Nitrogen dioxide
Template:Hatnote Template:Redirect Template:Chembox
Nitrogen dioxide is a chemical compound with the formula Template:Chem2. One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, Template:Chem2 is an intermediate in the synthesis of nitric acid, millions of tons of which are produced each year, primarily for the production of fertilizers.
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities.<ref name="EPAmain">Template:EPA content</ref> 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.<ref name=":0">Template:Cite web</ref><ref name=":2">Template:Cite web</ref> The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure.<ref name="NIOSH">Template:Cite web</ref> It is also included in the NOx family of atmospheric pollutants.
Properties
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above Template:Convert and becomes a yellowish-brown liquid below Template:Convert. It forms an equilibrium with its dimer, dinitrogen tetroxide (Template:Chem2), and converts almost entirely to Template:Chem2 below Template:Convert.<ref name="PGCH" />
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 (Template:Chem2) the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,<ref name="G&E">Template:Greenwood&Earnshaw2nd</ref> which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in Template:Chem2 also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as Template:Chem2.
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 Template:Chem2, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to Template:Chem2 results in ozone.
Preparation
Template:See also Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide. It is produced industrially by the oxidation of ammonia, the Ostwald Process. This reaction is the first step in the production of nitric acid:<ref name=Ull/>
It can also be produced by the oxidation of nitrosyl chloride:
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 Template:Chem2:<ref>Template:Greenwood&Earnshaw2nd </ref>
Alternatively, dehydration of nitric acid produces nitronium nitrate...
...which subsequently undergoes thermal decomposition:
Template:Chem2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Selected reactions
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
- 4 Template:Chem2 → 4 Template:Chem2 + 2 Template:Chem2 + Template:Chem2
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 Template:Chem2.
Thermal properties
At low temperatures, Template:Chem2 reversibly converts to the colourless gas dinitrogen tetroxide (Template:Chem2):
The exothermic equilibrium has enthalpy change Template:Nowrap.<ref name="Holleman">Holleman, A. F.; Wiberg, E. (2001) Inorganic Chemistry. Academic Press: San Diego. Template:ISBN.</ref>
At Template:Convert, Template:Chem2 decomposes with release of oxygen via an endothermic process (Template:Nowrap):
- 2 NO2 →2 NO + Template:Chem2
As an oxidizer
As suggested by the weakness of the N–O bond, Template:Chem2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.<ref>Template:Cite journal</ref>
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.<ref name=Ull>Template:Cite encyclopedia</ref> 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.<ref>Template:Cite journal</ref>
Conversion to nitrates
Template:Chem2 is used to generate anhydrous metal nitrates from the oxides:<ref name="Holleman" />
Alkyl and metal iodides give the corresponding nitrates:<ref name="G&E" />
With organic compounds
The reactivity of nitrogen dioxide toward organic compounds has long been known.<ref>Template:Cite journal</ref> For example, it reacts with amides to give N-nitroso derivatives.<ref>Template:Cite journal</ref> It is used for nitrations under anhydrous conditions.<ref>Template:March6th </ref>
Uses
Template:Chem2 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,<ref>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. Chapter 12: Nitrogen Dioxide in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007. Template:ISBN</ref>Template:Rp and as a room temperature sterilization agent.<ref name="Mechanism Overview, June 2012">Template:Cite web</ref> 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.<ref name="ChemWorld">Cotton, Simon (21 March 2013) Nitrogen dioxide. RSC Chemistry World.</ref>
Environmental presence
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):<ref name="Holleman" />
Template:Chem2 is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make Template:Chem2 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.<ref>WHO Air Quality Guidelines – Second Edition. Chapter 7.1 Nitrogen Dioxide.</ref>
Anthropogenic sources
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
For the general public, the most prominent sources of Template:Chem2 are internal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form Template:Chem2.<ref name="EPAmain" /> Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions.<ref>Template:Cite journal</ref>
Outdoors, Template:Chem2 can be a result of traffic from motor vehicles.<ref>Template:Cite web</ref> Indoors, exposure arises from cigarette smoke,<ref name="ATSDR">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.</ref> and butane and kerosene heaters and stoves.<ref>Template:Cite web</ref> Indoor exposure levels of Template:Chem2 are, on average, at least three times higher in homes with gas stoves compared to electric stoves.<ref name=":1">Template:Cite journal</ref><ref name="EPAbasic">Template:EPA content</ref>
Workers in industries where Template:Chem2 is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards.<ref name="PGCH" /> Workers in high voltage areas especially those with spark or plasma creation are at risk.Template:Cn Agricultural workers can be exposed to Template:Chem2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Toxicity
Template:Main Template:Chem2 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 Template:Chem2 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.<ref name="EPAassess2016">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. Federal Register Notice Jan 28, 2016 Free download available at Report page at EPA website.</ref>
Acute exposure
Acute harm due to Template:Chem2 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.<ref name="ToxnetHH">Toxnet Nitrogen dioxide: Human Health Effects Page accessed March 28, 2016.</ref><ref>CDC NIOSH International Chemical Safety Cards (ICSC): Nitrogen Dioxide Page last reviewed: July 22, 2015; Page last updated: July 1, 2014.</ref>
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.<ref>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, 2014</ref><ref>University of Kansas Hospital, Poison Control Center Poison Facts: Medium Chemicals: Nitrogen Dioxide Template:Webarchive page accessed March 28, 2016</ref>
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.<ref name="gov-right-know">Template:Cite web</ref>
Long-term
Exposure to low levels of Template:Chem2 over time can cause changes in lung function.<ref>Template:Cite journal</ref> Cooking with a gas stove is associated with 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.<ref name=":0" /><ref name=":2" /> Children exposed to Template:Chem2 are more likely to be admitted to hospital with asthma.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite web</ref>
Environmental effects
Interaction of Template:Chem2 and other Template:Chem2 with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.<ref>Template:Cite web</ref> Elevated levels of Template:Chem can also harm vegetation, decreasing growth, and reduce crop yields.<ref>Template:Cite web</ref>
See also
- Dinitrogen tetroxide (Template:Chem2)
- Nitric oxide (Template:Chem2) – pollutant that is short lived because it converts to Template:Chem2 in the presence of ozone
- Nitrite (Template:Chem2)
- Nitrous oxide (Template:Chem2) – "laughing gas", a linear molecule, isoelectronic with Template:Chem2 but with a nonsymmetric arrangement of atoms (Template:Chem2)
- Nitryl
References
Cited sources
External links
- 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
Template:Nitrogen compounds Template:Oxides Template:Oxygen compounds Template:Authority control