methane

Quick Reference

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Formula: CH₄
Molecular Weight: 16.04
Density (0°C, 1 atm): 1.432 g/L
Melting Point: -183°C
Boiling Point: -164°C
Solubility: 1.5 mM/L atm

Colorless
Odorless

Reacts with oxygen in the presence of heat forming carbon dioxide, water, heat, and sometimes carbon monoxide

Reacts with halogens in the presence of light forming methyl halides- CH_nX_(4-n)

Used to produce:

• Methanol
• Formaldehyde
• Nitromethane
• Chloroform
• Carbon tetrachloride
• Freons
• Energy from combustion

Common sources:

• Anaerobic bacteria metabolizing dead organisms
• Bacteria-assisted digestion of cellulose, esp. in ruminants and termites
• Wildfires and biomass burning
• Distillation of bituminous coal
• Heating carbon and hydrogen

Other names:

• Firedamp
• Natural gas
• Marsh gas

Also notable:

• Methane plays a role in the greenhouse effect. Though it is present in the atmosphere in much smaller concentrations than carbon dioxide is, molecule for molecule methane is 25 times more efficient as a greenhouse gas.
• Methane is nontoxic. Breathing methane will not poison you, but it will deprive you of the oxygen you should be breathing.
• If you smell a natural gas leak, you are actually smelling trace amounts of sulfur compounds artificially put into the gas- methane by itself is odorless. This scenting is done because an undetected leak could reach flammable concentrations (5.3% to 15%) or even pose a suffocation risk.
• Methane is found in nature in many places- emanating from landfills, wetlands, fossil fuel deposits, and even flatulence- but the methane from these natural sources really comes for the most part from the anaerobic metabolism that occurs in the bacteria present in all these places.
• Of all hydrocarbon fossil fuels, methane has the lowest energy yield from combustion. However, the chemical reaction of combustion is so simple for it that it is easier to achieve a complete, clean burn, and as a fuel it is renewed much more quickly by natural processes than larger, more complex hydrocarbons.

An Explanation

Most of the properties of methane follow rather simply from the shape and size of the methane molecule.

If we can take a molecule and draw an imaginary plane through it, so that on one side of the plane there is a different average amount of electrical charge than on the other side of the plane, we call that molecule "polar". So, for a molecule to be polar, it must have parts that are on average differently charged from each other, and those parts must be arranged so they could be on different sides of an imaginary plane.

There is a very slight difference in the average charge possessed by the hydrogen atoms as compared to the charge of the carbon atom. Methane is still nonpolar, though, because there is no way to separate the hydrogen atoms from the carbon atom by drawing a single plane. The hydrogen atoms, and with them their difference in average charge, are spread out as evenly and symmetrically around the molecule as they possibly could be!

The methane molecule is comparatively light- it has half the atomic mass of an oxygen molecule. This light atomic mass, along with its nonpolar nature, goes a long way in explaining why methane is the way it is.

Density: The density as measured at the top of this writeup is the density at standard temperature and pressure. In these conditions, methane is a gas. The density of a gas is directly proportional to the atomic mass of the molecules that make up the gas. Since the methane molecule has such a low atomic mass, its density at standard temperature and pressure is also low.

Melting Point, Boiling Point: Polar molecules (like water) feel attraction for each other. That's because the part of the molecule that is on average positively electrically charged is attracted to its corresponding opposite part on other molecules- the other part of the molecule that is on average negatively charged. Methane molecules, by contrast, feel very little attraction to each other. The hydrogen atoms that line the surface of the molecule "see" the like-charged hydrogen atoms on neighboring methane molecules and so can't be attracted to any such opposite charge.

Since methane molecules aren't attracted to each other, they have very little reason to clump together. It has to be damn cold to get these apathetic molecules to get close enough together to be a liquid or solid. That is why the melting point and boiling point temperatures are so low.

Solubility in water: A molecule interacting with methane must interact with the hydrogen atoms that encircle it- and all of those hydrogen atoms have the same (nearly neutral) average charge. The differently-charged parts of the water molecule would interact with oppositely-charged parts of the methane molecule- if there were any such parts. But the symmetrical methane molecule gives the water molecules no reason for attraction. The water molecules do, however, feel attraction for each other. The water then can better satisfy its polar attractions by keeping to itself than by mingling in any way with the unattractive (but not repulsive) methane. Therefore, methane (like other nonpolar molecules) is not very soluble at all in water.

