why nitric oxide is the most stable among the oxides of nitrogen?

Dear Abhishek,

Gaseous nitric oxide is the most thermally stable oxide of nitrogen and is also the simplest known thermally stable paramagnetic molecule--i.e., a molecule with an unpaired electron. It is one of the environmental pollutants generated by internal-combustion engines, resulting from the reaction of
nitrogen and oxygen in the air during the combustion process. At room temperature nitric oxide is a colourless gas consisting of diatomic molecules. However, because of the unpaired electron, two molecules can combine to form a dimer by coupling their unpaired electrons.

2NO N2O2

Thus, liquid nitric oxide is partially dimerized, and the solid consists solely of dimers.

When a mixture of equal parts of nitric oxide and nitrogen dioxide, NO2, is cooled to -21 C (-6 F), the gases form dinitrogen trioxide, a blue liquid consisting of N2O3 molecules. This molecule exists only in the liquid and solid states. When heated, it forms a mixture of NO and NO2. Nitrogen dioxide is prepared commercially by oxidizing NO with air, but it can be prepared in the laboratory by heating the nitrate of a heavy metal, as in the following equation,

2Pb(NO3)2 + heat 2PbO + 4NO2 + O2,

or by adding copper metal to concentrated nitric acid. Like nitric oxide, the nitrogen dioxide molecule is paramagnetic. Its unpaired electron is responsible for its colour and its dimerization. At low pressures or at high temperatures, NO2 has a deep brown colour, but at low temperatures the colour almost completely disappears as NO2 dimerizes to form dinitrogen tetroxide, N2O4. At room temperature an equilibrium between the two molecules exists.

2NO2 N2O4

Dinitrogen pentoxide, N2O5, is a white solid formed by the dehydration of nitric acid by phosphorus(V) oxide.

P4O10 + 4HNO3 4HPO3 + 2N2O5

Above room temperature N2O5 is unstable and decomposes to N2O4 and O2. Two oxides of nitrogen are acid anhydrides; that is, they react with water to form nitrogen-containing oxyacids.
Dinitrogen trioxide is the anhydride of nitrous acid, HNO2, and dinitrogen pentoxide is the anhydride of nitric acid, HNO3.

N2O3 + H2O 2HNO2

N2O5 + H2O 2HNO3

There are no stable oxyacids containing nitrogen with an oxidation number of +4.

Nitrogen dioxide reacts with water in one of two ways. In cold water NO2 disproportionates to form a mixture of HNO2 and HNO3, while at higher temperatures HNO3 and NO are formed.
In their chemical activity, the nitrogen oxides undergo extensive oxidation-reduction reactions. Nitrous oxide resembles oxygen in its behaviour when heated with combustible materials. It is a strong oxidizing agent that decomposes upon heating to form nitrogen and oxygen. Because one-third of the gas liberated is oxygen, nitrous oxide supports combustion better than air. All the nitrogen oxides are in fact good oxidizing agents. Dinitrogen pentoxide reacts violently with metals, nonmetals, and organic materials, as in the following reactions with potassium (K) and iodine gas (I2).

N2O5 + K KNO3 + NO2

N2O5 + I2 I2O5 + N2

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