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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 chemicals used in the fertilizer production plant and explosives manufacturing process. Nitric acid is a common laboratory reagent storage million tonnes of each year. Pure concentrated nitric acid is a high density toxic liquid to burn skin, freezing point -41.6°C, boiling point 82.6°C. The concentrated water solution of HNO3 has properties of photochemical decomposition. It turns brown by absorbing liberating nitrogen dioxide (NO2). The pH scale value of HNO3 is very low.

Where to find nitric acid?

Nitric acid manufacture 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 converted to yellow or red fuming acid by dissolved extra nitrogen dioxide in 98 percent acid solutions.

Nitric acid Lewis structure

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

In learning chemistry, the gas phase nitric acid (HNO3) has a planner structural formula but 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. The first two N-O chemical bond distances in HNO3 are equal but the third N-O polar bond distance 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 roughly sp2 hybridized orbital at which the nitrogen atom involving resonance hybrids with symmetrical planner structure, N-O bond distance 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 produced nitric acid.

Alternatively, 50 grams of potassium nitrate (KNO3) and 25 ml of concentrated sulphuric acid are taken in a round bottom flask in the laboratory (KNO3 + H2SO4 → HNO3 + KHSO4). It can be heated to about 200 °C to produce HNO3.

Nitric acid production process

Industrially, the production of pure nitric acid (molar mass or molecular weight = 63 gm/mol) is happened by heated ammonia with a suitable catalyst (Ostwald process) but 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.

Over a hot platinum gauze chemical catalyst, the mixture of ammonia and air (1:7) oxidized mostly to form nitrogen monoxide. The liberated specific heat keeps the catalyst hot. The heated nitrogen monoxide cooled and mixed with oxygen to get nitrogen dioxide. It passed into warm water or soluble under pressure to yield a 60 percent dissolved solution of nitric acid. Commercially available concentrated HNO3 is 98 percent concentrated. 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, 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

In 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 of the metals except gold, platinum, iridium, rhodium, 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 vol) and concentrated HCl (1 vol) is known as aqua regia. Aqua regia is a powerful oxidant due to the presence of free chlorine and NOCl. It dissolving gold or platinum metals through the action of nascent chlorine and ClNO. It is used for refining metal like gold or platinum, cleaning glassware in laboratories.

What is nitric acid used for?

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