Everything about Nitrogen Tetroxide totally explained
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Nitrogen tetroxide (
dinitrogen tetroxide or
nitrogen peroxide) is the
chemical compound N
2O
4. It is a powerful
oxidizer, highly toxic and corrosive. N
2O
4 is
hypergolic with various forms of
hydrazine, for example, 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
2, N
2O
4 is
diamagnetic. It is also colorless but can appear brownish yellow liquid due to the presence of NO
2 according to the following equilibrium:
» N
2O
4 ⇌ 2NO
2
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 rockets 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 rockets.
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).
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Power generation using N2O4
The tendency of N
2O
4 to reversibly break into NO
2 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 cycles have the potential to considerably increase efficiencies of power conversion equipment.
Chemical reactions
N
2O
4 has a very rich chemistry.
Intermediate in the manufacture of nitric acid
Nitric acid is manufactured on a large scale via N
2O
4. This species reacts with water to give both
nitrous acid and
nitric acid:
» N
2O
4 + H
2O → HNO
2 + HNO
3
The coproduct HNO
2 upon heating
disproportionates to
NO and more nitric acid.
Synthesis of metal nitrates
N
2O
4 behaves as the salt [NO
+][NO
3−], the former being a strong oxidant:
» 2 N
2O
4 + M → 2 NO + M(NO
3)
2
(M = Cu, Zn, Sn).
N
2O
4
see:
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