Nitric Acid

Nitric acid is the most important and valuable oxoacid of nitrogen. Its molecular formula is HNO₃, and its molar mass is 63.01 g/mol. Nitric acid is a highly corrosive mineral acid, also called the spirit of nitre and aqua fortis. It is colourless in its pure form.

1.0What is Nitric Acid?

Nitric acid is a highly corrosive, fuming, and colourless mineral acid. Known as the spirit of nitre and aqua fortis, it is essential in manufacturing fertilisers and explosives. Although pure nitric acid is colourless, it turns yellow with age due to the decomposition into nitrogen oxides and water. It is highly corrosive and toxic, causing severe skin burns, and reacts with hydroxides, metals, and oxides to form nitrate salts.

HNO₃ is a strong oxidising agent produced by the catalytic oxidation of ammonia. It is a standard laboratory reagent and an important chemical in industries for making explosives and fertilisers. Nitric acid's pH is approximately 1.

2.0Structure Of Nitric Acid

The chemical formula of nitric acid is HNO₃. A nitric acid molecule contains three oxygen atoms, one nitrogen atom, and one hydrogen atom. The structure includes:

• One oxygen atom double-bonded to the central nitrogen atom.
• Another oxygen atom is single-bonded to the nitrogen atom and bonded to the hydrogen atom.
• The third oxygen atom is singly bonded to the nitrogen and carries a negative charge.

The nitrogen atom, with four covalent bonds, carries a +1 charge, balancing the -1 charge on the oxygen atom, resulting in a zero net charge for the molecule. Resonance can alter these charges, explaining the molecule's stability. The nitric acid molecule is planar, with the nitrogen atom bonded to three oxygen atoms, one of which holds the proton. The other two N-O bonds show resonance with double bond character.

3.0Preparation of Nitric Acid

1. Laboratory Preparation

In the laboratory, nitric acid is prepared by heating potassium nitrate (KNO₃) or sodium nitrate (NaNO₃) with concentrated sulfuric acid (H₂SO₄) in a glass retort.

The reaction is as follows:

OR

     NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃

2. Large-Scale Preparation (Ostwald Process)

On an industrial scale, nitric acid is primarily produced by the Ostwald process, which involves the catalytic oxidation of ammonia (NH₃) by atmospheric oxygen.

• Oxidation of Ammonia: 4NH₃(g)  + 5O₂(g)  →   Pt/Rh gauge, 500K, nine bar 4NO(g)  + 6H2O(g)

• Formation of Nitrogen Dioxide: 2NO(g) + O₂(g) → 2NO₂(g)
• Formation of Nitric Acid: 3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)
• The NO produced is recycled back into the process.
• The aqueous HNO₃ can be concentrated by mass distillation up to ~68%.
• Further concentration to 98% is achieved by dehydration with concentrated sulfuric acid (H₂SO₄).

4.0Physical Properties of Nitric Acid

• State: Liquid
• Odour: Pungent and suffocating
• Colour: Typically colourless, but can be yellow or red depending on concentration.
• Concentration:
• • Industrial Grade: Approximately 68% nitric acid in water.
  • Commercial Grade: Typically between 52% and 68%.
  • Fuming Nitric Acid: About 86% or higher.
  • White/Red Fuming Nitric Acid: Concentrations above 95%.
• pH Value: Approximately 3.01
• Molar Mass: 63.01 g/mol
• Density: 1.51 g/cm³
• Boiling Point: 83°C
• Melting Point: −42°C

5.0Chemical Properties of Nitric Acid

1. Nature of Nitric Acid

• Type: Strong monoprotic acid
• pH Effect: Turns blue litmus paper red.

2. Reactions of Nitric Acid

• Hydrates: Common solid hydrates include monohydrate (HNO₃·H₂O) and trihydrate (HNO₃·3H₂O).
• Decomposition: Nitric acid decomposes upon heating or exposure to light, forming brown nitrogen dioxide (NO₂). Over time, nitric acid becomes brownish due to this decomposition. The reaction is:4HNO₃ → 2H₂O + 4NO₂ + O₂

3. Reactions with Metals:

• Copper:

With Dilute Nitric Acid: 3Cu + 8HNO₃(dilute) → 3Cu(NO₃)₂ + 2NO +4H₂O

With Concentrated Nitric Acid: Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O

• Zinc:

With Dilute Nitric Acid: 4Zn + 10HNO₃(dilute) → 4Zn(NO₃)₂ + 5H₂O + N₂O

With Concentrated Nitric Acid: Zn + 4HNO₃(conc.) → Zn(NO₃)₂+2H₂O+2NO₂

• Behaviour with Certain Metals: Metals such as chromium (Cr) and aluminium (Al) do not dissolve in concentrated nitric acid because they form a passive oxide film on their surfaces.
• Oxidizing Agent: Nitric acid is a strong oxidising agent and a potent electron acceptor in various redox reactions. Acid reacts vigorously with many non-metallic compounds.

4. Reactions with Non-Metals:

• Iodine: Oxidized to iodic acid (HIO₃).
• Carbon: Oxidized to carbon dioxide (CO₂).
• Sulfur: Oxidized to sulfuric acid (H₂SO₄).
• Phosphorus: Oxidized to phosphoric acid (H₃PO₄).

5. Ionisation: Nitric acid is a strong mineral acid and ionises completely in aqueous solution to form hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻):

HNO₃ + H₂O → H₃O⁺ + NO₃⁻

6.0Uses of Nitric Acid

1. Industrial Uses of Nitric Acid

• Chemical Manufacturing: Nitric acid is crucial for producing polymers (e.g., polyamides, polyurethane), nitrogen-based compounds (e.g., nylon), and explosives (e.g., TNT, nitroglycerin).
• Fertilisers: These are used to make nitrogenous fertilisers, such as calcium nitrate and ammonium nitrate.
• Purification: Essential for purifying precious metals like platinum, gold, and silver.
• Other Uses: Involves making dyes, coal tar products, and drugs.

2. Uses in Daily Life

• Laboratory Reagent: Commonly used in school labs.
• Wood Treatment: Applied to age maple and pine wood.
• Food Industry: Used for cleaning equipment.
• Testing: Spot tests for alkaloids and colourimetric tests for drugs like morphine and heroin.