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
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
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−
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
HNO2 + NH3 → N2 + 2H2O
HNO2 + NH2OH → N2O + 2H2O
HNO2 + HNO3 → 2NO2 + H2O
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
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
4Mg + 10HNO3 → 4Mg(NO3 )2 + NH4 NO3 + 3H2O
If the acid is diluted we get N2O
4Mg + 10HNO3 → 4Mg(NO3 )2 + N2O + 5H2O
· 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)