|Jmol-3D images||Image 1
|Molar mass||46.0055 g mol-1|
|Appearance||Vivid orange gas|
|Density||2.62 g dm-3|
21 °C, 294 K, 70 °F
|Solubility in water||Hydrolyses|
|Solubility||soluble in CCl4 and nitric acid|
|Vapor pressure||98.80 kPa (at 20 °C)|
|Refractive index (nD)||1.449 (at 20 °C)|
|Molecular shape||Dihedral digonal|
|Std enthalpy of
|GHS signal word||Danger|
|GHS hazard statements||H270, H314, H330|
|GHS precautionary statements||P220, P260, P280, P284, P305+351+338, P310|
|R-phrases||R26, R34, R8|
|S-phrases||(S1/2), S9, S26, S28, S36/37/39, S45|
|Main hazards||Poison, oxidizer|
|Related Nitrogen oxides||Dinitrogen pentoxide
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Nitrogen dioxide is the chemical compound with the formula NO2. It is 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 a paramagnetic, bent molecule with C2v point group symmetry.
Molecular properties 
Unlike ozone, O3, the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron, which decreases the alpha effect compared to nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO2 also means that this compound is a free radical.
Preparation and reactions 
- 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
Alternatively, reduction of concentrated nitric acid by metal (such as copper).
- 4 HNO3 + Cu → Cu(NO3)2 + 2 NO2 +2 H2O
Or finally by adding concentrated nitric acid over tin. Stannic acid is produced as byproduct.
- 4HNO3 + Sn → H2O + H2SnO3 + 4 NO2
Main reactions 
Basic thermal properties 
NO2 exists in equilibrium with the colourless gas dinitrogen tetroxide (N2O4):
- 2 NO2 N2O4
The equilibrium is characterized by ΔH = −57.23 kJ/mol, which is exothermic. NO2 is favored at higher temperatures, while at lower temperatures, dinitrogen tetroxide (N2O4) predominates. Colourless dinitrogen tetroxide (N2O4) can be obtained as a solid with melting point −11.2 °C. NO2 is paramagnetic due to its unpaired electron, while N2O4 is diamagnetic.
The chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, NO2 decomposes with release of oxygen via an endothermic process (ΔH = 114 kJ/mol):
- 2 NO2 → 2 NO + O2
As an oxidizer 
As suggested by the weakness of the N–O bond, NO2 is a good oxidizer. Consequently, it will combust, sometimes explosively, with many compounds, such as hydrocarbons.
- 2 NO2/N2O4 + H2O →HNO2 + 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 + 2 H2O + O2
Conversion to nitrates 
NO2 is used to generate anhydrous metal nitrates from the oxides:
- MO + 3 NO2 → 2 M(NO3)2 + NO
Alkyl and metal iodides give the corresponding nitrites:
- 2 CH3I + 2 NO2 → 2 CH3NO2 + I2
- TiI4 + 4 NO2 → Ti(NO2)4 + 2 I2
Safety and pollution considerations 
Nitrogen dioxide is toxic by inhalation. However, as the compound is acrid and easily detectable by smell at low concentrations, inhalation exposure can generally be avoided. One potential source of exposure is fuming nitric acid, which spontaneously produces NO2 above 0 °C. Symptoms of poisoning (lung edema) tend to appear several hours after inhalation of a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.
There is some evidence that long-term exposure to NO2 at concentrations above 40–100 µg/m3 may decrease lung function and increase the risk of respiratory symptoms.
- O2 + N2 → 2 NO
Nitric oxide can be oxidized in air to form nitrogen dioxide. At normal atmospheric concentrations this is a very slow process.
- 2 NO + O2 → 2 NO2
The most prominent sources of NO2 are internal combustion engines, thermal power stations and, to a lesser extent, pulp mills. Butane gas heaters and stoves are also sources. 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 (NOx). Limiting NOx production demands the precise control of the amount of air used in combustion. In households, kerosene heaters and gas heaters are sources of nitrogen dioxide.
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 Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome.
Nitrogen dioxide is also produced naturally during electrical storms. The term for this process is "atmospheric fixation of nitrogen". The rain produced during such storms is especially good for the garden as it contains trace amounts of fertilizer. (Henry Cavendish 1784, Birkland -Eyde Process 1903, et-al)
See also 
- Dinitrogen tetroxide
- Nitrous oxide (N2O), "laughing gas", a linear molecule, isoelectronic with CO2 but with a nonsymmetric arrangement of atoms (NNO)
- Nitric oxide (NO), a problematic pollutant that is short lived because it converts to NO2 in the presence of free oxygen.
- International Chemical Safety Card 0930
- National Pollutant Inventory - Oxides of nitrogen fact sheet
- NIOSH Pocket Guide to Chemical Hazards
- WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF) and "Answer to follow-up questions from CAFE (2004) (PDF)
- Nitrogen Dioxide Air Pollution
- Nitrogen dioxide pollution in the world (image)
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