Is methane gas harmful to humans?|| Why Methane is a good fuel

Methane is widely distributed in nature. Methane is the simplest organic substance, which is natural gas, biogas and pit gas the main ingredient, etc., is commonly known as gas. 

The minimum carbon content is (the maximum amount of hydrogen) of a hydrocarbon, which is natural gas the main component, methane, oil field gas and coal gas in the tunnel. It can be used as fuel and raw materials for the production of hydrogen, carbon black, carbon monoxide, acetylene, hydrocyanic acid and formaldehyde.

Methane

On April 2, 2018, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory directly demonstrated for the first time that methane causes an increasing greenhouse effect on the Earth’s surface.

Methane, chemical formula CH4, is the simplest hydrocarbon. It is composed of one carbon and four hydrogen atoms by sp3 hybridization.

Therefore, the structure of the methane molecule is a regular tetrahedron structure, the four bonds have the same bond length and the same angle. . Methane is a colorless and odorless gas under standard conditions. 

The biogas produced when some organic matter decomposes in the absence of oxygen is actually methane. Theoretically, the methane bond line can be expressed as a dot “·”, but this usage has not been seen in practice, probably because the “·” sign can also mean electrons. So in middle school, methane is considered to have no bond wire type.

Methane is mainly used as fuel, such as natural gas and gas, and is widely used in civil and industrial applications. As chemical raw materials, it can be used to produce acetylene, hydrogen, ammonia, carbon black, nitroxymethane, carbon disulfide, mono chloromethane, dichloromethane, chloroform, carbon tetrachloride, and hydrocyanic acid.

Physical properties

colour colorless
Melting point -182.5 ℃
Boiling point -161.5 ℃
Solubility (normal temperature and pressure) 0.03
Molecular Structure Regular tetrahedral nonpolar molecule
Molecular diameter 0.414nm
Relative density (water = 1) 0.42 (-164 ℃)
Relative density (air = 1) 0.5548 (273.15K, 101325Pa)
Critical temperature (℃) -82.6
Critical pressure (MPa) 4.59
Upper explosion limit% (V / V) 15.4
Lower explosion limit% (V / V) 5.0
Flash point (℃) -188
Ignition temperature (℃) 538
Heat of combustion 890.31KJ / mol
Total heating value (product liquid water) 55900kJ / kg (40020kJ / m3)
Net calorific value (product gaseous water) 50200kJ / kg (35900kJ / m3)
H—C—H bond angle 109 ° 28 ′
C-H 413kJ / mol
Crystal type Molecular crystal
IUPAC name methane
Alias Natural gas, biogas, biogas
Solubility (water) 3.5 mg / 100 mL (17 ° C)
Molar mass 16.043 g / mol
Warning term R: R12
Safety term S: S2-S9-S16-S33
Density (standard case) 0.717g / L
Special properties Extremely difficult to dissolve in water

Chemical Properties and Reactions

Under normal circumstances, methane is relatively stable, does not react with strong oxidants such as potassium permanganate, and does not react with strong acids and alkalis. But under certain conditions, some reactions of methane also occur.

Substitution Reaction

Methane is mainly chlorinated and brominated. The reaction between methane and fluorine is exothermic. Once the reaction occurs, a large amount of heat is difficult to remove, destroying the generated fluoromethane, and only carbon and hydrogen fluoride are obtained. Therefore, the direct fluorination reaction is difficult to achieve and needs to be diluted with a rare gas. The reaction between iodine and methane requires

With higher activation energy, the reaction is difficult to proceed. Therefore, iodine cannot directly react with methane to generate methyl iodide. But its reverse reaction is easy to carry out.

Take chlorination as an example: you can see that the yellow-green gas of chlorine in the test tube gradually fades, and white mist is generated. Oily droplets are formed on the inner wall of the test tube.

This is the monochloro-methane and dichloromethane produced by the reaction of methane and chlorine A mixture of methane, chloroform (or chloroform), carbon tetrachloride (or tetrachloromethane), hydrogen chloride, and a small amount of ethane (impurities).

