Methane (CH4) is a colorless, odorless gas with a tetrahedral geometry. Its chemical properties make it useful as a common fuel source, in producing hydrogen gas for fertilizers and explosives, and in synthesizing valuable chemicals. However, methane is also a potent greenhouse gas.
Methane Formula and Structure
Methane has the chemical formula of CH4 and a molecular weight of 16.043 g/mol. The methane molecule is tetrahedral, with the carbon atom at the center and the four hydrogen atoms on the corners of the tetrahedron. Each C-H bond is equivalent, and each bond is separated by an angle of 109.5°.
Physical Properties of Methane
Lighter than air, methane gas has a density of 0.657 g/L at 25 °C and 1 atmospheric pressure. It transforms into a liquid below -162 °C and a solid below -182.5 °C. Methane is barely soluble in water, with a solubility of 22.7 mg/L, but is soluble in various organic solvents such as:
- diethyl ether
Some of the most important chemical reactions involving methane are combustion and halogenation.
Combustion of methane releases substantial heat (891 kJ/mol). It is a multi-step oxidation reaction and can be summarized by the equation as follows:
One molecule of gaseous methane reacts with two molecules of oxygen gas under combustion conditions to form one molecule of carbon dioxide gas, two molecules of water vapor and energy.
Releasing only carbon dioxide and water, methane is the cleanest burning fossil fuel and constitutes most of natural gas. Although methane is relatively stable, it can be explosive when its content is between 5 to 14 percent in air, and it has been the cause of many mine disasters.
Although challenging in industrial scale, methane can be partially oxidized to the methanol by methane monooxygenase enzyme. Interestingly, a group of N-DAMO bacteria was found to adopt anaerobic oxidation of methane with nitrite as oxidant.
Methane can also react with halogen under radical conditions as follows:
The chlorine radical is first generated by a radical initiator such as ultraviolet light. This chlorine radical abstracts a hydrogen atom from methane to form a hydrogen chlorine and a methyl radical. The methyl radical then reacts with a chlorine molecule (Cl2), resulting in chloromethane and a chlorine radical, which goes through another cycle of reaction unless terminated by another radical.
There are many industrial uses for methane, thanks to its versatile chemical properties. It is an important source of hydrogen and carbon for various organic materials.
Methane is the primary component of natural gas, which is a common fuel source. It is widely used to power homes, turbines, automobiles and other things. Methane can also be liquefied for the ease of storage or transport. When combined with liquid oxygen, refined liquid methane can serve as a source of fuel for rockets.
Natural gas is also used to produce hydrogen gas on the industrial scale since methane can react with steam at high temperatures (700 to 1,100 °C) to yield carbon monoxide and hydrogen gas in the presence of a catalyst. Hydrogen is then used for manufacturing ammonia, which is precursor for fertilizers and explosives. As a good source of carbon, methane is also used to synthesize chloroform, carbon tetrachloride, nitromethane and methanol. The carbon black generated upon incomplete combustion of methane is a reinforcing agent for rubber in tires.
Methane as a Greenhouse Gas
In a sustainable system, methane released into the atmosphere is taken up by natural methane sinks such as the soil and the methane oxidation process in troposphere.
However, increased methane emissions in the past decades have contributed to the greenhouse effect. Despite its small concentration, methane warms up the planet 86 times as much as carbon dioxide, another greenhouse gas. Hopefully, efforts to control methane emissions could slow down the greenhouse effect before it is too late.