Nitrogen dioxide
Nitrogen dioxide is a chemical compound with the formula NO2. One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, NO2 is an intermediate in the synthesis of nitric acid, millions of tons of which are produced each year, primarily for the production of fertilizers.
Not to be confused with nitric oxide (formula NO), nitrous oxide (N2O), or generic nitrogen oxide pollutants NOx.Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities.[8] The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure.[9] It is also included in the NOx family of atmospheric pollutants.
Properties[edit]
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer, dinitrogen tetroxide (N2O4), and converts almost entirely to N2O4 below −11.2 °C (11.8 °F; 261.9 K).[6]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between one and two.
Unlike ozone (O3) the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,[10] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO2 also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as •NO2.
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O2 results in ozone.
Selected reactions[edit]
Thermal properties[edit]
At low temperatures, NO2 reversibly converts to the colourless gas dinitrogen tetroxide (N2O4):
Uses[edit]
NO2 is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in the manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, as a flour bleaching agent,[15]: 223 and as a room temperature sterilization agent.[16] It is also used as an oxidizer in rocket fuel, for example in red fuming nitric acid; it was used in the Titan rockets, to launch Project Gemini, in the maneuvering thrusters of the Space Shuttle, and in uncrewed space probes sent to various planets.[17]