is the chemical compound
with the formula NO2
. It is one of the several nitrogen oxides
. This reddish-brown toxic gas
has a characteristic sharp, biting odor. NO2
is one of the most prominent air pollutants.
Preparation and reactions
Nitrogen dioxide (NO2
) arises from the aerobic oxidation of nitric oxide
- 2 NO + O2 → 2 NO2
In the laboratory, NO2
can be prepared by treating of nitric acid
) with copper
metal. The reaction is the following:
- 4 HNO3 + Cu → Cu(NO3)2 + 2 NO2 + 2 H2O
NO2 exists in equilibrium with its dimer, N2O4, which is colourless and diamagnetic.
Safety and pollution considerations
Nitrogen dioxide is toxic by inhalation but the material is so acrid that accidental poisoning is easily avoided. For example, fuming nitric acid 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.
- 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−