Acetylene, preparation, properties, and chemical reactions.
Acetylene, C2H2 – organic matter class, alkynes, unsaturated hydrocarbons.
Acetylene (also – jene) – organic matter class, alkynes, unsaturated hydrocarbonconsisting of two carbon atoms and two hydrogen atoms.
The chemical formula of acetylene is C2H2. Structural formula of acetylene, CH≡CH. Isomers has not.
The structure of acetylene molecule:
Acetylene has a triple bond between atoms of carbon.
Acetylene – a colorless gas, without taste and smell. However, the technical acetylene contains impurities – phosphorous hydrogen, hydrogen sulfide, etc., which give it a pungent smell.
Easier air. Density compared to the density of the air of 0.9.
A very flammable gas. Flammable and explosive.
Acetylene is among the few compounds, combustion and explosion which is possible in the absence of oxygen or other oxidizing agents.
Mixtures of acetylene with air is explosive in a very wide range of concentrations. Explosion hazards are reduced when diluted acetylene other gases, for example nitrogen, methane or propane.
Acetylene requires great care in handling. May explode from shock, when heated to 500 °C or when compressed above 0.2 MPa at room temperature. A stream of acetylene released into outdoor air, can ignite from the smallest spark, including static electricity discharge from the finger. Storage of acetylene using special cylindersfilled with porous material soaked with acetone. They acetylene is stored in solution with acetone.
Slightly soluble in water. Very soluble in acetone. Soluble in other organic substances (gasoline, benzene, etc.)
Acetylene has a slight toxic effect.
|Aggregate state (at 20 °C and atmospheric pressure of 1 ATM.)||gas|
|Density (at 20 °C and atmospheric pressure of 1 ATM.) kg/m3||1,0896|
|Density (at 0 °C and atmospheric pressure of 1 ATM.) kg/m3||1,173|
|Melting point, °C||-80,8|
|Boiling point, °C||-80,55|
|Triple point, °C||335|
|The auto-ignition temperature, °C||335|
|The auto-ignition pressure, MPa||0,14-0,16|
|Critical temperature*, °C||Of 35.94|
|Critical pressure, MPa||6,26|
|Explosive concentration of the mixture of gas with air, % by volume||2.1 to 100|
|Specific heat of combustion, MJ/kg||56,9|
|Flame temperature, °C||3150-3200|
|Molar mass, g/mol||26,038|
* at temperatures above the critical temperature the gas cannot be condensed at any pressure.
Chemical properties of acetylene are similar to properties of other representatives of a number of alkynes. So it is characterized by the following chemical reactions:
1. galogenirovannami acetylene:
CH≡CH + Br2 → CHBr=CHBr (1,2-dibromethane);
CHBr=CHBr + Br2 → CHBr2-CHBr2 (1,1,2,2-tetrabromoethane).
The reaction proceeds by stages with the formation of derivatives of alkanes.
During this reaction, acetylene discolor bromine water.
2. hydrohalogenation acetylene:
CH≡CH + HBr → CH2=CHBr (bromate).
3. hydration of acetylene (reaction Michael G. Kucherov, 1881):
CH≡CH + H2O → [CH2=CH-OH] (enol) → CH3-CH=O (acetaldehyde ) (kat = HgSO4, Hg(NO3)2).
4. trimerization of acetylene (reaction Nikolaya Dmitrievicha zelinskogo, 1927):
3СН≡CH → C6H6 (benzene) (kat = activated carbon, to = 450-500 OS).
The reaction of the trimerization of acetylene is a special case of the reaction of polymerization of acetylene occurs by passing acetylene over activated charcoal at a temperature of 450-500 OS.
5. dimerization of acetylene:
CH≡CH + CH≡CH → CH2=CH-C≡CH (obtained vinyl acetylene) (kat = aqueous solution of CuCl and NH 4 CL).
The reaction of dimerization of acetylene is a special case of the reaction of polymerization of acetylene.
6. the combustion of acetylene:
2SN≡CH + 5О2 → 4СО2 + 2Н2О.
Acetylene burns with a white bright flame.
7. the oxidation of acetylene.
The reaction and its products are determined by the environment in which it occurs.
8. recovery of acetylene:
CH≡CH + H2 → C2H4 (ethylene) (kat = Ni, Pd or Pt, increased to);
CH≡CH + 2Н2 → C2H6 (ethane) (kat = Ni, Pd or Pt, increased to).
Acetylene in the laboratory is the result of the following chemical reactions:
1. the action of water on calcium carbide:
CaC2 + H2O → CA(Oh)2 + C2H2.
