Oxidizing Properties
Nitric acid is a strong oxidizing agent. When it undergoes thermal decomposition, it yields nascent oxygen as follows:
Remember:-
The nascent oxygen so formed oxidizes non-metals, metals, inorganic as well as organic compounds etc.
a) With Non-Metals
With hot concentrated nitric acid, non-metals are oxidized to their oxide while the acid itself gets reduced to nitrogen dioxide.
In all the reactions described below, the nascent oxygen is released during the thermal decomposition of the acid, which oxidizes the non-metals.
i) With carbon
Nascent oxygen reacts with carbon to form carbon dioxide.
The overall reaction is
ii) With sulphur
Nascent oxygen reacts with sulphur to form sulphur trioxide.
Sulphur trioxide in turn reacts with water to form sulphuric acid
The overall reactions is:
iii) With phosphorous
Nascent oxygen reacts with phosphorous to form phosphorous (V) oxide.
Phosphorous (V) oxide in turn reacts with water to form phosphoric acid.
The overall reaction is:
b) With Metals
Nitric acid behaves differently with different metals at different concentrations.
i) With sodium, potassium and calcium the reaction is highly explosive.
ii) With magnesium and manganese
With magnesium and manganese, cold and extremely dilute (1%) nitric acid, reacts to yield hydrogen as explained above.
iii) With Copper
- With cold dilute nitric acid:
Copper reacts with cold and dilute nitric acid to yield copper nitrate, water and nitric oxide.
The formed nitric oxide combines with the oxygen of air to give brown fumes of NO2.
- With concentrated nitric acid (cold or hot):
Copper reacts with cold or hot concentrated nitric acid to yield copper nitrate, water and nitrogen dioxide.
iv) With Zinc
- With cold dilute nitric acid:
Zinc reacts with cold and dilute nitric acid to yield zinc nitrate, water and nitric oxide.
- With concentrated nitric acid (cold or hot):
Zinc reacts with cold or hot concentrated nitric acid to yield zinc nitrate, water and nitrogen dioxide.
v) With Iron
- With cold and dilute nitric acid:
Iron reacts with cold and dilute nitric acid to give ferrous nitrate, water and nitric oxide.
- With concentrated nitric acid (hot or cold):
Iron reacts with cold or hot concentrated nitric acid to give ferric nitrate, water and nitrogen dioxide.
- With highly concentrated or fuming nitric acid:
When iron reacts with highly concentrated or fuming nitric acid, a film of an insoluble oxide is formed on it. This renders the iron passive.
vi) With Gold and Platinum
Gold and platinum do not dissolve in nitric acid. But they dissolves in "Aqua Regia" which is a mixture of concentrated nitric acid and concentrated hydrochloric acid in the proportion of 1:3 by volume.
The formed nascent chlorine reacts with these metals to produce their respective chlorides.
c) With Inorganic Compounds
i) With hydrogen sulphide
Nascent oxygen liberated from the decomposition of concentrated nitric acid oxidizes hydrogen sulphide to sulphur.
ii) With sulphur dioxide
When sulphur dioxide is treated with hot concentrated nitric acid, it gets oxidized to sulphur trioxide with the help of nascent oxygen.
iii) With ferrous sulphate
Nitric acid oxidizes ferrous sulphate to ferric sulphate in the presence of sulphuric acid.
The excess of ferrous sulphate absorbs the formed nitric oxide. It then forms a brown unstable compound, which appears like a brown ring.
iv) With concentrated hydrochloric acid:
The nascent oxygen from nitric acid reacts with hydrochloric acid to oxidize it to chlorine and water.
The overall reaction is
OR
d) With Organic Compounds
Feather, silk, skin and vegetable tissues that are organic compounds are stained yellow or brown by nitric acid. This colouration may be due to Xanthoproteic acid present as the protein part of these compounds.
i) With Cellulose
Cellulose is the chief constituent of wood. The chipped part of it results in saw dust. When hot concentrated nitric acid is poured on heated saw dust it catches fire spontaneously (Fig.6.16).
ii) With Turpentine
Turpentine is a combustible hydrocarbon. When a few drops of turpentine are added to fuming or very concentrated nitric acid, it bursts into flames forming water, nitrogen dioxide and carbon dioxide.
