Nitric Acid: Nitrogen forms numerous oxoacids. The common oxoacids of nitrogen are hyponitrous acid \(\left( {{{\rm{H}}_2}{{\rm{N}}_2}{{\rm{O}}_2}} \right){\rm{,}}\) nitrous acid \(\left( {{\rm{HN}}{{\rm{O}}_{\rm{2}}}} \right){\rm{,}}\) nitric acid \(\left( {{\rm{HN}}{{\rm{O}}_{\rm{3}}}} \right){\rm{,}}\) peroxynitric acid \(\left( {{\rm{HN}}{{\rm{O}}_{\rm{4}}}} \right){\rm{,}}\) etc. Out of which nitric acid is the most important. In the article, Nitric Acid, let’s learn everything about nitric acid, its structure, physical and chemical properties and uses.

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What is Nitric Acid?

Nitric acid is the most important and useful oxoacid of nitrogen. It contains one hydrogen, one nitrogen, and three oxygen atoms. Its molecular formula is \({\rm{HN}}{{\rm{O}}_{\rm{3}}}{\rm{,}}\) and its molar mass is \(63.01\,{\rm{ g/mol}}.\) It is a very strong oxidising agent and is quite useful. Nitrogen is shown with an oxidation state of \(+5\) in nitric acid.

Physical Properties of Nitric Acid

1. Pure nitric acid is a colourless fuming liquid with a pungent odour. However, impure nitric acid is yellow due to the presence of dissolved oxides of nitrogen.
2. Pure nitric acid freezes at \(231.4\,{\rm{ K}}\) and boils at \({\rm{355}}{\rm{.6\, K}}{\rm{.}}\)
3. Laboratory grade nitric acid is a constant boiling \(\left( {{\rm{394\, K}}} \right)\) mixture containing \(68\%\) \({\rm{HN}}{{\rm{O}}_{\rm{3}}}\) and \(32\%\) \({{\rm{H}}_{\rm{2}}}{\rm{O}}\) by mass. It has a specific gravity of \(1.504.\)
4. Fuming nitric acid is a pure nitric acid containing oxides of nitrogen dissolved in it.
5. It has a corrosive action on the skin and produces painful blisters.

Structure of HNO₃ Molecules

Spectroscopic studies have shown that in a gaseous state, nitric acid exists as a planar molecule with a bond angle and bond length, as shown below.

Actually, nitric acid is a resonance hybrid of the following two structures:

Laboratory Preparation of Nitric Acid

Nitric acid is usually prepared by heating potassium nitrate or sodium nitrate with concentrated sulphuric acid. Heating is done in a glass retort, and the vapours of nitric acid are condensed in a receiver which is cooled by water.
\({\rm{KN}}{{\rm{O}}_{\rm{3}}}{\rm{ + Conc}}.{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}{\rm{ + }}\;{\rm{Heat}}\; \to \;{\rm{KHS}}{{\rm{O}}_{\rm{4}}}{\rm{ + HN}}{{\rm{O}}_{\rm{3}}}\)

Commercial Preparation of Nitric Acid – Ostwald Process

On the commercial (industrial) scale, nitric acid can be obtained by catalytic oxidation of ammonia. This process of manufacturing nitric acid is called the Ostwald process.
In this process, conversion of ammonia to nitric acid is done through the following 3 steps:

Step 1. Oxidation of ammonia to nitric oxide: Ammonia (1:8 by volume) is oxidised by air in the presence of a platinum catalyst at \(800\,^\circ {\rm{C}}.\)

Step 2. Oxidation of nitric oxide to nitrogen dioxide: The nitric oxide is oxidised to nitrogen dioxide by air at a temperature about \(100 – 150\,^\circ {\rm{C}}.\) \(2{\rm{NO + }}{{\rm{O}}_{\rm{2}}} \to \;\;2{\rm{N}}{{\rm{O}}_{\rm{2}}}\)

Step 3. Formation of nitric acid: Nitrogen dioxide is then converted to nitric acid by absorbing nitrogen dioxide in water in the presence of air.

