Properties, Uses, and Reactions of Nitric Acid

Nitric acid, also known as aqua fortis, is a colorless corrosive liquid with the chemical formula HNO3. It was first produced by alchemists looking for the philosopher's stone and is not naturally occurring on Earth. This strong oxidizing agent comes with several properties that contribute to its various uses and reactions.

Properties of Nitric Acid

Nitric acid is a highly polar molecule due to the electronegativity difference between nitrogen and oxygen atoms, leading to the formation of hydrogen bonds. It is also a strong acid because it can dissociate into its conjugate base, NO3-, and release a proton to water. The dissociation constant of nitric acid ranges from 10^-4 to 10^-7 depending on the concentration of the solution.

Physical Properties

At room temperature, nitric acid exists as a clear liquid with a boiling point of 95°C. It is highly soluble in water due to its polar nature, forming nitrate salts upon dissolution. On contact with water, nitric acid releases its proton, forming the conjugate base, which is then hydrated to form an ion pair that can dissociate into free ions. Nitric acid shows high reactivity due to its electron-withdrawing nature and strong electrophilicity of the NO2 group. It reacts violently with metals like iron, copper, magnesium, aluminum, etc., causing a rapid evolution of hydrogen gas. However, under controlled conditions, such reactions are used for material processing.

Chemical Properties

Nitric acid can act as both a reducing agent and an oxidizing agent depending on the reaction conditions. In an acidic environment, such as when nitric acid is mixed with acids like sulfuric or phosphoric acid, it acts as an oxidizing agent. When nitric acid is mixed with bases like sodium hydroxide or potassium hydroxide, it behaves as a reducing agent by releasing nitrogen dioxide, NO2, and converting them to nitrites, NO2-, and thus producing red precipitate.

Uses of Nitric Acid

• Manufacturing of Explosives: Nitric acid plays a crucial role in the production of explosives like ammonium nitrate and nitroglycerine. These explosives have wide applications in mining, construction, oil drilling, and demolition industries.
• Fuel Additives: Nitric acid is added to diesel fuel to improve combustion efficiency and reduce emissions. This practice is known as selective catalytic reduction or lean burn.
• Chemical Synthesis: Nitric acid is used in the synthesis of various organic compounds such as nitriles, nitrogen heterocycles, and nitrated carbonyl compounds.
• Pharmaceuticals and Medicine: Nitric acid is employed in small quantities as preservatives and stabilizers in pharmaceuticals like vaccines and insulin. It also finds use in anti-inflammatory arthritis medications.

Chemical Reactions of Nitric Acid

The main chemical reactions involving nitric acid include those with metals, acids, and bases. Some notable examples include:

With Metals

When nitric acid comes into contact with most metals, a violent exothermic reaction occurs, releasing hydrogen gas. The reaction can be summarized as follows:

3 HNO3 + Fe → Fe(OH)3 + 3 NO + H2

In controlled environments, this reaction is exploited in material processing for surface treatment, pickling, and passivation of ferrous components.

With Other Acids and Bases

Acid-base neutralization reactions occur between nitric acid and other acids like sulfuric acid:

HNO3 + H2SO4 → H2NO3 + SO2

In this reaction, sulfuric acid is partially oxidized by HNO3, producing sulfur dioxide gas. When nitric acid reacts with bases like sodium hydroxide, it behaves as a reducing agent by releasing nitrogen dioxide, NO2, which then forms nitrites:

HNO3 + NaOH → NaNO2 + H2O

This reaction yields a red precipitate upon warming or heating, which is then converted back into nitrite when salt is added.

Preparation of Nitric Acid

The preparation of nitric acid involves the oxidation of ammonia with atmospheric oxygen under pressure and at high temperature. The reaction is highly exothermic and produces ammonium nitrate and water as products:

NH3 + O2 → HNO3 + 2 H2O

The process takes place in a large steel converter where ammonia gas and excess oxygen are passed through an efficient heat exchange system to prevent local overheating. After formation, the product mixture is cooled, and the ammonium nitrate component is extracted by precipitation with water.

In summary, nitric acid is a versatile compound with unique properties that contribute to its various uses and reactions. Although it can be hazardous due to its corrosive nature, proper handling and safety measures enable its applications in explosives production, fuel additives, chemical syntheses, pharmaceuticals, and more.