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Nitric Acid

What is Nitric Acid?

Nitric acid (chemical formula HNO3) is colorless, fuming, highly corrosive liquid. It is the most important heavy chemical used in the fertilizer production plant and explosives manufacturing process. Pure concentrated nitric acid is a high density toxic solution that burns our skin. The pH scale value of HNO3 is very low.

Nitric acid is a common laboratory reagent storage million tonnes each year. The freezing point of HNO3 is −41.6 °C and the boiling point of 82.6 °C.

The concentrated water solution of HNO3 has properties of photochemical decomposition. It turns brown by absorbing liberating nitrogen dioxide (NO2).

Where to Find Nitric Acid?

Nitric acid is manufactured by catalytic oxidation of ammonia (NH3) by the Oswald process or distillation of sodium or potassium nitrate with concentrated sulfuric acid. It is a pollutant of our environment that causes acid rain or snow.

Commercially, 98 percent concentrated nitric acid is converted to yellow or red fuming acid by dissolving extra nitrogen dioxide in 98 percent acid solutions.

Nitric Acid Lewis Structure

In learning chemistry, the gas phase nitric acid (HNO3) has a planner structural formula but the nitrate ion has a planner symmetrical structure with a negative charge on resonance hybrid. The same general structure holds probably in solid forms of HNO3.

Nitric acid (chemical formula HNO3) and nitrate ion Lewis structure, uses, and production process

The first two N-O chemical bond distances in HNO3 are equal but the third N-O polar bond distance is longer than the first two due to low bond energy corresponding to the single bond.

Structure of Nitrite Ion

The resonance hybrid of nitrate ion consists of a roughly sp2 hybridized orbital at which the nitrogen atom involves resonance hybrids with a symmetrical planner structure. The N-O bond distance in HNO3 1.22, consists of a bond order of 1 and 1/3.

Laboratory Preparation of HNO3

In the laboratory, nitrogen dioxide can be produced by the thermal decomposition of copper(II) nitrate. It can be passed through water to produce nitric acid.

Alternatively, 50 g of potassium nitrate (KNO3) and 25 ml of concentrated sulphuric acid are taken in a round bottom flask in the laboratory. It can be heated to about 200 °C to produce HNO3.

KNO3 + H2SO4 → HNO3 + KHSO4

Nitric Acid Production Process

Industrially, the production of pure nitric acid (molar mass or molecular weight = 63 g/mol) happens by heated ammonia with a suitable catalyst (Ostwald process). Ammonia undergoes different types of oxidation reactions under various conditions. For example, in the air, ammonia reacts but little to form nitrogen.  In pure oxygen, it burns to form nitrogen and water.

The production of pure HNO3 by the Ostwald process is carried out by following steps:

  1. Over a hot platinum gauze chemical catalyst, the mixture of ammonia and air (1:7) oxidized ammonia to form nitric oxide. The liberated specific heat keeps the catalyst hot.
  2. The heated nitric oxide cooled and mixed with oxygen to get nitrogen dioxide.
  3. Nitrogen dioxide passed into warm water or soluble under pressure to yield a 60 percent dissolved solution of nitric acid.
  4. Commercially available concentrated HNO3 is 98 percent concentrated.
  5. Fuming HNO3 contains additionally dissolved NO2 with a yellow colour.

Properties of Nitric Acid

Pure HNO3 is a colourless liquid. The pure or concentrated aqueous solutions of nitric acid have the properties of photochemical decomposition with a very high dielectric constant, a good ionizing solvent for electrolytes. It is a powerful oxidizing agent for oxidation.

Properties of nitric acid (HNO3)
Chemical formula HNO3
Molar mass 63.012 g mol−1
Appearance colorless, yellow, or red fuming state
Melting point −42 °C or −44 °F or 231 K
Boiling point 83 °C or 181 °F or 356 K)
Density 1.51 g cm−3
Conjugate base pair nitrate ion (NO3)
Acidity (pKa) 1.4

A dilute aqueous solution of  HNO3 (molarity below 2M) behaves like a strong acid with little oxidizing power. Dissociation is nearly 93 percent at 0.1M concentration. Only magnesium and manganese liberate hydrogen from the dilute solution of HNO3.

If the concentration is greater than 2M, HNO3 is a powerful oxidizing agent.

  • It attracts most metals except gold, platinum, iridium, rhodium, and rhenium.
  • Chromium, aluminum, iron, and some extent of copper are rendered for liberating hydrogen due to the formation of the oxide film.
  • Concentrated HNO3 in the presence of sulfuric acid attacks aromatic hydrocarbon and their derivatives to form nitro compounds.

Aqua Regia Preparation

The mixture of concentrated HNO3 (3 volumes) and concentrated HCl (1 volume) is known as aqua regia. Aqua regia is a powerful oxidant due to the presence of free chlorine and NOCl

Aqua regia can dissolve gold or platinum metals through the action of nascent chlorine and ClNO. Therefore, such solutions are used for refining gold or platinum metals and cleaning glassware in laboratories.

Uses of  Nitric Acid

  • The main storage consumption is about 80 percent of strong nitric acid used in the manufacture of ammonium nitrate (NH4NO3) which is chiefly used as an acidic fertilizer plant, and explosive in mining.
  • Nitric acid is also used (5 -10 percent) in making cyclohexanone, and caprolactam as a route to nylon.
  • It is confined to nitration in chemical reactions for the production of nitroglycerine, nitrocellulose, and trinitrotoluene (TNT).
  • It is used in the pickling of metal surfaces, as an oxidizer in rocket fuel, and in the manufacture of various nitrate salts.
  • The formation of red-fuming nitric acid molecules is found by dissolving N2O4. It is a superior nitrating laboratory reagent in chemistry.
  • A mixture of concentrated HNO3 and H2SO4 is also used to make a strong nitrating chemical compound, name nitronium ion (NO2+).