Nitrogen dioxide

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.

Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide. Direct production of the monomer usually occurs on a laboratory scale.

Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:

4 HNO₃ → 4 NO₂ + 2 H₂O + O₂

The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO₂.

Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO₂:

Pb(NO₃)₂ → PbO + 2 NO₂ + 1⁄2 O₂

Alternatively, dehydration of nitric acid produces nitronium nitrate...

2 HNO₃ → N₂O₅ + H₂O

6 HNO₃ + 1⁄2 P₄O₁₀ → 3 N₂O₅ + 2 H₃PO₄

...which subsequently undergoes thermal decomposition:

N₂O₅ → 2 NO₂ + 1⁄2 O₂

NO₂ is generated by the reduction of concentrated nitric acid with a metal (such as copper):

4 HNO₃ + Cu → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O

Thermal properties

At low temperatures, NO₂ reversibly converts to the colourless gas dinitrogen tetroxide (N₂O₄):

2 NO₂ ⇌ N₂O₄

The exothermic equilibrium has enthalpy change ΔH = −57.23 kJ/mol.^[11]

At 150 °C (302 °F; 423 K), NO₂ decomposes with release of oxygen via an endothermic process (ΔH = 14 kJ/mol):

2 NO₂ →2 NO +  O₂

As an oxidizer

As suggested by the weakness of the N–O bond, NO₂ is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.^[12]

Hydrolysis

NO₂ reacts with water to give nitric acid and nitrous acid:

3 NO₂ + H₂O → HNO₃ + HNO₂

This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia.^[13] 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.^[14]

Conversion to nitrates

NO₂ is used to generate anhydrous metal nitrates from the oxides:^[11]

MO + 3 NO₂ → M(NO₃)₂ + NO

Alkyl and metal iodides give the corresponding nitrates:^[10]

TiI₄ + 8 NO₂ → Ti(NO₃)₄ + 4 NO + 2 I₂

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]

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):^[11]

2 NO + O₂ → 2 NO₂

NO₂ is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make NO₂ 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.^[18]

Anthropogenic sources

Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

N₂ + 2 O₂ → 2 NO₂

For the general public, the most prominent sources of NO₂ are internal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO₂.^[8]

Outdoors, NO₂ can be a result of traffic from motor vehicles.^[19] Indoors, exposure arises from cigarette smoke,^[20] and butane and kerosene heaters and stoves.^[21] Indoor exposure levels of NO₂ are, on average, at least three times higher in homes with gas stoves compared to electric stove.^[22][23]

Workers in industries where NO₂ is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards.^[6] Workers in high voltage areas especially those with spark or plasma creation are at risk.^{[citation needed]} Agricultural workers can be exposed to NO₂ arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".^[24][25]

NO₂ diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO₂ are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction, inflammation, reduced immune response, and may have effects on the heart.^[26]

Acute exposure

Acute harm due to NO₂ exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema, leading to bronchitis or pneumonia, and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.^[27][28]

For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia, a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered.^[29][30]

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.^[31]

Long-term

Exposure to low levels of NO₂ over time can cause changes in lung function.^[32] Chronic exposure to NO₂ can cause respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma.^{[citation needed]} A small number of correlational studies have suggested that the presence of a gas stove inside a home may increase asthma risk, a process believed to be mediated by indoor NO₂ concentration.^[22][33]

Interaction of NO₂ and other NO_x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.^[34] Elevated levels of NO
₂ can also harm vegetation, decreasing growth, and reduce crop yields.^[35]

• Dinitrogen tetroxide
• Nitric oxide (NO) – pollutant that is short lived because it converts to NO₂ in the presence of ozone
• Nitrite
• Nitrous oxide (N₂O) – "laughing gas", a linear molecule, isoelectronic with CO₂ but with a nonsymmetric arrangement of atoms (NNO)
• Nitryl

• Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1-4398-5511-9.

Wikimedia Commons has media related to Nitrogen dioxide.

• International Chemical Safety Card 0930
• National Pollutant Inventory – Oxides of nitrogen fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF) and "Answer to follow-up questions from CAFE (2004) (PDF)
• Nitrogen Dioxide Air Pollution
• Current global map of nitrogen dioxide distribution
• A review of the acute and long term impacts of exposure to nitrogen dioxide in the United Kingdom IOM Research Report TM/04/03
• Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition