4_2 Oxides of Nitrogen: N₂O, NO and NO₂ and the NO₂–N₂O₄ Equilibrium

Nitrogen forms oxides in every positive oxidation state from +1 to +5:

\mathrm{N_2O}

, NO,

\mathrm{N_2O_3}

,

\mathrm{NO_2}

(with its dimer

\mathrm{N_2O_4}

) and

\mathrm{N_2O_5}

. They exist because nitrogen forms strong multiple bonds to oxygen. The three most important are dinitrogen oxide, nitrogen monoxide and nitrogen dioxide. Two of them, NO and

\mathrm{NO_2}

, are free radicals, molecules with an unpaired electron, and that explains much of their chemistry.

Dinitrogen Oxide (N₂O)

Dinitrogen oxide, also called nitrous oxide or laughing gas, is a linear molecule, N=N=O, with nitrogen in the +1 state. It is made by gently heating ammonium nitrate to about 200 °C. Ammonium nitrate can explode if overheated, so a safer mixture of sodium nitrate and ammonium sulphate is often used instead:

[NH4+].[O-][N+](=O)[O-]>>[N-]=[N+]=O.O.O

It is a colourless gas with a faintly sweet smell, fairly soluble in cold water. Breathed with oxygen, it causes euphoria and dulls pain, so it is used as a mild anaesthetic in medicine and dentistry. It relights a glowing splint like oxygen does, because the heat splits it into

\mathrm{N_2}

and

\mathrm{O_2}

. Burning sulphur and phosphorus therefore keep burning in it. It is also a potent greenhouse gas.

Nitrogen Monoxide (NO)

Nitrogen monoxide, also called nitric oxide, has nitrogen in the +2 state and 11 valence electrons, so one electron is unpaired. In the laboratory it is made by reacting copper with dilute nitric acid:

\mathrm{3Cu + 8HNO_3(dil) \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O}

At very high temperatures, such as in lightning, an electric arc or a car engine, nitrogen and oxygen combine directly to form NO. It is a colourless gas that is only sparingly soluble in water. Its best-known reaction is with oxygen: as soon as NO meets air, it turns into brown nitrogen dioxide, as two radicals combine with

\mathrm{O_2}

:

[N]=O.[N]=O.O=O>>O=[N+][O-].O=[N+][O-]

NO combines with chlorine to form nitrosyl chloride, NOCl. It also forms a brown complex with iron(II) sulphate, which is the basis of the brown ring test for nitrates. In the body, tiny amounts of NO act as a signalling molecule that relaxes blood vessels.

Nitrogen Dioxide (NO₂)

Nitrogen dioxide has nitrogen in the +4 state. It is made by heating lead(II) nitrate, or by reacting copper with concentrated nitric acid:

\begin{aligned} \mathrm{2Pb(NO_3)_2} &\xrightarrow{\ \Delta\ } \mathrm{2PbO + 4NO_2 + O_2} \\ \mathrm{Cu + 4HNO_3(conc)} &\rightarrow \mathrm{Cu(NO_3)_2 + 2NO_2 + 2H_2O} \end{aligned}

It is a toxic, reddish-brown gas with a sharp smell, and a major air pollutant from vehicle exhausts. The molecule is bent (about 134°), with an unpaired electron mostly on nitrogen. It is a strong oxidizing agent, and burning phosphorus, carbon and potassium keep burning in it. With water it gives a mixture of nitric and nitrous acids, and in the presence of oxygen only nitric acid forms. This reaction is the last step in making nitric acid, and it also produces acid rain:

\begin{aligned} \mathrm{2NO_2 + H_2O} &\rightarrow \mathrm{HNO_3 + HNO_2} \\ \mathrm{4NO_2 + 2H_2O + O_2} &\rightarrow \mathrm{4HNO_3} \end{aligned}

Dimerization: NO₂ and N₂O₄

Because each

\mathrm{NO_2}

molecule has an unpaired electron, two molecules can pair up through an N–N bond to form dinitrogen tetroxide,

\mathrm{N_2O_4}

, which is colourless:

O=[N+][O-].O=[N+][O-]>>[O-][N+](=O)[N+]([O-])=O

Forming the N–N bond releases energy, so the forward reaction is exothermic. By Le Chatelier's principle, cooling shifts the equilibrium towards colourless

\mathrm{N_2O_4}

, and heating shifts it back towards brown

\mathrm{NO_2}

. Solid

\mathrm{N_2O_4}

is colourless, the liquid is pale yellow, and the gas darkens as it is warmed. Above about 150 °C,

\mathrm{NO_2}

itself starts breaking down into colourless NO and oxygen, and this is complete at about 620 °C.

Tips for remembering

• N₂O laughs, NO is colourless, NO₂ is brown: laughing gas is sweet-smelling and unreactive, NO turns brown on meeting air, and NO₂ is the brown gas.
• Odd electron, pair up: NO₂ has an unpaired electron, so two molecules pair up into N₂O₄, and cooling favours the colourless dimer.
• Dilute acid gives NO, concentrated acid gives NO₂: copper with dilute nitric acid gives colourless NO, and with concentrated acid gives brown NO₂.