2. dehydrogenation of methane:
2CH4 → C2H2 + 3H2 (subject to > 1500 OC).
3. dehydrogenation of ethylene:
CH2=CH2→ CH≡CH +H2 (kat = Pt, Ni, Al2O3, Cr2O3, to = 400-600 °C).
Acetylene in the industry receive in the following ways and methods:
4. carbide method:
First get lime from calcium carbonate.
CaCO3 → Cao + CO2. (to = 900-1200 oC).
Then get a carbide of calcium, fusing calcium oxide and coke in an electric furnace at a temperature of 2500-3000 °C.
CaO + 3C → CaC2 + CO. (to = 2500-3000 OS).
Further, calcium carbide is treated with water at a known reaction.
CaC2 + H2O → CA(Oh)2 + C2H2.
The result is a acetylene of high purity is 99.9 %.
5. high-temperature cracking of methane:
High-temperature cracking of methane is carried out according to known reaction degidrirovaniya of methane in an electric arc furnace at a temperature of 2000-3000 °C and a voltage between the electrodes of 1000 V. the Output of acetylene is 50 %.
6. the various modes of pyrolysis of methane:
A type of high-temperature cracking of methane are regenerative pyrolysis (Wulff process), oxidative pyrolysis (Sachs-process or BASF process), homogeneous pyrolysis, pyrolysis in the medium of low-temperature plasma.
Thus, during the regenerative pyrolysis first burn methane and heated up the nozzle of the furnace to 1350-1400 °C. Then through a heated nozzle to cut off flow of the methane, thereby forming acetylene.
In the course of oxidative pyrolysis of methane is mixed with oxygen and burned. The resulting heat serves to heat the remainder of the methane to 1600 °C, which digidrive in acetylene. The yield of acetylene is 30-32 %.
During the homogeneous pyrolysis of methane and oxygen is burned in a furnace at a temperature of 2000 °C. Then, preheated to 600 °C, the remainder of the methane is passed through the furnace, thereby forming acetylene.
During pyrolysis in an environment of low-temperature plasma methane is heated by a jet of ionized gas (argon or hydrogen).
– as a raw material in the chemical industry for the production of acetic acid, ethyl alcohol, solvents, plastics, synthetic rubbers, aromatic hydrocarbons,
– for gas welding and cutting metals,
– for technical carbon,
– as a source of very bright, white light in the Autonomous lamps, where it is obtained by the reaction of calcium carbide and water.
Explosiveness of acetylene and safety in handling:
Acetylene has explosive properties.
Therefore, treatment with acetylene requires strict adherence to safety rules.
Acetylene burns and explodes even in the absence of oxygen and other oxidizing agents.
Mixtures of acetylene with air is explosive in a very wide range of concentrations.
A jet of acetylenereleased into the open air, can ignite from the smallest spark, including static electricity discharge from the finger.
Vzryvaet acetylene depends on many factors: pressure, temperature, purity of acetylene, the content of moisture, the presence of catalysts and other substances and other reasons.
The ignition temperature of acetylene at normal – atmospheric pressure in the range 500-600 °C. When the pressure significantly decreases the ignition temperature of acetylene. Thus, at a pressure of 2 kgf/cm2 (0.2 MPa, 1,935682 ATM.) the ignition temperature of acetylene is equal to 630 °C. And at a pressure of 22 kgf/cm2 (2.2 MPa, 21,292502 ATM.) the ignition temperature of acetylene equal to 350 °C.
The presence of acetylene of particles of different substances increase the surface contact and thus reduces the ignition temperature at atmospheric pressure. For example, activated carbon lowers the ignition temperature of acetylene to 400 °C, the hydrate of iron oxide (rust) – up to 280-300 °C, iron shavings – up to 520 °C, brass shavings up to 500-520 °C, carbide of calcium, up to 500 °C, aluminium oxide – up to 490 °C, copper shavings – 460 °C, the iron oxide – 280 °C, copper oxide up to 250 °C.
Explosiveness of acetylene decreases with dilution of acetylene with other gases, for example nitrogen, methane or propane.
Under certain conditions acetylene reacts with copper, silver and mercury to form explosive compounds. Therefore, in the manufacture of acetylene equipment (e.g., valves, cylinders) do not use alloyscontaining more than 70 % Cu.
Storage and transportation of acetylene, using special steel cylinders white color (with a red inscription “A”) filled with an inert porous material (e.g., charcoal). Thus acetylene is stored and transported in these bags in the form of a solution of acetylene in acetone under pressure of 1.5-2.5 MPa.
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