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Nitrogen dioxide is the chemical compound with the formula . It exists as a radical in nature. One of several nitrogen oxides, is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide is a paramagnetic bent molecule with C2v point group symmetry.

Preparation and reactions

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:

2 NO + → 2

In the laboratory, can be prepared in a two step procedure by thermal decomposition of dinitrogen pentoxide, which is obtained by dehydration of nitric acid:

2 → +
2 → 4 +

The thermal decomposition of some metal nitrates also affords :

2 → 2 PbO + 4 +

Monomer-dimer equilibrium

 exists in equilibrium with  :


The equilibrium is characterized by , which is exothermic. Resulting from an endothermic reaction at higher temperatures, the paramagnetic monomer is favored. Colourless diamagnetic can be obtained as a solid melting at m.p. −11.2 °C.

Main reactions

The chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, decomposes with release of oxygen via an endothermic process (ΔH = 114 kJ/mol):

2 → 2 NO +

As suggested by the weakness of the N–O bond, is a good oxidizer and will sustain the combustion, sometimes explosively, with many compounds, such as hydrocarbons.

It hydrolyzes with disproportionation to give nitric acid:

3 + → NO + 2

This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia. Nitric acid decomposes slowly to nitrogen dioxide, which confers the characteristic yellow color of most samples of this acid:

4 → 4 + 2 +

 is used to generate anhydrous metal nitrates from the oxides:

MO + 3 → 2 + NO

Similarly, alkyl and metal iodides give the corresponding nitrates:

2 + 3 → 2 + NO +

+ 8 → + 4 NO + 2

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation, but this could be avoided as the material is acrid and easily detected by our sense of smell. One potential source of exposure is fuming nitric acid, which spontaneously produces above 0 °C. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to at concentrations above 40–100 µg/m3 causes adverse health effects.

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

2 + → 2

The most important sources of are internal combustion engines, thermal power stations and, to a lesser extent, pulp mills. Butane gas heaters and stoves is also a source. The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrogen oxides ( ). Limiting production demands the precise control of the amount of air used in combustion.

Nitrogen dioxide is also produced by atmospheric nuclear tests, and is responsible for the reddish colour of mushroom clouds.

Nitrogen dioxide is a large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m3, not far below unhealthy levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone.A 2005 study by researchers at the University of California, San Diegomarker, suggests a link between levels and Sudden Infant Death Syndrome.

See also

  • Nitryl
  • Nitrous oxide ( ), "laughing gas", a linear molecule, isoelectronic with but with a nonsymmetric arrangement of atoms (NNO)
  • Nitric oxide (NO), a problematic pollutant that is short lived because it converts to in the presence of free oxygen.


  1. Hong Kong Advanced Level Examination 2003 Chemistry Paper 2 Question 3(b)(ii)
  2. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. Michael Thiemann, Erich Scheibler, Karl Wilhelm Wiegand “Nitric Acid, Nitrous Acid, and Nitrogen Oxides” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim.

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