| Nitrogen dioxide |
| General |
| Molecular formula |
NO2 |
| Molar mass |
46.01 g/mol |
| Appearance |
brown gas |
| CAS number |
[10102-44-0] |
| Properties |
| Density and phase |
see text |
| Solubility in water |
reacts |
| Melting point |
see text |
| Boiling point |
see text |
| Structure |
| Molecular shape |
bent |
| Dipole moment |
? D |
| Thermodynamic data |
Std enthalpy of
formation ΔfHo298 |
+33.10 kJ/mol |
Standard molar
entropy So298 |
240.04 J·K−1·mol−1 |
| Hazards |
| MSDS |
External MSDS |
| EU classification |
Highly toxic (T+) |
| NFPA 704 |
|
| R-phrases |
R26, R34 |
| S-phrases |
S1/2, S9, S26, S28,
S36/37/39, S45 |
| Supplementary data page |
Structure and
properties |
n, εr, etc. |
Thermodynamic
data |
Phase behaviour
Solid, liquid, gas |
| Spectral data |
UV, IR, NMR, MS |
| Related compounds |
| Related nitrogen oxides |
Nitrous oxide
Nitric oxide
Dinitrogen tetroxide
Dinitrogen pentoxide |
| Related compounds |
Nitric acid
Nitrous acid |
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references |
The chemical compound nitrogen dioxide (NO2) is a reddish or orange/brown gas with a characteristic sharp, biting odor. It is one of the most prominent air pollutants and an insidious deadly poison by inhalation.
Nitrogen oxide nomenclature
The naming of the nitrogen oxides can be confusing. Since these species are important in everyday life, this confusion can lead to serious communication problems. Here are the main "actors":
- Nitrous oxide or N2O, "laughing gas"
- Nitric oxide or NO, a problematic pollutant, related to CO but with one additional electron.
- Nitrogen dioxide, NO2, also a serious pollutant, with a bent structure .
- 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 or NO2-, nitrate or NO3-, nitronium or NO2+, and nitrosonium or NO+. NO2 is intermediate between nitrite and nitronium:
-
- NO2+ + e → NO2
- NO2 + e → NO2-
Formation and occurrence
Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:
-
- 2 NO + O2 → 2 NO2
NO2 is generated by the action of nitric acid on a variety of metals, such as copper or silver.
-
- 2 HNO3 + Ag → AgNO3 + NO2 + H2O
"Red fuming nitric acid" owes its red color to the presence of NO2.
Reactions
Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.
-
- 2 NO2 ↔ N2O4
The equilibrium favors NO2 at higher temperatures. Solid NO2 can be obtained as a solid from NO2 by very rapid cooling (for example with liquid nitrogen), although it is commonly contains N2O4.
At −50 °C the crystals of N2O4 are colorless, but honey-yellow at the melting point. The vapour at −10 °C is pale yellow and deepens as the temperature rises up to a maximum at about 183 °C, at which point a 2 cm thick layer of the gas between glass is essentially black and opaque. The color of the gas is a good indicator of the concentration of NO2, which below 183 °C increases with increasing temperature. Above 183 °C, NO2 converts to NO and O2.
NO2 reacts with water, even at ambient temperatures and pressure, where it disproportionates to nitric and nitrous acid. That reaction is used in the Ostwald process, the industrial route to nitrate-based fertilizers and explosives.
Structure and bonding
NO2 is a radical having a lone - or unpaired - electron, which renders this molecule paramagnetic and is involved in the electronic transitions that give rise to its color. The dimer, N2O4, is diamagnetic.
Safety and pollution considerations
Nitriogen 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 anesthesize 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 emitting nitrogen oxides near people.
This map, 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.
NO
2 pollution levels in
Europe.
A recent study by researchers at the University of California, San Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome [2].
External links
