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Nitrogen dioxide
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Nitrogen dioxide is the chemical compound with the formula NO2. One of several nitrogen oxides, NO2 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 paramagnetic bent molecule with C2v point group symmetry.
ogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:
- 2 NO + O2 ? 2 NO2
In the laboratory, NO2 can be prepared in a two step procedure by thermal decomposition of dinitrogen pentoxide, which is obtained by dehydration of nitric acid:
- 2 HNO3 ? N2O5 + H2O
- 2 N2O5 ? 4 NO2 + O2
The thermal decomposition of some metal nitrates also affords NO2:
- 2 Pb(NO3)2 ? 2 PbO + 4 NO2 + O2
ub>2 exists in equilibrium with N2O4:
- 2 NO2 ? N2O4
The equilibrium is characterized by ?H = -57.23 kJ/mol.
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Nitrogen dioxide is the chemical compound with the formula NO2. One of several nitrogen oxides, NO2 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 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 + O2 ? 2 NO2
In the laboratory, NO2 can be prepared in a two step procedure by thermal decomposition of dinitrogen pentoxide, which is obtained by dehydration of nitric acid:
- 2 HNO3 ? N2O5 + H2O
- 2 N2O5 ? 4 NO2 + O2
The thermal decomposition of some metal nitrates also affords NO2:
- 2 Pb(NO3)2 ? 2 PbO + 4 NO2 + O2
Monomer-dimer equilibrium
NO2 exists in equilibrium with N2O4:
- 2 NO2 ? N2O4
The equilibrium is characterized by ?H = -57.23 kJ/mol. Resulting from an endothermic reaction, the paramagnetic monomer is favored at higher temperatures. Colourless diamagnetic N2O4 can be obtained as a solid melting at m.p. –11.2 °C.
Main reactions
The chemistry of nitrogen dioxide has been investigated intensively. At 150 °C, NO2 decomposes with release of oxygen via an endothermic process (?H = 114 kJ/mol):
- 2 NO2 ? 2 NO + O2
As suggested by the weakness of the N-O bond, 2 NO2 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 NO2 + H2O ? NO + 2 HNO3
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 HNO3 ? 4 NO2 + 2H2O + O2
NO2 is used to generate anhydrous metal nitrates from the oxides:
- MO + 3 NO2 ? 2 M(NO3)2 + NO
Similarly, alkyl and metal iodides give the corresponding nitrates:
- 2 CH3I + 3 NO2 ? 2 CH3NO3 + NO + I2
- TiI4 + 8 NO2 ? Ti(NO3)4 + 4 NO + 2 I2
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 is often contaminated with NO2. 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 NO2 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:
- 2O2 + N2 ? 2 NO2
The most important sources of NO2 are internal combustion engines , thermal power stations and, to a lesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible for the reddish colour of mushroom clouds The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrous oxides (NOx). Limiting NOx production demands the precise control of the amount of air used in combustion.
The map shown below, depicting results of satellite measurements over Europe, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone.
A recent study by researchers at the University of California, San Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome.
See also
- Nitryl
- Nitrous oxide or N2O, "laughing gas", a linear molecule, isoelectronic with CO2 but with a nonsymmetric arrangement of atoms (NNO)
- Nitric oxide or NO, a problematic pollutant that is short lived because it converts to NO2 in the presence of free oxygen.
- NOx = all of the above in unspecified proportions but tending toward NO2.
More esoteric nitrogen oxides include N2O5 and the blue species N2O3.
Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO2-), nitrate (NO3-), nitronium or NO2+, and nitrosonium or NO+. NO2 is intermediate between nitrite and nitronium:
NO2+ + e- ? NO2
NO2 + e- ? NO2-
External links
- WHO-Europe reports: (PDF) and " (PDF)
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