Dinitrogen tetroxide
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{{Chembox new
| Name = Dinitrogen tetroxide
| ImageFile = Dinitrogen-tetroxide-3D-vdW.png
<!-- | ImageSize = 200px -->
| ImageName = Dinitrogen tetroxide
| IUPACName = Dinitrogen Tetroxide
| Section1 = {{Chembox Identifiers
| CASNo = 10544-72-6
}}
| Section2 = {{Chembox Properties
| Formula = N<sub>2</sub>O<sub>4</sub>
| MolarMass = 92.011 [[atomic mass unit|u]]
| Appearance = Transparent gas
| Density = 1443 [[kilogram per cubic metre|kg/m³]] (liquid at 1.013 [[bar (unit)|bar]], boiling point)
| Solvent = other solvents
| SolubleOther = reacts with water
| MeltingPt = 261.9 [[kelvin|K]] (-11.2 °[[celsius|C]])
| BoilingPt = 294.3 K (21.1 °C)
| VaporPressure = 96 [[kPa]] (20 °C) [http://www.itcilo.it/english/actrav/telearn/osh/ic/10102440.htm]
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = -35.05 kJ/mol
| Entropy = 150.38 J/(mol•K)
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS = [http://encyclopedia.airliquide.com/sds/en/090_AL_EN.pdf Air Liquide MSDS (PDF)]
}}
| Section7 = {{Chembox Hazards
| MainHazards = [[Ingestion]]:<br /> ?<br />[[Inhalation]]:<br /> Corrosive & toxic<br />[[Skin]]:<br /> Corrosive<br />[[Eyes]]:<br/> Corrosive}}
}}
'''Nitrogen tetroxide''' ('''dinitrogen tetroxide''' or '''nitrogen peroxide''') is the [[chemical compound]] N<sub>2</sub>O<sub>4</sub>. It is a powerful [[oxidizer]], highly toxic and corrosive. N<sub>2</sub>O<sub>4</sub> is [[hypergolic]] with various forms of [[hydrazine]], i.e., they
burn on contact without a separate ignition source, making them popular [[bipropellant]] rocket fuels. It is a useful reagent in chemical synthesis.
==Structure and properties==
The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. Unlike NO<sub>2</sub>, N<sub>2</sub>O<sub>4</sub> is [[diamagnetic]].<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> It is also colorless but can appear brownish yellow liquid due to the presence of NO<sub>2</sub> according to the following equilibrium:
:N<sub>2</sub>O<sub>4</sub> {{unicode|⇌}} 2NO<sub>2</sub>
Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some nitrogen tetroxide is a component of [[smog]] containing nitrogen dioxide.
==Production==
Nitrogen dioxide is made by the [[catalytic]] [[oxidation]] of [[ammonia]]: steam is used as a diluent to reduce the combustion temperature. Most of the water is condensed out, and the gases are further cooled; the [[nitric oxide]] that was produced is oxidised to [[nitrogen dioxide]], and the remainder of the water is removed as [[nitric acid]]. The gas is essentially pure nitrogen tetroxide, which is condensed in a brine-cooled liquefier.
==Use as a rocket propellant==
Dinitrogen tetroxide is one of the most important rocket propellants ever developed, much like the German developed [[hydrogen peroxide]]-based [[T-Stoff]] oxidizer used in their World War II rocket propelled combat aircraft designs, like the [[Messerschmitt Me 163]] ''Komet'', and by the late 1950s it became the storable oxidizer of choice for rockets in both the [[USA]] and [[USSR]]. It is a [[hypergolic]] [[propellant]] often used in combination with a [[hydrazine]]-based [[rocket]] [[fuel]]. One of the earliest uses of this combination was on the [[Titan rocket]]s used originally as ICBM's and then as launch-vehicles for many spacecraft. Used on the U.S. Gemini and Apollo spacecraft, it continues to be used on the Space Shuttle, most geo-stationary satellites, and many deep-space probes. It now seems likely that NASA will continue to use this oxidiser in the next-generation 'crew-vehicles' which will replace the shuttle. It is also the primary oxidizer for Russia's [[Proton rocket]] and China's [[Long March rocket]]s.
