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Dinitrogen tetroxide

Dinitrogen tetroxide

Dinitrogen tetroxide
Systematic name Dinitrogen Tetroxide
Other Names Nitrogen Peroxide
Chemical formula N2O4
Molecular weight 92.011 u
Appearance Transparent gas
CAS number 10544-72-6
MSDS link Air Liquide MSDS (PDF)
Physical properties
Density 1443 kg/m³ (liquid at 1.013 bar, boiling point)
Solubility reacts with water
Thermal decomposition maintains equilibrium with
NO2 at room temperature
Solid properties
Standard enthalpy change of formation
-35.05 kJ/mol
Standard molar entropy
150.38 J/(mol•K)
Heat capacity
 ? J/(mol•K)
Density  ? g/cm3
Liquid properties
ΔfH0liquid -19.5 kJ/mol
S0liquid 209.2 J/(mol•K)
Cp 142.7 J/(mol•K)
Density 1.44 g/cm3
Viscosity 0.47e-3 Pas (293K) [1]
Viscosity 0.33e-3 Pas (315K)[1]
Gas properties
ΔfH0gas 11.1 kJ/mol
S0gas 304.4 J/(mol•K)
Cp 79.2 J/(mol•K)
Phase behavior
Melting point 261.9 K (-11.2 °C)
Boiling point 294.3 K (21.1 °C)
Triple point  ? K (? °C)
 ? kPa
Critical point 430.9 K (157.8 °C)
10.132 MPa
Vapor pressure 96 kPa (20 °C) [1] (?)
Heat of fusion
14.67 kJ/mol [2] (?)
Entropy of fusion
 ? J/mol•K
Heat of vaporization
39.60 kJ/mol [3] (?)
Ingestion  ?
Inhalation Corrosive & toxic
Skin Corrosive
Eyes Corrosive
OSHA Permissible Exposure Limit
5 ppm
NIOSH Immediate Danger to Life and Health
20 ppm
Personal protection
Skin: Prevent skin contact
Eyes: Prevent eye contact
Wash skin: When contaminated
Remove: When wet or contaminated
Change: No recommendation
Provide: Eyewash, Quick drench
Reacts with
combustible material
water (to form nitric acid)
chlorinated hydrocarbons
carbon disulfide
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

Nitrogen tetroxide (dinitrogen tetroxide or nitrogen peroxide) is the chemical compound N2O4. It is a powerful oxidizer, and is highly toxic and corrosive. N2O4 has received much attention as a rocket propellant. 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 NO2, N2O4 is diamagnetic.[2] It is also colorless but can appear brownish yellow liquid due to the presence of NO2 according to the following equilibrium:

N2O4 2NO2

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some nitrogen tetroxide is a component of smog containing nitrogen dioxide.


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, 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). [4]

Power generation using N2O4

The tendency of N2O4 to reversibly break into NO2 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

N2O4 has a very rich chemistry.[3]

Intermediate in the manufacture of nitric acid

Nitric acid is manufactured on a large scale via N2O4. This species reacts with water to give both nitrous acid and nitric acid:

N2O4 + H2O → HNO2 + HNO3

The coproduct HNO2 upon heating disproportionates to NO and more nitric acid.

Synthesis of metal nitrates

N2O4 behaves as the salt [NO+][NO3], the former being a strong oxidant:

2 N2O4 + M → 2 NO + M(NO3)2

(M = Cu, Zn, Sn). N2O4 see: NOBF4


  1. ^ a b Sutton, Biblarz; Rocket Propulsion Elements 7th. Edition; p. 244, 258; Wiley-Interscience Publication; 2001
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. ^ Addison, C. C. (1980). "Dinitrogen Tetroxide, Nitric Acid, and Their Mixtures as Media for Inorganic Reactions". Chemical Reviews 80: 21-39. doi:10.1021/cr60323a002.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Dinitrogen_tetroxide". A list of authors is available in Wikipedia.
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