Nitrogen dioxide is a chemical compound with the formula and is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use (primarily in the production of fertilizers). At higher temperatures, nitrogen dioxide is a reddish-brown gas. It can be fatal if inhaled in large quantities. The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure. Nitrogen dioxide is a paramagnetic, bent molecule with C2v point group symmetry.
It is included in the NOx family of atmospheric pollutants.
Properties
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above and becomes a yellowish-brown liquid below . It forms an equilibrium with its dimer, dinitrogen tetroxide (), and converts almost entirely to below .
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 () the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron, which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as .
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 , atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to results in ozone.
Preparation
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):
+
Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
In the laboratory, can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide:
which subsequently undergoes thermal decomposition:
The thermal decomposition of some metal nitrates also generates :
is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Formation from decomposition of nitric acid
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid.
Selected reactions
Thermal properties
exists in equilibrium with the colourless gas dinitrogen tetroxide ():
The equilibrium is characterized by , which is exothermic. is favored at higher temperatures, while at lower temperatures, predominates. can be obtained as a white solid with melting point −11.2 °C. is paramagnetic due to its unpaired electron, while is diamagnetic.
At , decomposes with release of oxygen via an endothermic process ():
As an oxidizer
As suggested by the weakness of the N–O bond, is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.
Hydrolysis
NO2 reacts with water to give nitric acid and nitrous acid:
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia. This reaction is negligibly slow at low concentrations of NO2 characteristic of the ambient atmosphere, although it does proceed upon NO2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO2 (often written as HONO) in outdoor and indoor environments.
Conversion to nitrates
is used to generate anhydrous metal nitrates from the oxides:
Alkyl and metal iodides give the corresponding nitrates:
Ecology
is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make a trace gas in the atmosphere of Earth, where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere, especially in determining ozone concentrations.