In the previous section, we discussed about Hydrogen chloride. In this section we will discuss about Nitric acid.
In the next section, we will see Carbon.
Nitric acid
Nitric acid is another industrially important compound. One of it's main uses is in the production of chemical fertilizers.
Nitric acid is produced in the laboratory by heating potassium nitrate and concentrated sulphuric acid in a retort. The arrangement is shown in the fig.7.5 below:
The equation of the reaction is:
KNO3 + H2SO4 → KHSO4 + HNO3. This is a balanced equation.
• Nitric acid is a volatile acid. That means, it easily changes to gaseous state from liquid state. So cold water is used to cool the vapours. When the vapours are cooled, they condense, and turn into liquid nitric acid.
• Pure nitric acid is a colourless liquid. But the nitric acid obtained in the laboratory is pale yellow in colour. Let us see the reason:
Nitric acid dissociates to produce nitrogen dioxide (NO2) gas. Let us write the equation for this dissociation: 4HNO3 → 4NO2 + 2H2O + O2.
The nitrogen dioxide thus produced is reddish brown in colour. This nitrogen dioxide dissolves in the acid, and as a thus the acid becomes pale yellow in colour.
Stage 1: Ammonia and oxygen reacts together to form nitric oxide. Platinum is used as a catalyst in this reaction. The equation is: 4NH3 + 5O2 → 4NO + 6H2O.
Stage 2: The nitric oxide reacts with oxygen to produce nitrogen dioxide. The equation is:
2NO + O2 → 2NO2.
Stage 3: The nitrogen dioxide is dissolved in water, in the presence of oxygen to produce nitric acid.
The equation is: 4NO2 + 2H2O + O2 → 4HNO3.
Reaction of nitric acid with metals
• We have seen that hydrochloric acid reacts with metals and produce hydrogen. Details here.
• The nitric acid also reacts with metals. But the products will depend upon the concentration of the acid, and also on the nature of the metal
2. Add equal volume of freshly prepared ferrous sulphate solution to it and mix well.
3. Hold the test tube containing the mixture in a slanting position.
4. Slowly pour concentrated sulphuric acid along the sides of the test tube.
5. If a brown ring is formed at the junction of the two liquids, the salt can be confirmed as a nitrate.
• In the manufacture of fertilizers • As an oxidising agent in rocket fuels • In the purification of gold • For etching alphabets and pictures on metals • To prepare aqua regia used for dissolving noble metals
Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid. They are taken in the ratio 1:3. That is., if we take 5 ml of nitric acid, we must take 15 ml of hydrochloric acid. This mixture can dissolve noble metals like gold and platinum. Let us see how this is possible:
The equation for the reaction is:
HNO3 + 3HCl → 2H2O. + NOCl + 2[Cl]
The chlorine atoms thus liberated, will react with gold, platinum etc., forming their chlorides and thus dissolving them.
Nitric acid is produced in the laboratory by heating potassium nitrate and concentrated sulphuric acid in a retort. The arrangement is shown in the fig.7.5 below:
 |
| Fig.7.5 |
KNO3 + H2SO4 → KHSO4 + HNO3. This is a balanced equation.
• Nitric acid is a volatile acid. That means, it easily changes to gaseous state from liquid state. So cold water is used to cool the vapours. When the vapours are cooled, they condense, and turn into liquid nitric acid.
• Pure nitric acid is a colourless liquid. But the nitric acid obtained in the laboratory is pale yellow in colour. Let us see the reason:
Nitric acid dissociates to produce nitrogen dioxide (NO2) gas. Let us write the equation for this dissociation: 4HNO3 → 4NO2 + 2H2O + O2.
The nitrogen dioxide thus produced is reddish brown in colour. This nitrogen dioxide dissolves in the acid, and as a thus the acid becomes pale yellow in colour.
Industrial production of nitric acid
This is possible through Ostwald process. This process was designed by the German scientist Wilhem Ostwald in 1932. Let us see the various stages in this process:Stage 1: Ammonia and oxygen reacts together to form nitric oxide. Platinum is used as a catalyst in this reaction. The equation is: 4NH3 + 5O2 → 4NO + 6H2O.
Stage 2: The nitric oxide reacts with oxygen to produce nitrogen dioxide. The equation is:
2NO + O2 → 2NO2.
Stage 3: The nitrogen dioxide is dissolved in water, in the presence of oxygen to produce nitric acid.
The equation is: 4NO2 + 2H2O + O2 → 4HNO3.
Reaction of nitric acid with metals
• We have seen that hydrochloric acid reacts with metals and produce hydrogen. Details here.
• The nitric acid also reacts with metals. But the products will depend upon the concentration of the acid, and also on the nature of the metal
Identification of nitrate salts
1. Take the aqueous solution of the given salt in a test tube.2. Add equal volume of freshly prepared ferrous sulphate solution to it and mix well.
3. Hold the test tube containing the mixture in a slanting position.
4. Slowly pour concentrated sulphuric acid along the sides of the test tube.
5. If a brown ring is formed at the junction of the two liquids, the salt can be confirmed as a nitrate.
Note that it is very important to handle acids carefully. All safety measures should be taken. Some of the compulsory safety measures are:
• Wearing lab coat or lab apron
• Wearing safety goggles for protection of eyes
• Using acid resistant gloves
Experiments should be performed only under the guidance of authorised professionals
Uses of nitric acid
• In the manufacture of fertilizers • As an oxidising agent in rocket fuels • In the purification of gold • For etching alphabets and pictures on metals • To prepare aqua regia used for dissolving noble metals
Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid. They are taken in the ratio 1:3. That is., if we take 5 ml of nitric acid, we must take 15 ml of hydrochloric acid. This mixture can dissolve noble metals like gold and platinum. Let us see how this is possible:
The equation for the reaction is:
HNO3 + 3HCl → 2H2O. + NOCl + 2[Cl]
The chlorine atoms thus liberated, will react with gold, platinum etc., forming their chlorides and thus dissolving them.
In the next section, we will see Carbon.
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