Introduction:
HNO3, is a chemical of great commercial importance. There are three important methods for its manufacture.
METHODS OF PREPARATION
Laboratory method:
From potassium nitrate (KNO3):
HNO3, can be prepared on large scale by beating Chile salt petre with conc. H2SO4
COMMERCIAL METHOD:
General principle:
The nitrogen and from the air can be made to combine by electric discharge. NO produced which is further oxidized to NO2, and is then dissolved in water.
Process:
N2 and O2 from atmosphere are allowed to combine with each other is the presence of electric are at 3000°C.
This reaction is endothermic and is favoured at high temperature.The gases which come out from this electric fieid contain 1. 25 % of NO. These gases are rapidly cooled down to 1000 °C.
At 600°C, NO combines with oxygen to form NO2.
NO2 is cooled down to 50°C by water pipes. NO2 is absorbed in water in a tower in which the water trickles down Fig (1.1)
HNO2 is oxidized to HNO3 and NO. NO is re-oxidized to NO2 and thus to HNO3.
Preparation of conc. HNO3:
- There are three stages to get 100 % HNO3. Dil HNO3, obtained by different methods can be concentrated to 68 % by the process of distillation. This percentage gives a constant boiling mixture. It is called commercial HNO3.
- If concentrated H2SO4, is used and distilled with 68 % HNO3,, then we get 98% HNO3, H2SO4, absorbs the water.
- If we want to get 100% HNO3, then 98% HNO3, is strongly cooled. Pure acid is deposited as colorless crystals at – 42°C. These crystals are separated and melted to get 100 % HNO3.
Physical properties:
- Pure HNO3, is a colourless liquid
- It has a characteristic choking smell
- its B is 86°C
- Its M.P. is – 42°C,
- The specific gravity of pure HNO3, at 15°C is 1.538.
- In the crystalline state, it is transparent
- It is soluble in water in all proportions.
- In moist air, it gives fumes.
- It is hygroscopic in nature.
CHEMICAL PROPERTIES OF NITRIC ACID:
Decomposition:
HNO3, decomposes in the presence of light even at ordinary temperature
Acidic property:
HNO3 is a strong acid. It ionizes in water to give sufficient protons. it’s dissociation constant is in the range of 10+3. On the basis of acidic nature, it reacts with many oxides, hydroxides, carbonates bicarbonates, sulphites and bisulphites. In this way metal nitrates are produced.
- Reaction with calcium oxide (CaO):
- Reaction with NaOH and KOH:
- Reaction with Na2 CO3:
- Reaction with NaHCO3:
