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Nitrogen dioxide is a chemical compound with the formula . One of several , nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the synthesis of nitric acid, millions of tons of which are produced each year, primarily for the production of .
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities. Cooking with a gas stove produces nitrogen dioxide which causes poorer indoor air quality. Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases. The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure. It is also included in the NOx family of air pollution.
The chemical bond length between the nitrogen atom and the oxygen atom is 119.7 picometre. This bond length is consistent with a bond order between one and two.
Unlike ozone () the ground state 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.
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 :
is generated by the reduction of concentrated nitric acid with a metal (such as copper):
The exothermic equilibrium has enthalpy change .Holleman, A. F.; Wiberg, E. (2001) Inorganic Chemistry. Academic Press: San Diego. .
At , decomposes with release of oxygen via an endothermic process ():
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 Nitrous acid (often written as HONO) in outdoor and indoor environments.
is used to generate anhydrous metal nitrates from the oxides:
Alkyl and metal iodides give the corresponding nitrates:
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,Subcommittee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants; Committee on Toxicology; Board on Environmental Studies and Toxicology; Division on Earth and Life Studies; National Research Council. [http://www.nap.edu/read/11170/chapter/12 Chapter 12: Nitrogen Dioxide] in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007. and as a room temperature sterilization agent. 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.Cotton, Simon (21 March 2013) [http://www.rsc.org/chemistryworld/2013/04/nitrogen-dioxide-podcast Nitrogen dioxide]. ''RSC Chemistry World''.
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.WHO Air Quality Guidelines – Second Edition. Chapter 7.1 [http://www.euro.who.int/__data/assets/pdf_file/0017/123083/AQG2ndEd_7_1nitrogendioxide.pdf Nitrogen Dioxide].
Outdoors, can be a result of traffic from motor vehicles. Indoors, exposure arises from cigarette smoke,US Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology. April 2002 ATSDR Nitrous Oxides. and butane and and stoves. Indoor exposure levels of are, on average, at least three times higher in homes with gas stoves compared to electric stoves. |left]]Workers in industries where is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards. Workers in high voltage areas especially those with spark or plasma creation are at risk. Farmworker can be exposed to arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".
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 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.U.S. EPA. Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/068, 2016. [https://www.federalregister.gov/articles/2016/01/28/2016-01548/integrated-science-assessment-for-oxides-of-nitrogen-health-criteria Federal Register Notice Jan 28, 2016] Free download available at [https://web.archive.org/web/20160205203630/http://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310879 Report page at EPA website].
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.Agency for Toxic Substances and Disease Registry via the CDC Medical Management Guidelines for Nitrogen Oxides Page last reviewed: October 21, 2014; Page last updated: October 21, 2014University of Kansas Hospital, Poison Control Center Poison Facts: Medium Chemicals: Nitrogen Dioxide page accessed March 28, 2016
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.
In 2019, the Court of Justice of the EU, found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO2) in 12 air quality zones.
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