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Nitrogen Dioxide (NO2)
Nitric Dioxide (NO2)
Nitrogen dioxide is the chemical compound with the formula NO2. It exists as a radical in nature. One ofseveral nitrogen oxides, NO2 is an intermediate in the industrial synthesis of nitric acid, millions oftons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is aprominent 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:
In the laboratory, NO2 can be prepared in a two step procedure by thermal decomposition of dinitrogenpentoxide, which is obtained by dehydration of nitric acid:
The thermal decomposition of some metal nitrates also affords NO2:
NO2 exists in equilibrium with N2O4:
The equilibrium is characterized by ΔH = -57.23 kJ/mol. Resulting from an endothermic reaction, theparamagnetic monomer is favored at higher temperatures. Colourless diamagnetic N2O4 can beobtained as a solid melting at m.p. –11.2 °C.
The chemistry of nitrogen dioxide has been investigated intensively. At 150 °C, NO2 decomposes withrelease of oxygen via an endothermic process (ΔH = 114 kJ/mol):
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:
This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia. Nitricacid decomposes slowly to nitrogen dioxide, which confers the characteristic yellow color of most samples of thisacid:
NO2 is used to generate anhydrous metal nitrates from the oxides:
Similarly, alkyl and metal iodides give the corresponding nitrates:
Safety and pollution considerations
Nitrogen dioxide is toxic by inhalation, but this could be avoided as the material is acrid and easily detected byour sense of smell. One potential source of exposure is fuming nitric acid, which is often contaminated withNO2. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low butpotentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential foroverexposure.
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 nitrogencombines with oxygen to form nitrogen dioxide:
The most important sources of NO2 are internal combustion engines, thermal power stations and, to alesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible forthe reddish colour of mushroom clouds The excess air required for complete combustion of fuels in these processesintroduces nitrogen into the combustion reactions at high temperatures and produces nitrous oxides (NOx). LimitingNOx 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 largescale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide playsa role in atmospheric chemistry, including the formation of tropospheric ozone. A recent study by researchers at the University of California, SanDiego, suggests a link between NO2 levels and Sudden Infant Death Syndrome.
Effects of Various NO2 Levels