\(4{\rm{N}}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + }}{{\rm{O}}_{\rm{2}}}\; \to \;4{\rm{HN}}{{\rm{O}}_{\rm{3}}}\)

The plant used for the manufacture of nitric acid is described as follows:

1. Converter: The converter is made of aluminium and fitted with a platinum-rhodium gauze cylinder, which is closed at the bottom with a silica lid. The gauze is initially electrically heated to \(800\,^\circ {\rm{C}}.\) A mixture of ammonia and clear air is passed through the gauze from the top, and the product leaves from the bottom.
   

1. Oxidation tower: The nitric oxide containing nitrogen and some water vapour coming out of the converter is cooled at about \(100 – 150\,^\circ {\rm{C}}\) by passing through coolers. In an oxidation tower nitric oxide is mixed with more air, and it gets converted into nitrogen dioxide.
   \(2{\rm{NO + }}{{\rm{O}}_2} \to 2{\rm{N}}{{\rm{O}}_{\rm{2}}}\)
2. Absorption tower: Nitrogen dioxide is allowed to enter the absorption tower from the lower end. Water is sprinkled from the top of the tower packed with quartz pieces. Here, nitrogen dioxide is absorbed into the water in the presence of air to yield nitric acid.
   \({\rm{4N}}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + }}{{\rm{O}}_{\rm{2}}} \to 4{\rm{HN}}{{\rm{O}}_3}\)

Concentrating nitric acid: Distillation can concentrate the aqueous nitric acid generated by this process to around \(68.5\) percent by mass. Dehydration using concentrated sulphuric acid can increase the concentration to \(98\) percent acid.
Distillation of concentrated aqueous nitric acid with phosphorus pentoxide yields anhydrous nitric acid.

Chemical Properties of Nitric Acid

1. Thermal stability: On heating, nitric acid decomposes to give nitrogen dioxide, oxygen and water.
   \({\rm{4HN}}{{\rm{O}}_{\rm{3}}}{\rm{ + Heat}} \to 4{\rm{N}}{{\rm{O}}_2}{\rm{ + }}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\)
2. Acidic nature: Nitric acid is a strong monobasic acid. In aqueous solution, it ionises as:
   \({\rm{HN}}{{\rm{O}}_{\rm{3}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{3}}}{{\rm{O}}^ + }{\rm{ + NO}}_3^ – \)
   Being a monobasic acid, it reacts with metallic oxides, hydroxides, carbonates and bicarbonates, giving only one series of salts called nitrates.
   \({\rm{CaO + 2HN}}{{\rm{O}}_{\rm{3}}}\; \to {\rm{Ca}}{\left( {{\rm{N}}{{\rm{O}}_{\rm{3}}}} \right)_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O}}\)
   \({\rm{NaHC}}{{\rm{O}}_{\rm{3}}}{\rm{ + HN}}{{\rm{O}}_{\rm{3}}}\; \to {\rm{NaN}}{{\rm{O}}_{\rm{3}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O + C}}{{\rm{O}}_{\rm{2}}}\)
   \({\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}{\rm{ + 2HN}}{{\rm{O}}_3}\; \to {\rm{2NaN}}{{\rm{O}}_3}{\rm{ + }}{{\rm{H}}_2}{\rm{O + C}}{{\rm{O}}_2}\)
3. Oxidising agent: Nitric acid is a strong oxidising agent since it readily gives nascent oxygen both in the concentrated as well as in the dilute form.
   \({\rm{2HN}}{{\rm{O}}_3}\left( {{\rm{Conc}}.} \right) \to {\rm{2N}}{{\rm{O}}_2}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O + }}\left[ {\rm{O}} \right]\)
   \({\rm{2HN}}{{\rm{O}}_3}\left( {{\rm{dil}}.} \right) \to {\rm{2NO + }}{{\rm{H}}_2}{\rm{O + 3}}\left[ {\rm{O}} \right]\)
   Therefore, nitric acid oxidises many non-metals and compounds.
   Example: \({\rm{C + 4HN}}{{\rm{O}}_3}\; \to \;{\rm{4N}}{{\rm{O}}_2}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + C}}{{\rm{O}}_2}\)
   \({\rm{3}}{{\rm{H}}_2}{\rm{S}}\;{\rm{ + 2HN}}{{\rm{O}}_3}\; \to \;{\rm{3S + }}\;{\rm{4}}{{\rm{H}}_{\rm{2}}}{\rm{O + 2NO}}\)

