Nitrogen dioxide at −196 °C, 0 °C, 23 °C, 35 °C, and 50 °C. (NO
2) converts to the colorless dinitrogen tetroxide (N
4) at low temperatures, and reverts to NO
2 at higher temperatures.
Nitrogen(IV) oxide, Deutoxide of nitrogen
|Jmol 3D model||Interactive image
|Molar mass||46.0055 g mol−1|
|Appearance||Vivid orange gas|
|Density||1.88 g dm−3|
|Melting point||−11.2 °C (11.8 °F; 261.9 K)|
|Boiling point||21.2 °C (70.2 °F; 294.3 K)|
|Solubility||soluble in CCl
4, nitric acid, chloroform
|Vapor pressure||98.80 kPa (at 20 °C)|
Refractive index (nD)
|1.449 (at 20 °C)|
|37.5 J/mol K|
|240 J mol−1 K−1|
Std enthalpy of
|+34 kJ mol−1|
|Main hazards||Poison, oxidizer|
|Safety data sheet||ICSC 0930|
|GHS signal word||Danger|
|H270, H314, H330|
|P220, P260, P280, P284, P305+351+338, P310|
EU classification (DSD)
|R-phrases||R26, R34, R8|
|S-phrases||(S1/2), S9, S26, S28, S36/37/39, S45|
|Lethal dose or concentration (LD, LC):|
LC50 (median concentration)
|30 ppm (guinea pig, 1 hr)
315 ppm (rabbit, 15 min)
68 ppm (rat, 4 hr)
138 ppm (rat, 30 min)
1000 ppm (mouse, 10 min)
LCLo (lowest published)
|64 ppm (dog, 8 hr)
64 ppm (monkey, 8 hr)
|US health exposure limits (NIOSH):|
|C 5 ppm (9 mg/m3)|
|ST 1 ppm (1.8 mg/m3)|
IDLH (Immediate danger)
Related Nitrogen oxides
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Nitrogen dioxide is the chemical compound with the formula NO
2. It is one of several nitrogen oxides. NO
2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. At higher temperatures it is a reddish-brown gas that has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide is a paramagnetic, bent molecule with C2v point group symmetry.
- 1 Properties
- 2 Preparation and reactions
- 3 Ecology
- 4 Uses
- 5 Human-caused sources and exposure
- 6 Toxicity
- 7 See also
- 8 References
- 9 External links
Nitrogen dioxide is a reddish-brown gas above 70 °F (21 °C; 294 K) with a pungent, acrid odor, becomes a yellowish-brown liquid below 70 °F (21 °C; 294 K), and converts to the colorless dinitrogen tetroxide (N
4) below 15 °F (−9 °C; 264 K).
It has a molar mass of 46.0055, which makes it heavier than air, whose average molar mass is 28.8.
Unlike ozone, O3, the ground 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 NO
2 also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as •NO
Preparation and reactions
- 2 NO + O
2 → 2 NO
2 + N
2 → 2 NO
In the laboratory, NO
2 can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide, which subsequently undergoes thermal decomposition:
- 2 HNO
3 → N
5 + H
- 2 N
5 → 4 NO
2 + O
The thermal decomposition of some metal nitrates also affords NO
- 2 Pb(NO
2 → 2 PbO + 4 NO
2 + O
Alternatively, reduction of concentrated nitric acid by metal (such as copper).
- 4 HNO
3 + Cu → Cu(NO
2 + 2 NO
2 + 2 H
Or finally by adding concentrated nitric acid over tin; hydrated tin dioxide is produced as byproduct.
- 4 HNO3 + Sn → H2O + H2SnO3 + 4 NO2
2 is highly reactive.
