Nitrogen dioxide

From Freepedia

Nitrogen dioxide
Image:Nitrogen dioxide.png
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 Δ_fH^o₂₉₈	+33.10 kJ/mol
Standard molar entropy S^o₂₉₈	240.04 J·K⁻¹·mol⁻¹
Hazards
MSDS	External MSDS
EU classification	Highly toxic (T+)
R-phrases	R26, R34
S-phrases	S1/2, S9, S26, S28, S36/37/39, S45
Supplementary data page
Structure and properties	n, ε_r, 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 red or orange/brown gas with a characteristic sharp, biting odor. It is considered an insidious deadly poison by inhalation.

Nitric oxide (NO) spontaneously produces the dioxide when exposed to air:

2 NO + O₂ → 2NO₂

It is also generated by various nitric acid reactions, e.g. when metals, such as copper or silver, are exposed to nitric acid.

It reacts readily with water, even at normal temperature and pressure, where it disproportionates to nitric and nitrous acid. That reaction is used in the Ostwald process, which in turn allows the industrial production of fertilizers and explosives.

Red fuming nitric acid has a certain percentage of dissolved nitrogen dioxide, hence its red color.

Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide.

2NO₂ ↔ N₂O₄

This reaction is pushed towards dinitrogen tetroxide as temperatures go down. Nitrogen dioxide rarely exists as a liquid or solid because almost all of it will be converted to dinitrogen tetroxide.

Nitrogen dioxide is one of the most prominent air pollutants. Long-term exposure to concentration levels above 40–100 µg/m³ causes adverse health effects [1]. From a health point of view, the most important source of nitrogen dioxide is road traffic, emitting nitrogen oxides very near people and causing levels of concern in streets and cities, including violations of European limit values. Heat sources, including vehicle engines, will oxidize some nitrogen. While nitrogen oxides become thermodynamically unstable at lower temperatures, the activation energy required to reduce them is not present.

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 crucial role in atmospheric chemistry, including the formation of tropospheric ozone.

Image:NO2 pollution europe hires.jpg

A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome [2].