plants.
4. As a cleaning agent for furniture and glass surfaces.
5. As an important reagent and is used as a solvent in the laboratory.

Q.5. What type of reaction is the synthesis of ammonia by Haber’s process?
Ans: The synthesis of ammonia by Haber’s process is a reversible, exothermic reaction and occurs with a decrease in volume.