Nitric Acid

Nitric acid is a strong acid. Its chemical formula is HNO₃ and is also called the spirit of niter and aqua fortis. It is colorless when exists in pure form but turns yellow when it gets older. The yellow color is the result of the decomposition of nitric acid into oxides of nitrogen and water.

Nitric Acid Structure

The molecule of nitric acid is made of 3 oxygen atoms, 1 nitrogen atom, and 1 hydrogen atom. In its molecule, one of the oxygen atoms is bonded to the central nitrogen atom through double bonds. While the other oxygen atom is bonded to the central nitrogen atom and hydrogen atom through a single bond. The third and the last oxygen molecule has a -1 charge and is bonded to central nitrogen through a single bond.

As the central nitrogen atom takes part in four covalent bonds with three oxygen atoms, it has a +1 charge. So, the nitric acid has 0 net charge as the positive charge on the nitrogen atom is cancelled or neutralized by the negative charge on the third oxygen atom.

The molecule of Nitric acid has a planar shape in which the nitrogen atom is bonded to three oxygen atoms; out of which, one oxygen atom holds the proton. The remaining two N-O bonds are equivalent and display the resonance with double bond character.

Physical properties of Nitric Acid (HNO₃)

• In liquid form, it has a pungent and suffocation odor.
• The color of nitric acid, depending on its concentration, can be yellow, red or colorless.
• The industrial-grade nitric acid is 68% in water, whereas, the commercial-grade is 52% to 68% in water.
• The fuming nitric acid is around 86% in water, whereas, white fuming or red fuming nitric acid is more than 95% in water.
• Its pH is nearly 3.0.
• The molecular weight (molar mass) of nitric acid is 63.01 g/mol.
• Its density is 1.51 g/cm³
• Its boiling point is 83 degrees Celsius.
• Its melting point is -42 degrees Celsius.

Chemical Properties of Nitric Acid

1. It is a strong monoprotic or monobasic acid. It ionizes in an aqueous solution as shown below:

HNO₃ + H₂O → H₃O⁺ + NO^-3

It also reacts with metallic oxides, hydroxides, carbonates and bicarbonates to produce nitrates, for example:

CaO + 2HNO₃ → Ca (NO₃)₂ + H₂O

NaHCO₃ + HNO₃ → NaNO₃ + H₂O + CO₂

Na₂CO₃ + 2HNO₃ → 2NaNO₃ + H₂O + CO₂

2. It changes blue litmus to red. It easily forms solid hydrates like monohydrate HNO₃.H₂O and the trihydrate HNO₃.3H₂O

3. In the presence of light or when heated, it decomposes easily and forms brown nitrogen dioxide. So, fresh nitric acid, which is white, turns brownish over time. The reaction is given below:

4HNO₃ + heat → 2H₂O + 4NO₂ + O₂

4. It is a strong oxidizing agent as it produces nascent oxygen both in the concentrated as well as in the dilute form. For example:

2HNO₃ (conc.) → 2NO₂ + H₂O + (O)

2HNO₃ (dil.) → 2NO + H₂O + 3 (O)

Thus, it also quickly reacts with various non-metallic compounds and oxidizes them. For example:

C+4HNO₃ → 4NO₂ + 2H₂O + CO₂

3H₂S + 2HNO₃ → 3S + 4H₂O + 2NO

5. Hydrogen gas is released when nitric acid reacts with metals located above hydrogen in the metal activity series. For example:

Mg + 2HNO₃ → Mg (NO₃)2 + H₂

Mn + 2HNO₃ → Mn (NO₃)2 + H₂

Uses of the Nitric Acid

1. It is widely used in the production of fertilizers such as ammonium nitrate and the polymers like Nylon.
2. It is used as a nitrating agent, which introduces a nitro group when used in combination with sulphuric acid.
3. It can be sued to produce explosives like TNT.
4. It is also used as an oxidizer in liquid-fueled rockets.
5. It can remove the wart when used in its pure form.
6. In electrochemistry, it is useful as the chemical doping agent.
7. It is also used to purify gold and silver.

Preparation of Nitric Acid

Laboratory preparation of Nitric Acid:

In this method, nitric acid is prepared in the lab by heating a nitrate salt such as sodium nitrate, potassium nitrate, etc., with concentrated sulphuric acid, in a glass reservoir. The chemical reaction is shown below:

NaNO₃ + H₂SO₄ + Heat → NaHSO₄ + HNO₃

The nitric acid vapors produced in the reaction are condensed to form brown liquid using a reservoir that is cooled under cold water.

Commercial production of Nitric Acid

As nitric acid is very important commercially, it is also produced in large quantities through commercial production. Generally, the cost-effective methods that ensure higher yield are preferred. Some of which are described below:

1) Birkland and Eyde Process:

This process works as described below:

i) The air is passed through the electric arc to make nitrogen react with oxygen. The reaction is shown below:

N₂ + O₂ → 2NO + energy

The reaction that takes place between N₂ and O₂ is endothermic and reversible. Further, high temperature is preferred for the formation of NO as per the Le Chatelier Principle. So, a temperature of around 3000 degrees Celsius is maintained by the electric arc.

ii) NO that is formed is quickly cooled to 1000 degrees Celsius to prevent its decomposition.

iii) Now, NO, which is produced combines with oxygen to form NO₂ as shown below:

2NO + O₂ → 2NO₂

iv) The NO₂ vapors are made to pass through water to produce HNO₃.

2NO₂ + H₂O → HNO₃ + HNO₂

3HNO₂ → HNO₃ + 2NO + H₂O

2) Ostwald's Process

The set-up of Ostwald's process comprises a catalyst chamber or converter, oxidation tower and absorption tower.

Converter: It is made of aluminum, it contains a platinum-rhodium gauze cylinder that is closed at the bottom using a silica lid. The gauze is heated to 800 degrees Celsius. Then the mixture of ammonia and air mixture enters through the gauze from the top and exit is present at the bottom to release the product.

Cooler: The nitric oxide coming out of the converter is cooled down here.

Oxidation Chamber: In this chamber, oxidation of nitric oxide (NO) to nitrogen dioxide (NO₂) takes place.

Absorption Tower: NO₂ enters the absorption tower from the lower part and water is made to enter from the top. In this tower, nitrogen dioxide is absorbed into the water.

The reactions involved in Ostwald's process are as follows:

i) The mixture of ammonia and air in the ratio of 1:10 is passed into the catalyst chamber over a Pt gauze catalyst at 750 to 900 degrees Celsius. It leads to the oxidation of ammonia to NO, whose yield is more than 90%. The reaction is shown below:

4NH₃ + 5O₂ → 4NO + 6H₂O + Energy

Further, the oxidation of ammonia in this reaction is exothermic as it occurs with the release of energy.

ii) The NO produced is cooled and is mixed with air in the oxidizing chamber. It causes oxidation of NO to NO₂, which is then cooled to 50 degrees Celsius.

2NO + O₂ → 2NO₂

iii) Now, NO₂ is absorbed into the water in the absorption tower in the presence of air.

4NO₂ + H₂O + O2 → 4HNO₃

The Nitric acid produced in the above step is concentrated to about 68% by distillation. Further, it can be made more concentrated up to 98% by distillation with concentrated H₂SO₄.