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For characterstics see [[1]]

The simplest hydrocarbon, methane, is a gas with a chemical formula of CH4.

PropertiesEdit

At room temperature and pressure, methane is a colourless, odourless gas. It has boiling point of −162°C at 1 atmosphere pressure and is highly flammable.

Pure methane is odorless, but when used commercially is usually mixed with small quantities of odorants, strongly-smelling sulfur compounds such as ethyl mercaptan to enable the detection of leaks.

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.

Methane has a wide range of thermodynamic stability. It has a mass of 16.05.

Potential health effectsEdit

Methane is not toxic by any route. The immediate health hazard is that it may cause thermal burns. It is flammable and may form mixtures with air that are flammable or explosive. Methane is violently reactive with oxidizers, halogens, and some halogen compounds. Methane is an asphyxiant and may displace oxygen in a workplace atmosphere. Asphyxia may result if the oxygen concentration is reduced to below 18% by displacement. The concentrations at which flammable or explosive mixtures form are much lower than the concentration at which asphyxiation risk is significant.

Reactions of methane Edit

The reactions with methane are: combustion, hydrogen activation, and halogen reaction.

CombustionEdit

Methane is a significant fuel. In the combustion of methane several steps are involved:

Methane forms a methyl radical (C H_3), which reacts with oxygen forming formaldehyde (H C H O or H_2 C O). The formaldehyde gives a formyl radical (H C O), which then forms carbon monoxide (C O). The process is called oxidative pyrolysis:

C H_4 + O_2 \to CO + H_2 + H_2 O

Following oxidative pyrolysis, the H_2 oxidizes, forming H_2 O, replenishing the active species, and releasing heat. This occurs very quickly, usually in less than a millisecond.

H_2 + \frac{1}{2} O_2 \to H_2 O

Finally, the C O oxidizes, forming C O_2 and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.

C O + \frac{1}{2} O_2 \to C O_2

Hydrogen activationEdit

The strength of the carbon-hydrogen covalent bond[[2]] in methane is among the strongest in all hydrocarbons, and thus its use as a chemical feedstock is limited. Despite the high activation barrier for breaking the C-H bond, CH4 is still the principal starting material for manufacture of hydrogen. 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.

Reactions with halogensEdit

Methane undergoes reactions with all the halogens give correct conditions. The reactions occur as follows:

C H_4 + X_2 \to C H_3 X +  H X

Where X is either Fluorine (F), Chlorine (Cl), Bromine (Br) or sometimes Iodine (I).

This mechanism for this process is called free radical substitution[[3]] and it occurs as follows:

Initiation:

X_2 \to \cdot 2 X

Propagation:

C H_4 + \cdot X \to \cdot C H_3 +  H X
\cdot C H_3 + \cdot X_2 \to C H_3 X + \cdot X

Termination:

2 \cdot X \to X_2
\cdot C H_3 + X \to C H_3 X
\cdot C H_3 + \cdot C H_3 \to C H_3 C H_3

Sources of methane Edit

Methane is a principal component of natural gas.

Large amounts of methane are emitted to atmosphere through Mud volcanoes which are connected with deep geological faults.

Natural gas fieldsEdit

The major source of methane is extraction from geological deposits known as natural gas fields. It is associated with other hydrocarbon fuels and sometimes accompanied by helium and nitrogen. The gas at shallow levels (low pressure) is formed by anaerobic decay of organic matter deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those which give oil generate natural gas.

Alternative sourcesEdit

Apart from gas fields an alternative method of obtaining methane is via biogas generated by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste, or any other biodegradable feedstock, under anaerobic conditions. Industrially, methane can be created from common atmospheric gases and hydrogen (produced, perhaps, by electrolysis) through chemical reactions such as the Sabatier process[[4]], Fischer-Tropsch process[[5]]. Coal bed methane extraction is a method for extracting methane from a coal deposit.

Methane in the earth's atmosphere is an important greenhouse gas with a Global warming potential of 23 over a 100 year period.

Emissions of methaneEdit

Houweling et al. (1999) give the following values for methane emissions [6]:

Origin CH4 emission (Tg/yr)
Natural emissions
Wetlands (incl rice production) 225
Ocean 20
Termites 15
Hydrates 10
Natural total 290
Anthropogenic emissions
Energy 110
Landfills 40
Ruminants 115
Waste treatment 25
Biomass Burning 40
Anthropogenic total 330

Slightly over half of the total emission is due to human activity [7].

Living Plants (e.g. forests) have recently been identified as a potentially important source of methane. The recent paper calculated emissions of 62–236 Tg yr-1, and "this newly identified source may have important implications". [8], [9]. However the authors stress "our findings are preliminary with regard to the methane emission strength".[10]

Uses Edit

Fuel Edit

For more on the use of methane as a fuel, see: natural gas

Methane is an important fuel for electrical generation. Compared to other Hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. Also, Methane's heat of combustion is about 902 kJ/mol, which is lower than any other hydrocarbon, but if a ratio is made with the atomic weight (16 g/mol) divided by the heat of combustion (902 kJ/mol) it is found that methane, being the simplest hydrocarbon, actually produces the most heat per gram than other complex hydrocarbons. In many cities, methane is piped into homes for domestic heating and cooking purposes. In this context it is usually known as natural gas.

Industrial uses Edit

Methane is used in industrial chemical processes and may be transported in liquid or refrigerated liquid form. While leaks from a liquid container are initially heavier than air, the gas is lighter than air. Gas pipelines distribute large amounts of natural gas, of which methane is a significant component.

In the chemical industry, methane is the feedstock of choice for the production of hydrogen, methanol, acetic acid, and acetic anhydride. When used to produce any of these chemicals, methane is first converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. In this process, methane and steam react on a nickel catalyst at high temperatures (700–1100 °C).

C H_4 + H_2 O \to C O + 3 H_2

The ratio of carbon monoxide to hydrogen in synthesis gas can then be adjusted via the water gas shift reaction to the appropriate value for the intended purpose.

C O + H_2 O \overleftarrow{\rightarrow} C O_2 + H_2

Less significant methane-derived chemicals include acetylene, prepared by passing methane through an electric arc, and the chloromethanes (chloromethane, dichloromethane, chloroform, and carbon tetrachloride), produced by reacting methane with chlorine gas. However, the use of these chemicals is declining, acetylene as it is replaced by less costly substitutes, and the chloromethanes due to health and environmental concerns.

See alsoEdit

External linksEdit

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