When used as a propellant, dinitrogen tetroxide is usually referred to simply as 'Nitrogen Tetroxide' and the abbreviation 'NTO' is extensively used. Additionally, NTO is often used with the addition of a small percentage of [[nitric oxide]], which inhibits stress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to as "[[Mixed Oxides of Nitrogen]]" or "MON". Most spacecraft now use MON instead of NTO, for example, the Space Shuttle reaction control system uses MON3 (NTO containing 3wt%NO).
[http://www.friends-partners.org/partners/mwade/props/rocindex.htm]
==Power generation using N<sub>2</sub>O<sub>4</sub>==
The tendency of N<sub>2</sub>O<sub>4</sub> to reversibly break into NO<sub>2</sub> has led to research into its use in advanced power generation systems as a so-called dissociating gas. "Cool" nitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen dioxide at half the molecular weight. This hot nitrogen dioxide is expanded through a turbine, cooling it and lowering the pressure, and then cooled further in a heat sink, causing it to recombine into nitrogen tetroxide at the original molecular weight. It is then much easier to compress to start the entire cycle again. Such dissociative gas [[Brayton cycle]]s have the potential to considerably increase efficiencies of power conversion equipment.
==Chemical reactions==
N<sub>2</sub>O<sub>4</sub> has a very rich chemistry.<ref>{{cite journal| author=Addison, C. C.|title= Dinitrogen Tetroxide, Nitric Acid, and Their Mixtures as Media for Inorganic Reactions| journal=Chemical Reviews|year= 1980| volume= 80| pages= 21–39|doi=10.1021/cr60323a002}}</ref>
===Intermediate in the manufacture of nitric acid===
Nitric acid is manufactured on a large scale via N<sub>2</sub>O<sub>4</sub>. This species reacts with water to give both [[nitrous acid]] and [[nitric acid]]:
:N<sub>2</sub>O<sub>4</sub> + H<sub>2</sub>O → HNO<sub>2</sub> + HNO<sub>3</sub>
The coproduct HNO<sub>2</sub> upon heating [[Disproportionation|disproportionate]]s to [[Nitric oxide|NO]] and more nitric acid.
===Synthesis of metal nitrates===
N<sub>2</sub>O<sub>4</sub> behaves as the salt [NO<sup>+</sup>][NO<sub>3</sub><sup>−</sup>], the former being a strong oxidant:
:2 N<sub>2</sub>O<sub>4</sub> + M → 2 NO + M(NO<sub>3</sub>)<sub>2</sub>
(M = Cu, Zn, Sn).
N<sub>2</sub>O<sub>4</sub>
see: [[Nitrosyl fluoroborate|NOBF<sub>4</sub>]]
==External links==
* [http://www.npi.gov.au/database/substance-info/profiles/67.html National Pollutant Inventory - Oxides of nitrogen fact sheet]
* [http://www.cdc.gov/niosh/npg/npgd0454.html NIOSH Pocket Guide to Chemical Hazards]: Nitrogen tetroxide
* [http://encyclopedia.airliquide.com/encyclopedia.asp?LanguageID=11&CountryID=19&Formula=N2O4&GasID=0&UNNumber=&btnFormula.x=0&btnFormula.y=0#MSDS Air Liquide Gas Encyclopedia: NO<sub>2</sub> / N<sub>2</sub>O<sub>4</sub>]
==References==
{{reflist}}
[[Category:Nitrogen compounds]]
[[Category:Oxides]]
[[Category:Rocket oxidizers]]
[[Category:Nitrates]]
[[da:Dinitrogentetroxid]]
[[de:Distickstofftetroxid]]
[[es:Tetróxido de dinitrógeno]]
[[fr:Peroxyde d'azote]]
[[ja:四酸化二窒素]]
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[[pl:Tetratlenek diazotu]]
[[pt:Tetróxido de nitrogênio]]
[[ru:Тетраоксид диазота]]
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[[zh:四氧化二氮]]