Nitrogen dioxide

Nitrogen dioxide
Nitrogen dioxide
CAS number 10102-44-0 YesY
PubChem 3032552
ChemSpider 2297499 YesY
EC number 233-272-6
UN number 1067
ChEBI CHEBI:33101 YesY
RTECS number QW9800000
Gmelin Reference 976
Jmol-3D images Image 1
Image 2
Image 3
Molecular formula NO2
Molar mass 46.0055 g mol-1
Exact mass 45.992903249 g mol-1
Appearance Vivid orange gas
Density 2.62 g dm-3
Boiling point

21 °C, 294 K, 70 °F

Solubility in water Reacts
Vapor pressure 98.80 kPa (at 20 °C)
Refractive index (nD) 1.449 (at 20 °C)
Molecular shape Dihedral digonal
GHS pictograms The flame-over-circle pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) The skull-and-crossbones pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
GHS hazard statements H270, H314, H330
GHS precautionary statements P220, P260, P280, P284, P305+351+338, P310
EU Index 007-002-00-0
EU classification Very Toxic T+
R-phrases R26, R34, R8
S-phrases (S1/2), S9, S26, S28, S36/37/39, S45
NFPA 704
NFPA 704.svg
Related compounds
Related Nitrogen oxides Dinitrogen pentoxide

Dinitrogen tetroxide
Dinitrogen trioxide
Nitric oxide
Nitrous oxide

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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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

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.


NO2 exists in equilibrium with dinitrogen tetroxide (N2O4):

2 NO2 is in equilibrium with N2O4

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 N2O4 can be obtained as a solid melting at m.p. −11.2 °C.[2]

Preparation and reactions

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:[2]

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 HNO3N2O5 + 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

Main reactions

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 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.

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.[3] 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

NO2 is used to generate anhydrous metal nitrates from the oxides:[2]

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 NO2Ti(NO2)4 + 2 I2

The pure gas is produced by adding concentrated nitric acid over tin. Stannic acid is produced as byproduct.

4HNO3 + Sn → H2O + H2SnO3 + 4 NO2

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.

The 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.[4]

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitric oxide:

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 important sources of NO2 are internal combustion engines,[5] 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 2009 tropospheric column density.

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 NO2 levels and Sudden Infant Death Syndrome.[6]

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

Nitrogen dioxide (NO2) gas converts to the colorless gas dinitrogen tetroxide (N2O4) at low temperatures, and converts back to NO2 at higher temperatures. The bottles in this photograph contain equal amounts of gas at different temperatures.

External links


  1. ^ a b "nitrogen dioxide (CHEBI:33101)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Retrieved 4 October 2011. 
  2. ^ a b c Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. ^ 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.
  4. ^ Health Aspects of Air Pollution with Particulate Matter,Ozone and Nitrogen Dioxide. World Health Organization. 13–15 January 2003. p. 48. Retrieved 2011-11-19. 
  5. ^ Son, Busoon; Wonho Yang, Patrick Breysse, Taewoong Chung and Youngshin Lee (March 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. 
  6. ^ "Sids Linked to Nitrogen Dioxide Pollution". Retrieved 2008-02-25. 

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Look at other dictionaries:

  • nitrogen dioxide — n a poisonous strongly oxidizing reddish brown gas NO2 * * * a brownish, irritant gas, NO2, generated by the decomposition of nitrogen tetroxide or the reaction of metals with concentrated nitric acid. Exposure to heavy concentrations causes lung …   Medical dictionary

  • nitrogen dioxide — n. a poisonous, reddish brown gas, NO2, used in making nitric acid, as a rocket fuel oxidizer, etc.: it is also an air pollutant formed from automobile exhausts …   English World dictionary

  • nitrogen dioxide — noun a highly poisonous brown gas (NO2) • Hypernyms: ↑dioxide, ↑gas * * * ˌnitrogen diˈoxide 7 [nitrogen dioxide] noun uncountable …   Useful english dictionary

  • nitrogen dioxide — azoto(IV) oksidas statusas T sritis chemija formulė NO₂ atitikmenys: angl. nitrogen dioxide; nitrogen(IV) oxide rus. азота двуокись; азота диоксид; азота(IV) оксид ryšiai: sinonimas – azoto dioksidas …   Chemijos terminų aiškinamasis žodynas

  • nitrogen dioxide — azoto dioksidas statusas T sritis radioelektronika atitikmenys: angl. nitrogen dioxide vok. Stickstoffdioxid, n rus. двуокись азота, f pranc. bioxyde d azote, m; peroxyde d azote, m …   Radioelektronikos terminų žodynas

  • nitrogen dioxide — azoto dioksidas statusas T sritis apsauga nuo naikinimo priemonių apibrėžtis Dusinamoji pavojingoji cheminė medžiaga. Rusvos spalvos dujos. Lydymosi temperatūra 11,2 °C, virimo temperatūra 21,3 °C. 1,5 karto sunkesnis už orą. Gerai tirpsta… …   Apsaugos nuo naikinimo priemonių enciklopedinis žodynas

  • nitrogen dioxide — azoto dioksidas statusas T sritis ekologija ir aplinkotyra apibrėžtis Azoto junginys su deguonimi (NO₂) – rudos, troškios dujos. atitikmenys: angl. nitrogen dioxide vok. Stickstoffdioxid, n rus. двуокись азота, f …   Ekologijos terminų aiškinamasis žodynas

  • Nitrogen Dioxide —   This compound of nitrogen and oxygen is formed by the oxidation of nitric oxide (NO) which is produced by the combustion of solid fuels …   Energy terms

  • Nitrogen dioxide —   A compound of nitrogen and oxygen formed by the oxidation of nitric oxide (NO) which is produced by the combustion of solid fuels.   U.S. Dept. of Energy, Energy Information Administration s Energy Glossary …   Energy terms

  • nitrogen dioxide — noun Date: 1869 a toxic reddish brown gas NO2 that is a strong oxidizing agent, is produced by combustion (as of fossil fuels), and is an atmospheric pollutant (as in smog) …   New Collegiate Dictionary

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