Uses of Nitric Acid

Some important uses of nitric acid are given below:

1. In the manufacture of ammonium nitrate and basic calcium nitrate which are used as fertilizers.
2. In the manufacture of explosives and pyrotechnics such as gun cotton, nitroglycerine, trinitrotoluene (T.N.T), picric acid, etc.
3. In preparation of nitro compounds which are used as perfumes, dyes and medicines.
4. In the manufacture of artificial silk.
5. For pickling (cleaning) of stainless steel and etching of metals.
6. As an oxidiser in rocket fuels.
7. As a reagent in the laboratory.
8. In the purification of gold and silver as aqua regia.

Summary

Nitric acid is one of the important oxoacids of nitrogen. In this article, Nitric Acid, you have gained knowledge on what is nitric acid, its physical properties, structure, and different preparation methods with the emphasis on Ostwald’s process. Apart from this, chemical properties such as thermal stability, oxidising agent, acidic nature and uses of nitric acid are clear from this article.

FAQs on Nitric Acid

Let’s look at some of the commonly asked questions about Nitric Acid:

Q.1. What is the structure of nitric acid?
Ans: Spectroscopic studies have shown that in a gaseous state, nitric acid exists as a planar molecule with the bond angle and one length as shown below.

Q.2. What are the uses of nitric acid?
Ans: Some important uses of nitric acid are:
1. In the manufacture of ammonium nitrate and basic calcium nitrate which are used as fertilizers.
2. In the manufacture of explosives and pyrotechnics such as gun cotton, nitroglycerine, trinitrotoluene (T.N.T), picric acid, etc.
3. In preparation of nitro compounds which are used as perfumes, dyes and medicines.
4. In the manufacture of artificial silk.

Q.3. What are the preparation methods of nitric acid?
Ans: Nitric acid is usually prepared in the laboratory by heating potassium nitrate or sodium nitrate with concentrated sulphuric acid. Heating is done in a glass retort, and the vapours of nitric acid are condensed in a receiver which is cooled by water.

On a commercial (industrial) scale, nitric acid is prepared by the Ostwald process. It occurs through the following 3 steps:
Step 1: Ammonia (1:8 by volume) is oxidised to nitric oxide by air in the presence of a platinum catalyst at \(800\,^\circ {\rm{C}}.\)

Step 2: The nitric oxide is oxidised to nitrogen dioxide by air at a temperature about \(100\,^\circ {\rm{C}}.\)
\({\rm{2NO  +  }}{{\rm{O}}_{\rm{2}}} \to {\rm{2N}}{{\rm{O}}_{\rm{2}}}\)
Step 3: Nitrogen dioxide is then converted to nitric acid by absorbing nitrogen dioxide in water in the presence of air.
\({\rm{4N}}{{\rm{O}}_{\rm{2}}}{\rm{ + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O + }}{{\rm{O}}_{\rm{2}}} \to 4{\rm{HN}}{{\rm{O}}_{\rm{3}}}\)

Q.4. How is nitric acid prepared in the laboratory?
Ans: Nitric acid is usually prepared in the laboratory by heating potassium nitrate or sodium nitrate with concentrated sulphuric acid. Heating is done in a glass retort, and the vapours of nitric acid are condensed in a receiver which is cooled by water.
\({\rm{KN}}{{\rm{O}}_3}{\rm{ + Conc}}.{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{KHS}}{{\rm{O}}_4}{\rm{ + HN}}{{\rm{O}}_{\rm{3}}}\)

Q.5. What is the symbol of nitric acid?
Ans: The symbol of nitric acid is \({\rm{HN}}{{\rm{O}}_{\rm{3}}}\)

Q.6. How many elements are present in nitric acid?
Ans: Three elements are present in nitric acid \(\left( {{\rm{HN}}{{\rm{O}}_{\rm{3}}}} \right),\) i.e., nitrogen (N), hydrogen (H) and oxygen (O).

Q.7. What neutralises nitric acid?
Ans: The bases like sodium hydroxide \(\left( {{\rm{NaOH}}} \right){\rm{,}}\) calcium oxide \(\left( {{\rm{CaO}}} \right)\) can be used to neutralise nitric acid.

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