|
NITROGEN DIOXIDE
| Nitrogen dioxide |
 |
| General |
| Molecular formula |
NO2 |
| Molar mass |
46.01 g/mol |
| Appearance |
brown gas |
| CAS number |
[10102-44-0] |
| Properties |
| Density and phase |
1443 kg/m³, liquid
3.4 kg/m³, gas at 294.25 K |
| Solubility in water |
reacts |
| Melting point |
-11.2°C (261.95 K) |
| Boiling point |
21.1°C (293.25 K) |
| 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 |
Nitrogen dioxide is the chemical compound NO2. It is one of several nitrogen oxides (NOx). This orange/brown gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants and a poison by inhalation.
Formation and occurrence
Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:
-
- 2NO + O2 → 2NO2
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 ↔ N2O4 ΔG = 45.53 kJ/mol
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, 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. 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
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.
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.
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−
Reference
- ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
External links
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