1. General Information and Introduction
- Formula and Molecular Mass: The chemical formula for nitric acid is HNO3. Its molecular mass is approximately 63 g/mol.
- Physical Properties: Pure nitric acid is a colorless, pungent-smelling liquid. However, it often appears yellowish due to the presence of dissolved nitrogen dioxide (NO2), which is formed by its decomposition in the presence of light. It’s a strong, monobasic acid.
- Fuming Nitric Acid: This is a term for concentrated nitric acid containing dissolved nitrogen oxides, giving it a yellowish-brown color and causing it to emit reddish-brown fumes.
2. Laboratory Preparation
Nitric acid is prepared in the lab by heating potassium nitrate (KNO3) or sodium nitrate (NaNO3) with concentrated sulfuric acid (H2SO4). The reaction is carried out in an all-glass apparatus because the vapors of nitric acid are highly corrosive and would attack rubber or cork.
KNO3+H2SO4<200°CKHSO4+HNO3
The temperature is kept below 200°C to prevent the formation of a hard crust of potassium sulfate (K2SO4), which is difficult to remove. The nitric acid vapors are then condensed in a flask cooled by water.
3. Industrial Manufacturing: The Ostwald Process
The Ostwald process is the industrial method for producing nitric acid and involves three main steps:
- Catalytic Oxidation of Ammonia: Ammonia gas is oxidized by atmospheric oxygen in the presence of a platinum-rhodium gauze catalyst at a high temperature (~800°C) to form nitric oxide (NO). This step is a highly exothermic reaction.4NH3+5O2Pt/Rh gauze4NO+6H2O
- Oxidation of Nitric Oxide: The nitric oxide gas is then cooled and oxidized by oxygen from the air to form nitrogen dioxide (NO2).2NO+O2→2NO2
- Absorption of Nitrogen Dioxide: Nitrogen dioxide is absorbed in water in an absorption tower to form dilute nitric acid.4NO2+2H2O+O2→4HNO3
4. Chemical Properties
- As an Acid: Dilute nitric acid reacts with bases, basic oxides, and carbonates to form a salt, water, and carbon dioxide (in the case of carbonates).NaOH+HNO3→NaNO3+H2O
- Reaction with Metals: This is a crucial property. Nitric acid is a strong oxidizing agent, so it rarely produces hydrogen gas when it reacts with metals. Instead, the hydrogen formed is oxidized to water, and the nitric acid itself is reduced to various nitrogen oxides (NO2, NO, N2O).
- With Metals like Copper:Cu+4HNO3 (conc)→Cu(NO3)2+2NO2+2H2O3Cu+8HNO3 (dilute)→3Cu(NO3)2+2NO+4H2O
- Passivity: Certain metals like iron, aluminum, and chromium become unreactive (passive) when treated with concentrated nitric acid due to the formation of a thin, protective oxide layer on their surface.
- Brown Ring Test: This is a standard test for the presence of nitrate ions (NO3−). In this test, a freshly prepared solution of ferrous sulfate is added to the nitrate solution, followed by the careful addition of concentrated sulfuric acid along the side of the test tube. A brown ring forms at the junction of the two liquids, confirming the presence of nitrate ions.
- As a Nitrating Agent: Nitric acid is used for the nitration of organic compounds, a reaction where a nitro group (−NO2) is introduced. For example, in the preparation of explosives like TNT (Trinitrotoluene).
5. Uses of Nitric Acid
- Fertilizers: A major use is in the production of ammonium nitrate, a key component of many fertilizers.
- Explosives: It’s a crucial raw material for manufacturing explosives like TNT and picric acid.
- Dyes, Drugs, and Plastics: It’s used in the synthesis of a wide range of organic compounds.
- Aqua Regia: A mixture of concentrated nitric acid and hydrochloric acid (in a 1:3 ratio) called aqua regia is used to dissolve noble metals like gold and platinum.
