NITROGEN DIOXIDE


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Nitrogen dioxide
Nitrogen dioxide

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| Density and phase
| 1443 kg/m³, liquid
3.4 kg/m³, gas at 294.25 K
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| NFPA 704
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| R-phrases
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'Nitrogen dioxide' is the chemical compound NO2. It is one of the several nitrogen oxides. This reddish-brown gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants and a poison by inhalation.

Contents
Formation and occurrence
Reactions
Structure and bonding
Safety and pollution considerations
See also
Reference
External links

Formation and occurrence


Nitrogen dioxide in a test tube

Nitric oxide (NO), a common pollutant, oxidizes in air to the dioxide:
::2NO + O2 → 2NO2
Ammonia oxidizing in air at 850°C, with a platinum gauze catalyst, produces NO2:
::4NH3 + 7O2 → 4NO2 + 6H2O
NO2 is generated by the action of nitric acid on a variety of metals, such as copper or silver.
::2HNO3 + Ag → AgNO3 + NO2 + H2O
"Red fuming nitric acid" owes its red color to the presence of NO2.
NO2 is generated in biological settings from decomposition of peroxynitrite (ONOO−), a potent oxidizing and nitrating agent formed from the reaction of nitric oxide with superoxide.

Reactions


Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.
::2 NO2
ightleftharpoons N2O4 ΔG = 45.53 kJ/mol
The equilibrium favors NO2 at higher temperatures. Solid NO2 can be obtained from NO2 by very rapid cooling (for example with liquid nitrogen), although it commonly contains N2O4.
At −50 °C the crystals of N2O4, which is diamagnetic, are colorless, but they become honey-yellow at the melting point. The vapour at −10 °C is pale yellow and deepens as the temperature rises.

Structure and bonding


NO2 is a radical, having one unpaired electron, which renders this molecule paramagnetic.
Canonical forms of nitrogen dioxide resonating

Low energy electronic transitions give rise to the visible color of this molecule. The molecule is nonlinear with bond distances and angles intermediate between those for the corresponding anion, nitrite, and the cation, nitronium.[1]
Species O-N-O angle (°)
 N-O distance (Å)
NO2+
180
1.154
NO2
134
1.197
NO2−
115
1.236

Safety and pollution considerations


NO2 pollution levels in Europe, from January 2003 to June 2004.

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]. The most important source of NO2 is internal combustion engines, which emit nitrogen oxides near people. A major industrial source is pulp mills.
The map shown to the right, depicting results of satellite measurements, 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 [2].

See also



★ 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, related to CO but with one additional electron.

★ NOx = 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−

★ Birkeland-Eyde process

Reference


1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

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)

★ Computational Chemistry Wiki

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