Nitric Acid
Nitric Acid (HNO3)
Nitric acid is a highly corrosive and toxic strong acid that is colorless when pure but tends to acquire a yellow cast due to decomposition into nitrogen oxides and water. It is an important industrial chemical used for the production of fertilizers, explosives, and many organonitrogen compounds.
Properties of Nitric Acid
| Property | Description |
|---|---|
| Chemical formula | HNO3 |
| Molar mass | 63.01 g/mol |
| Density | 1.51 g/cm³ (at 68°F or 20°C for 70% HNO3) |
| Melting point | -42°C |
| Boiling point | 83°C (at 1 atm for 68% HNO3) |
| pH | <-1 (for 1M solution, indicating strong acidic character) |
| Solubility | Miscible with water in all proportions |
Nitric acid is a strong oxidizing agent, especially in its concentrated form. It can react violently with many organic materials and metals, releasing toxic fumes of nitrogen oxides.
Production of Nitric Acid
The industrial production of nitric acid is primarily through the Ostwald process, which involves the following steps:
Ammonia oxidation: Ammonia is oxidized to produce nitrogen monoxide (NO) and water. [ 4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g) ]
Nitrogen monoxide oxidation: The nitrogen monoxide produced is further oxidized to nitrogen dioxide (NO2). [ 2NO(g) + O_2(g) \rightarrow 2NO_2(g) ]
Absorption: Nitrogen dioxide reacts with water to form nitric acid. [ 3NO_2(g) + H_2O(l) \rightarrow 2HNO_3(aq) + NO(g) ]
The last step is a reversible reaction, and the NO produced can be recycled back into the process.
Uses of Nitric Acid
- Fertilizer Production: Nitric acid is used to produce ammonium nitrate and calcium ammonium nitrate, which are key components in fertilizers.
- Explosives: Nitroglycerin, trinitrotoluene (TNT), and nitrocellulose are all made using nitric acid.
- Metal Processing: It is used in etching and pickling of metals.
- Organic Synthesis: Nitric acid is used in the production of various organic compounds, including dyes and drugs.
Reactions Involving Nitric Acid
Reaction with Metals
Nitric acid reacts with most metals, but the products depend on the concentration of the acid and the nature of the metal.
- With non-reactive metals like gold and platinum, no reaction occurs in the absence of an oxidizing agent.
- With metals above hydrogen in the electrochemical series, hydrogen gas is not evolved. Instead, nitric acid acts as an oxidizing agent, forming nitrogen oxides.
For example, the reaction with copper: [ Cu(s) + 4HNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2NO_2(g) + 2H_2O(l) ]
Reaction with Non-metals
Nitric acid reacts with non-metals such as carbon (in the form of graphite or diamond) and sulfur.
Reaction with Organic Compounds
Nitric acid can perform nitration, which is the substitution of a hydrogen atom in an organic molecule with a nitro group (-NO2). This is a key step in the synthesis of many explosives and pharmaceuticals.
For example, the nitration of benzene: [ C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O ]
Safety and Handling
Nitric acid is highly corrosive and can cause severe burns. It is also a strong oxidizer and can cause fires or explosions if it comes into contact with flammable materials. Proper personal protective equipment (PPE) such as gloves, goggles, and acid-resistant clothing should be worn when handling nitric acid.
Environmental Impact
Nitric acid can contribute to the formation of acid rain when released into the atmosphere. It can also lead to the eutrophication of water bodies if it enters the water system due to its use in fertilizers.
In conclusion, nitric acid is a versatile and widely used chemical with applications ranging from agriculture to the military. Its handling requires careful consideration due to its corrosive and oxidizing properties, and its environmental impact must be managed to prevent negative effects on ecosystems.