  • CH4 + Cl2 → (light) CH3 Cl (gas) + HCl
  • CH3 Cl + Cl2 → (light) CH2 Cl2 (oil) + HCl
  • CH2 Cl2 + Cl2 → (light) CHCl3 (oil) + HCl
  • CHCl3 + Cl2 → (light) CCl4 (oil) + HCl

The liquid level rose in the test tube, and white crystals precipitated in the saline solution. This is because the hydrogen chloride generated in the reaction was dissolved in water. Because hydrogen chloride is easily soluble in water, the concentration of chloride ions in the water is increased after dissolving in water, so that sodium chloride crystals are precipitated. 

Hold the test tube nozzle with your thumb and raise the liquid level. With the nozzle up, drop purple litmus test solution or zinc particles into the test tube to verify that it is dilute hydrochloric acid.

If the amount of chlorine is controlled, a large amount of methane is used to obtain mainly methyl chloride; if a large amount of chlorine is used, carbon tetrachloride is mainly obtained. Industrially, the mixture is separated one by one by rectification. The above-chlorinated products are all important solvents and reagents.

Features: ① No reaction occurs in a dark place at room temperature;

The reaction occurs at 250°C;

can react under the effect of light at room temperature;

Use light to initiate the reaction, and absorb a photon to generate thousands of methyl chloride molecules;

If there is oxygen or some impurities that can capture free radicals, the reaction has an induction period. The length of the induction period is related to the presence of these impurities.

According to the characteristics of the above facts, it can be judged that the chlorination of methane is a radical type of substitution reaction. As shown in the figure.

Oxidation reaction

The most basic oxidation reaction of methane is combustion:

CH4 + 2O2 → CO2 + 2H2O

The hydrogen content of methane is the highest of all hydrocarbons, reaching 25%, so gaseous hydrocarbons of the same mass are completely burned, and the methane consumption is the highest.

Ignite pure methane, place a dry beaker above the flame, and soon you can see water vapor condensing on the beaker wall. Invert the beaker, add a small amount of clear lime water, shake, and the lime water becomes cloudy. It shows that the combustion of methane produces water and carbon dioxide. 

Collect methane gas in a tall glass tube, stand upright on the table, remove the glass sheet, and quickly put the burning spoon containing the burning candle into the tube. Light blue flame. This shows that methane can burn quietly in the air, but does not support combustion. 

Using a large test tube to drain the water, first input 2/3 volume of oxygen from the oxygen tank and then pass 1/3 volume of methane. Plug it with a rubber stopper and take it out of the water. Invert the test tube several times to fully mix the gas. 

Cover the test tube with a cloth, tilt the test tube mouth slightly, remove the plug, and quickly ignite the test tube mouth with a burning wooden stick, that is, a sharp crackling sound occurs. Although this experiment is simple, it is also prone to failure. 

Ignite the methane emitted from the mouth of the glass tube, put it in a bottle filled with chlorine gas, and the methane will continue to burn, emitting a red-yellow flame, while seeing black smoke and white mist. Black smoke is carbon black, and white mist is a drop of hydrochloric acid mist formed by hydrogen chloride gas and water vapor.

Thermal decomposition

Under the condition of blocking air and heating to 1000 ℃, methane decomposes to produce carbon black and hydrogen

CH4 = (1000 ℃) = C + 2H2

Hydrogen is a raw material for industries such as ammonia and gasoline, carbon black is a raw material for the rubber industry

Hydrate formation

Methane can form cage-like hydrates, and methane is enclosed in “cages.” This is what we often call flammable ice. It is ice-like, non- stoichiometric, non- stoichiometric, composed of water and natural gas when mixed under medium and high pressure and low-temperature conditions under appropriate conditions (appropriate temperature, pressure, gas saturation, water salinity, PH value, etc.) Cage-shaped crystalline compounds (carbon has a large electronegativity and can attract similar hydrogen atoms to form hydrogen bonds under high pressure to form a cage structure). 

It can be expressed by m CH4 · nH2Om represents the gas molecules in the hydrate, and n is the hydration index (that is, the number of water molecules).

