Nitrogen dioxide

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Nitrogen dioxide
Skeletal formula of nitrogen dioxide with some measurements Spacefill model of nitrogen dioxide
Nitrogen dioxide at different temperatures
Identifiers
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
Properties
Molecular formula NO
2
Molar mass 46.0055 g mol−1
Appearance Vivid orange gas
Density 2.62 g dm−3
Boiling point 21.2 °C (70.2 °F; 294.3 K)
Solubility in water Hydrolyses
Solubility soluble in CCl
4
and nitric acid[2]
Vapor pressure 98.80 kPa (at 20 °C)
Refractive index (nD) 1.449 (at 20 °C)
Structure
Molecular shape Dihedral digonal
Thermochemistry
Std molar
entropy
So298
240 J·mol−1·K−1[3]
Std enthalpy of
formation
ΔfHo298
+34 kJ·mol−1[3]
Hazards
MSDS ICSC 0930
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
Main hazards Poison, oxidizer
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
Related compounds
Related Nitrogen oxides Dinitrogen pentoxide

Dinitrogen tetroxide
Dinitrogen trioxide
Nitric oxide
Nitrous oxide

Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

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.[4] Nitrogen dioxide is a paramagnetic, bent molecule with C2v point group symmetry.

Molecular properties[edit]

Nitrogen dioxide has a molar mass of 46.0055, which makes it heavier than air, whose average molar mass is 28.8.

The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between one and two.

Unlike ozone, O3, the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,[5] which decreases the alpha effect compared to nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO
2
also means that this compound is a free radical.

Preparation and reactions[edit]

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air:[6]

2 NO + O
2
→ 2 NO
2

In the laboratory, NO
2
can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide, which subsequently undergoes thermal decomposition:

2 HNO
3
N
2
O
5
+ H
2
O
2 N
2
O
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[edit]

Basic thermal properties[edit]

NO
2
exists in equilibrium with the colourless gas dinitrogen tetroxide (N
2
O
4
):

2 NO
2
is in equilibrium with N
2
O
4

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. Dinitrogen tetroxide (N
2
O
4
) can be obtained as a white solid with melting point −11.2 °C.[6] NO2 is paramagnetic due to its unpaired electron, while N2O4 is diamagnetic.

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 an oxidizer[edit]

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.

Hydrolysis[edit]

It hydrolyses to give nitric acid and nitrous acid:

2 NO
2
/N
2
O
4
+ H
2
O
HNO
2
+ HNO
3

This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia.[7] 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
2
O
+ O
2

Conversion to nitrates[edit]

NO
2
is used to generate anhydrous metal nitrates from the oxides:[6]

MO + 3 NO
2
→ 2 M(NO
3
)
2
+ NO

Alkyl and metal iodides give the corresponding nitrites:

2 CH
3
I
+ 2 NO
2
→ 2 CH
3
NO
2
+ I
2
TiI
4
+ 4 NO
2
Ti(NO
2
)
4
+ 2 I
2

Safety and pollution considerations[edit]

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 white 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.[8]

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.

2 NO + O
2
→ 2 NO
2

The most prominent sources of NO
2
are internal combustion engines,[9] 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[10] are sources of nitrogen dioxide.

Nitrogen Dioxide 2011 tropospheric column density.

Nitrogen dioxide is also produced by atmospheric nuclear tests, and is responsible for the reddish colour of mushroom clouds.[11]

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.[12]

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[edit]

Nitrogen dioxide (NO
2
) gas converts to the colorless gas dinitrogen tetroxide (N
2
O
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.

External links[edit]

References[edit]

  1. ^ "nitrogen dioxide (CHEBI:33101)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Retrieved 4 October 2011. 
  2. ^ Mendiara, S. N.; Sagedahl, A.; Perissinotti, L. J. (2001). "An electron paramagnetic resonance study of nitrogen dioxide dissolved in water, carbon tetrachloride and some organic compounds". Applied Magnetic Resonance 20: 275. doi:10.1007/BF03162326.  edit
  3. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X. 
  4. ^ "Nitrogen dioxide". 
  5. ^ Chemistry of the Elements, N.N. Greenwood, A. Earnshaw, p.455
  6. ^ a b c Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  7. ^ 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.
  8. ^ 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. 
  9. ^ 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. 
  10. ^ "The Impact of Unvented Gas Heating Appliances on Indoor Nitrogen Dioxide Levels in 'TIGHT' Homes". Retrieved 2013-04-11. 
  11. ^ Effects of Nuclear Explosions. Nuclearweaponarchive.org. Retrieved on 2010-02-08.
  12. ^ "Sids Linked to Nitrogen Dioxide Pollution". Retrieved 2008-02-25.