The most important and useful oxoacid of nitrogen is nitric acid. Its molecular formula is HNO3 and molar mass 63.01 g mol-1.
Fig: 13.6 - Laboratory preparation of nitric acid
Fig: 13.8 - Converter for the Ostwald's process
Preparation of Nitric Acid
Nitric acid is usually prepared by heating potassium nitrate with concentrated sulphuric acid. This heating is done in a glass retort and vapors of nitric acid are condensed in a receiver, which is cooled by water. The reaction is:
On a commercial scale, nitric acid is manufactured through the Ostwald's process - the process of catalytic oxidation of ammonia.
The conversion of ammonia into nitric acid in this process is done through the following steps:
Oxidation of ammonia to nitric oxide
Ammonia is oxidized by air in the presence of Pt catalyst at 800°C to give nitric oxide.
Oxidation of NO to NO
The nitric oxide is oxidised by air at temperature below 100°C, to give nitrogen dioxide (NO2)
Formation of nitric acid
Nitrogen dioxide is then converted to nitric acid by absorbing NO2 in water, in the presence of air.
The manufacturing plant for the production of nitric acid by the Ostwald's process, has the following components and processes.
Fig: 13.7 - Ostwald's process for the manufacture of nitric acid
The converter is made of aluminium and fitted with platinum - rhodium gauze cylinder (23 cm x 34 cm). The cylinder is closed at the bottom with a silica lid. The gauze is initially heated to 800°C electrically. Thereafter no external heating is required as oxidation of ammonia is an exothermic reaction. A mixture of ammonia and clear air (volume ratio = (1:8) is then passed through the gauze from the top, and the products leave from the bottom. Every 1000 cm2 area of the gauze produces about 500 kg of nitric oxide, every 24 hours.
When nitric oxide, containing nitrogen and some water vapors, come out of the converter, they are cooled to about 100°C by passing them through coolers. In the oxidation tower, nitric oxide is mixed with more air and it gets converted to NO2.
NO2 is allowed to enter the absorption tower from the lower end. The tower is packed with quartz pieces and water is sprinkled from the top. Here, NO2 is absorbed into water in the presence of air to yield nitric acid.
Commercial nitric acid has a brown color due to dissolved NO2. The procedure of bubbling dry air through warm commercial nitric acid, is to drive away the dissolved nitrogen dioxide so that the acid becomes colorless.
Concentrating nitric acid
Aqueous nitric acid obtained by this method can be concentrated by distillation to 68.5% by mass. Further concentration to 98% acid can be achieved by dehydration with concentrated sulphuric acid. Anhydrous nitric acid can be obtained by distillation of concentrated aqueous nitric acid with phosphorus pentoxide (P2O5 or P4O10).
Physical properties of nitric acid
- Pure nitric acid is a colorless fuming liquid with a pungent odor. Impure acid or pure acid on standing develops yellow color due to the presence of dissolved oxides of nitrogen (mainly NO2).
- It is completely soluble in water and forms a constant boiling mixture (120.5°C) with water, containing 68% (by mass) of nitric acid.
- Pure acid has a density of 1.54 g/mL. The constant boiling mixture has a density of 1.4 g/mL at 20°C.
- Anhydrous nitric acid boils at 355.6 K (83.6°C) and freezes to a white solid at 231.4 K (- 41.7°C).
- It has a corrosive action on the skin and causes yellow blisters.
Chemical properties of nitric acid
Pure nitric acid is not very stable. Even at ordinary temperature, in presence of sunlight it undergoes slight decomposition. As the temperature increases, the rate of decomposition also increases. On strong heating it decomposes completely to give nitrogen dioxide, water and oxygen.
The nitrogen dioxide, which is reddish brown in colour, may dissolve in the undecomposed acid to give it an yellowish brown colour.The above reaction also shows that nitric oxide contains oxygen.
To show that Nitric Acid contains Oxygen
The apparatus is set up as shown in figure 6.15.
The combustion tube is heated strongly and concentrated nitric acid is allowed to flow into it. Due to the heat, it decomposes, liberating the corresponding gases. Reddish yellow fumes of nitrogen dioxide, water vapour and a colourless gas are obtained. Nitrogen dioxide and water vapour dissolve in the water while the colourless gas gets collected in the gas jar. This gas rekindles a glowing splint, which proves it to be oxygen.
Nitric acid is a strong monobasic acid. It ionizes in water readily as follows:
a) Reactions with Basic Oxides
b) Reaction with Bases (Hydroxides)
c) Reaction with Carbonates and Hydrogen Carbonates
d) Reaction with Metals
Nitric acid usually does not behave as an acid, with metals to form the corresponding salt and liberate hydrogen.However, magnesium and manganese are the only two metals, which react with cold and very dilute (1%) nitric acid to evolve hydrogen.
e) Reaction with Metallic Sulphites
Structure of nitric acid
Nitric acid is a monobasic acid and may be structurally represented as,
Gaseous nitric acid is a planar molecule. The bond length and bond angles in nitric acid molecule are shown above.
Structure of nitrate ion
The nitrate ion, NO3- is isoelectronic with carbonate ion, CO32-. It is also a planar ion. The nitrate ion can be represented by the resonance structures shown below:
Uses of nitric acid
The important uses of nitric acid are as follows:
1) Nitric acid plays a significant role in the manufacture of various products such as:
- Explosives like trinitrotoluene (T.N.T.) nitro glycerine, gun cotton, ammonal etc.
- Fertilizers such as calcium nitrate, ammonium nitrate etc.
- Nitrate salts such as calcium nitrate, silver nitrate, ammonium nitrate.
- Dyes, perfumes, drugs etc. from coal tar products.
- Sulphuric acid by Lead Chamber process.
2) It is used in the purification of silver, gold, platinum etc.
3) Nitric acid is used in etching designs on copper, brass, bronze ware etc.
4) It is used to prepare "aqua regia" to dissolve the noble elements.
5) It is used as a laboratory reagent.
Ammonal is a mixture of ammonium nitrate and aluminium powder.
The uses of nitric acid are summarized in figure 6.18.