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- CHnX(4-n)
Used to produce:
Common sources:
Other names:
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