Colorless: An explanation of the nuts and bolts of this is way, way out of the scope of this meager article. Here's a short explanation.

The electrically charged "particles" that make up methane, by virtue of the interactions they have with each other, interact with and absorb certain frequencies of light. It just so happens that the frequencies that methane absorbs are so evenly distributed that when white light passes through the methane and so is filtered by that absorption, the light that remains and strikes our eyes does not contain more energy for any one of our color-discriminating cells to absorb than any of the others. Since all of the kinds of our color-discriminating cells are equally stimulated by this light, it still registers to our brains as "white". Since the perceived color of what has passed through the methane is the same as the perceived color of the original light source, we judge the methane to be "colorless".

Odorless: Methane is no more poisonous to our human bodies than helium. Our native environments never contained dangerous quantities of it. Whatever force(s) put us here likely had no "incentive" to give us the ability to smell it.

Sources:

• Methane MSDS
• http://scifun.chem.wisc.edu/chemweek/methane/methane.html
• http://www.chem.uidaho.edu/~honors/gassol.html
• http://icp.giss.nasa.gov/research/methane/greenhouse.html
• Thanks be to Oolong and jk for much-needed advice

Methane is constantly produced in the earth's biosphere, by the decomposition of organic material, sometimes assisted by the fermentation processes of various bacteria and archaea, sometimes living freely, sometimes living in symbiosis with other creatures, ranging from termites to cattle. Since organic materials are carbon chains, and methane is just the simplest form of carbon chain, it makes sense that it would be a byproduct.

But once it reaches our atmosphere, it only exists for a limited time. The quickest source I could get lists the half-life of atmospheric methane as six years. This has probably varied across the earth's history, depending on temperature and atmospheric oxygen levels. But, overall, since methane is a reducing chemical, and we have an atmosphere that is an oxidizing atmosphere, at some point, they will have to meet and neutralize and produce water and carbon dioxide.

This is basic chemistry and earth science. Where methane gets interesting is in the rapidly growing field of astrobiology. Because methane's existence involves being pushed up a thermodynamic hill, there is a limited amount of reasons it could exist inside a planet's atmosphere.

• It is constantly being created by some process. One of which would be life, but it can also be created by, for example, ultraviolet light interacting with water and carbon dioxide. However, Life is the most likely process to create methane.
• The planet is cold enough that even if methane and oxygen are both present, the reaction between them is occurring at a slow enough rate that they can both exist indefinitely. This is the case, for example, on the gas giants of our own solar system.
• The methane exists with no oxidizer to counterbalance it. It forms the atmosphere, and is thus stable.

When the Curiosity Rover went to Mars, it carried equipment to detect atmospheric methane. If methane does exist in quantity on Mars, it might be a byproduct of deeply buried microbial life, slowly releasing it into the atmosphere. Of course, since on Mars there is no oxidizing atmosphere, the only way it would be destroyed is through photodisintegration. Currently, the experiments are not turning up elevated levels of methane, but if they do, the argument about whether it is biogenic will certainly be fascinating.

Much further away, the hunt for exoplanets is now reaching the point where scientists are beginning to be able to take a spectrum of an exoplanet's atmosphere, which is an incredible feat. And if methane shows up in the spectrum of some exoplanets, it would be a sign that the planet could have life. A small, rocky planet in the habitable zone of a star with both oxygen and methane in its spectrum would either have some form of life, or else very unusual non-biological chemical processes. NASA currently has a mission, called FINESSE, to analyze atmospheric spectrums for signs of life.

And so it is that the first sign of life in the universe we might ever get is the spectrum of methane, one of the simplest and most common of molecules, barely detectable in the atmosphere of a planet orbiting a star scores of lightyears away.

Meth"ane (?), n. [See Methal.] Chem.

A light, colorless, gaseous, inflammable hydrocarbon, CH4; marsh gas. See Marsh gas, under Gas.

Methane series Chem., a series of saturated hydrocarbons, of which methane is the first member and type, and (because of their general chemical inertness and indifference) called also the paraffin (little affinity) series. The lightest members are gases, as methane, ethane; intermediate members are liquids, as hexane, heptane, etc. (found in benzine, kerosene, etc.); while the highest members are white, waxy, or fatty solids, as paraffin proper.

© Webster 1913.