Combustible ice is mainly stored in the permafrost zone on the ocean floor or in cold areas, which is difficult to find and explore. The newly developed highly sensitive instrument can measure the precise content of various trace amounts of methane, ethane, propane and hydrogen in the soil and rocks on the seafloor in real-time, so as to determine the existence of combustible ice resources and the number of resources.

Natural gas hydrates with a methane content of over 99% are also called methane hydrates.

Greenhouse effect

On April 2, 2018, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory used the comprehensive observations of the Earth’s atmosphere obtained for ten years from the Southern Great Plains Observatory of Oklahoma to directly demonstrate for the first time that methane causes greenhouses on the surface of the earth. The effect is increasing.

Researchers said that at the beginning of the 21st century, the concentration of methane in the atmosphere stagnated, and the greenhouse effect followed the same pattern; but since 2007, as the concentration of methane began to rise, the greenhouse effect caused by it also rose. 

Main application

Methane is an important fuel and the main component of natural gas, accounting for about 87%. In a standard pressure room temperature environment, methane is colorless and odorless; the special taste of domestic natural gas is an artificial odor added for safety, usually using methyl mercaptan or ethyl mercaptan. 

In an atmosphere of atmospheric pressure, the boiling point of methane is −161°C. As long as the gas content in the air exceeds 5% to 15%, it is very flammable. Liquefied methane will not burn unless in a high-pressure environment (usually 4 to 5 atmospheres). Chinese national standards stipulate that methane cylinders are brown and white.

Carbon black can be obtained by pyrolysis of methane, which is used as an additive for pigments, inks, paints and rubbers; chloroform and CCl4 are important solvents. Methane is widely distributed in nature and is one of the main components of natural gas, biogas and pit gas. It can be used as fuel and raw materials for the production of hydrogen, carbon monoxide, carbon black, acetylene, hydrocyanic acid and formaldehyde.

Methane is used as a standard fuel for heating value testing of water heaters and gas stoves. Production of standard and calibration gases for flammable gas alarms. It can also be used as a carbon source for the vapor deposition of solar cells and amorphous silicon films. And methane is used as a raw material for pharmaceutical chemical synthesis.

In addition to being used as fuel, it is widely used in the synthesis of ammonia, urea, and carbon black. It can also be used to produce methanol, hydrogen, acetylene, ethylene, formaldehyde, carbon disulfide, nitromethane, hydrocyanic acid, and 1,4-butanediol. Methane can be chlorinated to obtain mono-, di-, trichloromethane and carbon tetrachloride.

Preparation of Methane

Methane is a flammable gas and can be manufactured artificially, so after the oil is used up, it will become an important energy source.

Its main sources are:

  1. Decomposition of organic waste.
  2. Natural sources (such as swamps): 23%.
  3. Extracted from fossil fuels: 20%.
  4. Animal (such as cattle) digestion process: 17%.
  5. Bacteria in rice fields: 12%.
  6. Anaerobic heating or burning of biological substances.

The main methane production methods are as follows:

Bacterial decomposition

The organic matter into biogas tank, control the temperature and humidity, the methane bacteria multiply rapidly, the decomposition of organic matter to methane, carbon dioxide, hydrogen, hydrogen sulfide, carbon monoxide, methane, wherein 60% -70%. After low-temperature liquefaction, methane can be extracted to produce cheap methane.

Synthetic method

The carbon dioxide and hydrogen are reacted by a catalyst to generate methane and oxygen, and then purified.

CO2 + 2H2 = CH4 + O2

Reacts carbon vapor with hydrogen directly, and high purity methane can also be obtained.

Laboratory method

Anhydrous sodium acetate (CH3COONa) and soda lime (NaOH and CaO as desiccant)

Reaction equation: CH3COONa + NaOH === Na2CO3 + CH4 ↑

Collection: drainage method (cannot be collected by downward air method)

Features and precautions:

  1. Methane must be prepared by reacting anhydrous sodium acetate with dry soda lime. If sodium acetate crystals or lime are not dried, methane gas will hardly be produced.
  2. The operating precautions and collection methods for this experiment are exactly the same as for oxygen.

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