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Methane

Properties
Structure of Methane

General

Name Methane

Lewis Structure:

    H  
    |  
  H-C-H
    |  
    H  
Chemical formula CH4
Formula weight 16.04 amu
Synonyms Marsh gas; Methyl hydride
CAS number 74-82-8

Phase behavior

Melting point 90.6 K (-182.5C)
Boiling point 111.55 K (-161.6C)
Triple point 90.67 K (-182.48C)
0.117 bar
Critical point 190.6 K (-82.6C)
46 bar
ΔfusH 1.1 kJ/mol
ΔvapH 8.17 kJ/mol

Gas properties

ΔfH0gas -74.87 kJ/mol
S0gas 188 J/molK
Cp 35.69 J/molK

Safety

Acute effects Asphyxia; in severe cases unconsciousness, cardiac arrest or CNS injury. The compound is transported as a cryogenic liquid, exposure to this will obviously cause frostbite.
Chronic effects ???
Flash point -188C
Autoignition temperature 600C
Explosive limits 5-15%

More info

Properties NIST WebBook
MSDS Hazardous Chemical Database

SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

The simplest hydrocarbon, methane, is a gas with a chemical formula of CH4. Pure methane is odorless, but when used commercially is usually mixed with small quantities of strongly-smelling sulfur compounds such as ethyl mercaptan to enable the detection of leaks.

A principal component of natural gas, methane is a significant fuel. Burning one molecule of methane in the presence of oxygen releases one molecule of CO2 (carbon dioxide) and two molecules of H2O (water):

CH4 + 2O2 → CO2 + 2H2O

Methane is a greenhouse gas with a global warming potential of 21.

Contents

Sources of methane

Principal methane sources are

Methane is extracted from geological deposits as a mineral fuel which is associated with other hydrocarbon fuels.

60% of the world emissions are from sources affected by humans. They come primarily from agricultural and other human activities. During the past 200 years, the concentration of this gas in the atmosphere doubled, passing from 0.8 to 1.7 ppm.

Methane is also classified as a biogas because it can be created by the (anaerobic) decomposition of certain organic matters.

  • Industrial sources

Methane can be created and used industrially, and perhaps in nature, by chemical reactions such as the Sabatier process, Fischer-Tropsch process, and steam reforming. Similar gases and materials are often present in geologic and volcanic processes.

  • At high pressures, such as are found on the bottom of the ocean, methane forms a solid clathrate with water. An unknown but possibly very large quantity of methane is trapped in this form in ocean sediments. The sudden release of large volumes of methane from such sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the earth's distant past, such as the Paleocene-Eocene thermal maximum of 55 million years ago.

Reactions of methane

In the combustion of methane several steps are involved:

Methane forms to a methyl radical (CH3), which reacts to formaldehyde (HCHO or H2CO). The formaldehyde reacts to a formal radical (HCO), which then forms carbon monoxide (CO). The process is called oxidative pyrolysis:

CH4 + O2 → CO + H2 + H2O

Following oxidative pyrolysis, the H2 oxidizes, forming H2O, replenishing the active species, and releasing heat. This occurs very quickly, usually in less than a millisecond.

H2 + ½ O2 → H2O

Finally, the CO oxidizes, forming CO2 and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.

CO + ½ O2 → CO2
  • Hydrogen activation

The strength of the carbon-hydrogen covalent bond in methane is among the strongest in all hydrocarbons, and thus its use as a chemical feedstock is limited. The search for catalysts which can facilitate C-H bond activation in methane and other low alkanes is an area of research with considerable industrial significance.

Methane not on Earth

Methane has been detected or is believed to exist in several locations of the solar system. It is believed to have been created by abiotic processes, with the possible exception of Mars.

Traces of methane gas are present in the thin atmosphere of the Earth's Moon.

Methane has also been detected in interstellar clouds.

Units of measure

See also

External links


Last updated: 05-13-2005 07:56:04