Nitrogen Dioxide

Nitrogen dioxide
Identifiers
CAS number	[10102-44-0]
Properties
Molecular formula	NO2
Molar mass	46.0055
Appearance	brown gas
Density	1443 kg/m³, liquid 3.4 kg/m³, gas at 294.25 K
Melting point	-11.2°C (261.95 K)
Boiling point	21.1°C (293.25 K)
Hazards
EU classification	Highly toxic (T+)
NFPA 704	0 3 0
R-phrases	R26, R34
S-phrases	S1/2, S9, S26, S28,S36/37/39, S45
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Nitrogen dioxide is a chemical compound with the formula NO₂. It is one of the several nitrogen oxides. At ordinary temperatures and atmospheric pressure, it is a reddish-brown gas with a characteristic sharp, biting odor. It is one of the most prominent air pollutants and a poison by inhalation. This gas is present in small quantities in smog and automobile exhaust fumes. However, chemists find it useful as a catalyst, nitrating agent, and oxidizing agent.

Preparation

Nitrogen dioxide (NO₂) is prepared by simple reaction of nitric acid (HNO₃) over copper metal. The reaction is the following:

4HNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(L)

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects^[1].

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

2O₂ + N₂ → 2 NO₂

The most important sources of NO₂ are internal combustion engines ^[2], thermal power stations and, to a lesser extent, pulp mills.^[3]

The map shown below, depicting results of satellite measurements over Europe, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone. A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome. ^[4]

Oxides of nitrogen

• Nitrous oxide or N₂O, "laughing gas," a linear molecule, isoelectronic with CO₂ but with a nonsymmetric arrangement of atoms (NNO)
• Nitric oxide or NO, a problematic pollutant that is short lived because it converts to NO2 in the presence of free oxygen.
• NO_x = all of the above in unspecified proportions but tending toward NO₂.

More esoteric nitrogen oxides include N₂O₅ and the blue species N₂O₃.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO₂⁻), nitrate (NO₃⁻), nitronium or NO₂⁺, and nitrosonium or NO⁺. NO₂ is intermediate between nitrite and nitronium:

NO₂⁺ + e⁻ → NO₂

NO₂ + e⁻ → NO₂⁻

See also

• Air pollution
• Nitric acid
• Nitric oxide
• Nitrogen
• Nitrous oxide
• Oxygen
• Pollutant

Notes

References

ISBN links support NWE through referral fees

• Chang, Raymond. 2006. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031.

• Cotton, F. Albert, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann. 1999. Advanced Inorganic Chemistry 6th ed. New York: Wiley. ISBN 0471199575.

• Sloss, Leslie L. 1992. Nitrogen Oxides Control Technology Fact Book. Park Ridge, NJ: Noyes Data Corp. ISBN 0815512945.

External links

All links retrieved November 15, 2022.

• NIOSH Pocket Guide to Chemical Hazards
• Health Aspects of Air Pollution (2003) – WHO-Europe reports: (PDF)
• Nitrogen Dioxide Air Pollution