Nitrogen dioxide
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| Names | |||
|---|---|---|---|
| Systematic IUPAC name
Dioxidonitrogen(•)[1] (additive) | |||
| Other names | |||
| Identifiers | |||
3D model (JSmol)
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| ChEBI | |||
| ChemSpider | |||
| ECHA InfoCard | 100.030.234 | ||
| EC Number |
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| 976 | |||
PubChem CID
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| RTECS number |
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| UN number | 1067 | ||
CompTox Dashboard (EPA)
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| Properties | |||
| NO 2• | |||
| Molar mass | 46.0055 g mol-1 | ||
| Appearance | Vivid orange gas | ||
| Density | 2.62 g dm-3 | ||
| Boiling point | 21 °C (70 °F; 294 K) | ||
| Reacts | |||
| Vapor pressure | 98.80 kPa (at 20 °C) | ||
Refractive index (nD)
|
1.449 (at 20 °C) | ||
| Structure | |||
| Dihedral digonal | |||
| Thermochemistry | |||
Std molar
entropy (S⦵298) |
240 J·mol−1·K−1[2] | ||
Std enthalpy of
formation (ΔfH⦵298) |
−34 kJ·mol−1[2] | ||
| Hazards | |||
| GHS labelling: | |||
  
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| Danger | |||
| H270, H314, H330 | |||
| P220, P260, P280, P284, P305+P351+P338, P310 | |||
| NFPA 704 (fire diamond) | |||
| Related compounds | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Nitrogen dioxide is the chemical compound with the formula NO
2. It is one of several nitrogen oxides. NO
2 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.[3] Nitrogen dioxide is a paramagnetic, bent molecule with C2v point group symmetry.
Molecular properties
Nitrogen dioxide has a molar mass of 46.0055, which makes it heavier than air, whose average molar mass is 28.8. According to the ideal gas law, NO2 is therefore more dense than air.
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order of one and a quarter, as in ozone (O3).
The ground electronic state of nitrogen dioxide is a doublet state, since there is one unpaired bonding electron delocalised over both bonds, hence the bond order of one and a quarter.
Occurrence
NO
2 exists in equilibrium with the colourless gas dinitrogen tetroxide (N
2O
4):
- 2 NO
2 ⇌ N
2O
4
The equilibrium is characterized by ΔH = −57.23 kJ/mol, which is exothermic. Resulting from an endergonic reaction at higher temperatures, the paramagnetic monomer is favored. Colourless diamagnetic N
2O
4 can be obtained as a solid melting at m.p. −11.2 °C.[4]
Preparation and reactions
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:[4]
- 2 NO + O
2 → 2 NO
2
In the laboratory, NO
2 can be prepared in a two step procedure by thermal decomposition of dinitrogen pentoxide, which is obtained by dehydration of nitric acid:
- 2 HNO
3 → N
2O
5 + H
2O - 2 N
2O
5 → 4 NO
2 + O
2
The thermal decomposition of some metal nitrates also affords NO
2:
- 2 Pb(NO3)2 → 2 PbO + 4 NO
2 + O
2
Alternatively, reduction of concentrated nitric acid by metal (such as copper).
- 4 HNO
3 + Cu → Cu(NO3)2 + 2 NO
2 +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
The chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, NO
2 decomposes with release of oxygen via an endothermic process (ΔH = 114 kJ/mol):
- 2 NO
2 → 2 NO + O
2
As suggested by the weakness of the N–O bond, NO
2 is a good oxidizer. Consequently, it will combust, sometimes explosively, with many compounds, such as hydrocarbons.
It hydrolyzes to give nitric acid and nitrous acid:
- 2 NO
2/N
2O
4 + H
2O →HNO
2 + HNO
3
This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia.[5] Nitric acid decomposes slowly to nitrogen dioxide, which confers the characteristic yellow color of most samples of this acid:
- 4 HNO
3 → 4 NO
2 + 2 H
2O + O
2
NO
2 is used to generate anhydrous metal nitrates from the oxides:[4]
- MO + 3 NO
2 → 2 M(NO
3)
2 + NO
Alkyl and metal iodides give the corresponding nitrites:
- 2 CH
3I + 2 NO
2 → 2 CH
3NO
2 + I
2
- TiI
4 + 4 NO
2 → Ti(NO
2)
4 + 2 I
2
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 NO
2 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 NO
2 at concentrations above 40–100 µg/m3 may decrease lung function and increase the risk of respiratory symptoms.[6]
Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitric oxide:
- O
2 + N
2 → 2 NO
Nitric oxide can be oxidized in air to form nitrogen dioxide. At normal atmospheric concentrations this is a very slow process[dubious – discuss]
- 2 NO + O
2 → 2 NO
2
The most prominent sources of NO
2 are internal combustion engines,[7] 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 (NO
x). Limiting NO
x 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 also produced by atmospheric nuclear tests, and is responsible for the reddish colour of mushroom clouds.[citation needed]
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 NO
2 levels and Sudden Infant Death Syndrome.[8]
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
2) gas converts to the colorless gas dinitrogen tetroxide (N
2O
4) at low temperatures, and converts back to NO
2 at higher temperatures. The bottles in this photograph contain equal amounts of gas at different temperatures.
- Nitrite
- Dinitrogen tetroxide
- Nitryl
- Nitrous oxide (N
2O), "laughing gas", a linear molecule, isoelectronic with CO
2 but with a nonsymmetric arrangement of atoms (NNO) - Nitric oxide (NO), a problematic pollutant that is short lived because it converts to NO
2 in the presence of free oxygen.
External links
- 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)
- A review of the acute and long term impacts of exposure to nitrogen dioxide in the United Kingdom IOM Research Report TM/04/03
References
- ^ a b "nitrogen dioxide (CHEBI:33101)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Retrieved 4 October 2011.
- ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X.
- ^ "Nitrogen dioxide".
- ^ a b c Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ 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.
- ^ Health Aspects of Air Pollution with Particulate Matter,Ozone and Nitrogen Dioxide (PDF). World Health Organization. 13–15 January 2003. p. 48. Retrieved 2011-11-19.
- ^ Son, Busoon (2004). "Estimation of occupational and nonoccupational nitrogen dioxide exposure for Korean taxi drivers using a microenvironmental model". Environmental Research. 94 (3): 291–296. doi:10.1016/j.envres.2003.08.004. PMID 15016597. Retrieved 2008-02-25.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ "Sids Linked to Nitrogen Dioxide Pollution". Retrieved 2008-02-25.