Basic thermal properties
2 exists in equilibrium with the colourless gas dinitrogen tetroxide (N
- 2 NO
2 ⇌ N
The equilibrium is characterized by ΔH = −57.23 kJ/mol, which is exothermic. NO2 is favored at higher temperatures, while at lower temperatures, dinitrogen tetroxide (N2O4) predominates. Dinitrogen tetroxide (N
4) can be obtained as a white solid with melting point −11.2 °C. NO2 is paramagnetic due to its unpaired electron, while N2O4 is diamagnetic.
The chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, NO
2 decomposes with release of oxygen via an endothermic process (ΔH = 114 kJ/mol):
- 2 NO
2 → 2 NO + O
As an oxidizer
As suggested by the weakness of the N–O bond, NO
2 is a good oxidizer. Consequently, it will combust, sometimes explosively, with many compounds, such as hydrocarbons.
- 2 NO
4 + H
2O → HNO
2 + HNO
This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia. Nitric acid decomposes slowly to nitrogen dioxide, which confers the characteristic yellow color of most samples of this acid:
- 4 HNO
3 → 4 NO
2 + 2 H
2O + O
Conversion to nitrates
2 is used to generate anhydrous metal nitrates from the oxides:
- MO + 3 NO
2 → M(NO
2 + NO
Conversion to nitrites
Alkyl and metal iodides give the corresponding nitrites:
- 2 CH
3I + 2 NO
2 → 2 CH
2 + I
4 + 4 NO
2 → Ti(NO
4 + 2 I
2 is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make NO
2 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.
2 is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, and as a flour bleaching agent.:223 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 unmanned space probes sent to various planets.
Human-caused sources and exposure
Workers in industries where NO
2 is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards. Astronauts in the Apollo–Soyuz Test Project were almost killed when NO
2 was accidentally vented into the cabin. Agricultural workers can be exposed to NO
2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "Silo-filler's disease".
2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves, and chemically reacts with antioxidant and lipid molecules in the ELF; NO
2's health effects 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.:Section 4
Acute harm due to NO
2 exposure is only likely to arise in occupational settings. Direct exposure to the skin can cause irritations and burns. Only very high concentrations of the gaseous form cause immediate distress: 10–20 ppm can cause mild irritation of the nose and throat, 25–50 ppm can cause edema leading to bronchitis or pneumonia, and levels above 100 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.
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.
For the public, chronic exposure to NO
2 can cause respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma. NO
2 creates ozone which causes eye irritation and exacerbates respiratory conditions, leading to increased visits to emergency departments and hospital admissions for respiratory issues, especially asthma.
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.
The U.S. EPA has set safety levels for environmental exposure to NO
2 at 100 ppb, averaged over one hour, and 53 ppb, averaged annually. As of February 2016, no area of the US was out of compliance with these limits and concentrations ranged between 10–20 ppb, and annual average ambient NO2 concentrations, as measured at area-wide monitors, have decreased by more than 40% since 1980.
2 concentrations in vehicles and near roadways are appreciably higher than those measured at monitors in the current network. In fact, in-vehicle concentrations can be 2–3 times higher than measured at nearby area-wide monitors. Near-roadway (within about 50 metres (160 ft)) concentrations of NO2 have been measured to be approximately 30 to 100% higher than concentrations away from roadways. Individuals who spend time on or near major roadways can experience short-term NO2 exposures considerably higher than measured by the current network. Approximately 16% of U.S. housing units are located within 300 feet (91 m) of a major highway, railroad, or airport (approximately 48 million people). This population likely includes a higher proportion of non-white and economically-disadvantaged people. Studies show a connection between breathing elevated short-term NO2 concentrations, and increased visits to emergency departments and hospital admissions for respiratory issues, especially asthma. NO2 exposure concentrations near roadways are of particular concern for susceptible individuals, including asthmatics, children, and the elderly.
- Dinitrogen tetroxide
- Nitric oxide (NO) – a problematic pollutant that is short lived because it converts to NO
2 in the presence of free oxygen
- Nitrous oxide (N
2O) – "laughing gas", a linear molecule, isoelectronic with CO
2 but with a nonsymmetric arrangement of atoms (NNO)
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