Nitric acid

Nitric acid

Preparation

Nitric acid is prepared by heating equal amounts of potassium or sodium nitrate with concentrated sulphuric acid.

KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

The temperature is kept as low as possible to avoid decomposition of nitric acid. The acid condenses to a fuming liquids which is coloured brown by the presence of a little nitrogen dioxide which is formed due to the decomposition of nitric acid.

4HNO₃ → 4NO₂ + 2H₂O + O₂

Commercial method of preparation

Nitric acid prepared in large scales using Ostwald's process. In this method ammonia from Haber’s process is mixed about 10 times of air. This mixture is preheated and passed into the catalyst chamber where they come in contact with platinum gauze. The temperature rises to about 1275 K and the metallic gauze brings about the rapid catalytic oxidation of ammonia resulting in the formation of NO, which then oxidised to nitrogen dioxide.

4NH₃ + 5O₂ → 4NO + 6H₂O + 120 kJ

2NO + O₂ → 2NO₂

The nitrogen dioxide produced is passed through a series of adsorption towers. It reacts with water to give nitric acid. Nitric acid formed is bleached by blowing air.

6NO₂ + 3H₂O → 4HNO₃ + 2NO + H₂O

Properties

Pure nitric acid is colourless. It boils at 86 °C. The acid is completely miscible with water forming a constant boiling mixture (98% HNO₃, Boiling point 120.5 °C). Fuming nitric acid contains oxides of nitrogen. It decomposes on exposure to sunlight or on being heated, into nitrogen dioxide, water and oxygen.

4HNO₃ → 4NO₂ + 2H₂O + O₂

Due to this reaction pure acid or its concentrated solution becomes yellow on standing.

In most of the reactions, nitric acid acts as an oxidising agent. Hence the oxidation state changes from +5 to a lower one. It doesn’t yield hydrogen in its reaction with metals. Nitric acid can act as an acid, an oxidizing agent and an nitrating agent.

As an acid: Like other acids it reacts with bases and basic oxides to form salts and water

ZnO + 2HNO₃ → Zn(NO₃ )₂ + H₂O

3FeO + 10HNO₃ → 3Fe(NO₃ )₃ + NO + 5 H₂O

As an oxidising agent: The nonmetals like carbon, sulphur, phosphorus and iodine are oxidised by nitric acid.

C + 4HNO₃ → 2H₂O + 4NO₂ + CO₂

S + 2HNO₃ → H₂SO₄ + 2NO

P₄ + 20HNO₃ → 4H₃PO₄ + 4H₂O + 20NO₂

3I₂ + 10HNO₃ → 6HIO₃ + 10NO + 2H₂O

HNO₃ + F₂ → HF + NO₃F

3H₂S + 2HNO₃ → 3S + 2NO + 4H₂O

As an nitrating agent: In organic compounds replacement of a –H atom with –NO₂ is often referred as nitration. For example.

C₆H₆ + HNO₃ → ^{H 2 SO4} → C₆H₅ NO₂ + H₂O

Nitration takes place due to the formation of nitronium ion

HNO₃ + H₂SO₄ → NO₂⁺ + H₃O⁺ + HSO₄⁻

Action of nitric acid on metals

All metals with the exception of gold, platinum, rhodium, iridium and tantalum reacts with nitric acid. Nitric acid oxidises the metals. Some metals such as aluminium, iron, cobalt, nickel and chromium are rendered passive in concentrated acid due to the formation of a layer of their oxides on the metal surface, which prevents the nitric acid from reacting with pure metal.

With weak electropositive metals like tin, arsenic, antimony, tungsten and molybdenum, nitric acid gives metal oxides in which the metal is in the higher oxidation state and the acid is reduced to a lower oxidation state. The most common products evolved when nitric acid reacts with a metal are gases NO₂, NO and H₂O. Occasionally N₂, NH₂OH and NH₃ are also formed.

The reactions of metals with nitric acid are explained in 3 steps as follows:

Primary reaction: Metal nitrate is formed with the release of nascent hydrogen

M + HNO₃ → MNO₃ + (H)

Secondary reaction: Nascent hydrogen produces the reduction products of nitric acid.

Tertiary reaction: The secondary products either decompose or react to give final products

Decomposition of the secondary:

Reaction of secondary products:

HNO₂ + NH₃ → N₂ + 2H₂O

HNO₂ + NH₂OH → N₂O + 2H₂O

HNO₂ + HNO₃ → 2NO₂ + H₂O

Examples:

Copper reacts with nitric acid in the following manner

3Cu + 6HNO₃ → 3Cu(NO₃ )₂ + 6(H)

6(H) + 3HNO₃ → 3HNO₂ + 3H₂O

3HNO₂ → HNO₃ + 2NO + H₂O

overall reation

3Cu + 8HNO₃ → 3Cu(NO₃ )₂ + 2NO + 4H₂O

The concentrated acid has a tendency to form nitrogen dioxide

Cu + 4HNO₃ → Cu(NO₃ )₂ + 2NO₂ + 2H₂O

Magnesium reacts with nitric acid in the following way

4Mg + 8HNO₃ → 4Mg(NO₃ )₂ + 8[H]

HNO₃ + 8H → NH₃ + 3H₂O

HNO₃ + NH₃ → NH₄ NO₃

overall reaction

4Mg + 10HNO₃ → 4Mg(NO₃ )₂ + NH₄ NO₃ + 3H₂O

If the acid is diluted we get N₂O

4Mg + 10HNO₃ → 4Mg(NO₃ )₂ + N₂O + 5H₂O

Uses of nitric acid:

·           Nitric acid is used as a oxidising agent and in the preparation of aquaregia.

·           Salts of nitric acid are used in photography (AgNO₃) and gunpowder for firearms. (NaNO₃)