C2H2 acetylene. Properties of acetylene. Preparation of acetylene.

Structural isomerism

Alkynes are characterized by carbon skeleton isomerism, multiple bond position isomerism, and interclass isomerism.
Structural isomers are compounds with the same composition that differ in the order of bonding of atoms in the molecule, i.e. the structure of molecules.

Carbon skeleton isomers differ in the structure of the carbon skeleton.

Interclass isomers are substances of different classes with different structures, but the same composition. Alkynes are interclass isomers with alkadienes. The general formula of alkynes and alkadienes is CnH2n-2.

Isomers with different triple bond positions differ in the position of the triple bond in the carbon skeleton.

Use of acetylene in welding

Acetylene is the main flammable gas used in gas welding and is also widely used for gas cutting (oxy-fuel cutting). The temperature of the oxy-acetylene flame can reach 3300°C. Thanks to this, acetylene, compared to more accessible flammable gases (propane-butane, natural gas, etc.), provides higher welding quality and productivity.

Stations can be supplied with acetylene for gas welding and cutting

• from acetylene cylinders and
• from an acetylene generator.

To store acetylene, standard cylinders with a capacity of 40 liters, painted white, with the inscription “Acetylene” in red are usually used (PB 10-115-96, GOST 949-73). According to GOST 5457-75, technical dissolved acetylene grade B and gaseous are used for gas-flame processing of metals.

Table. Characteristics of grades of technical acetylene (GOST 5457-75) used in welding and cutting.

The cylinders are filled with a porous mass soaked in acetone. Acetylene is highly soluble in acetone: at normal temperature and pressure, 23 liters of acetylene are dissolved in 1 liter of acetone (5.7 liters of acetylene are dissolved in 1 liter of gasoline, 1.15 liters of acetylene are dissolved in 1 liter of water). The porous mass performs the following functions:

• increases safety when working with a cylinder - due to the porous mass, the total volume of acetylene is divided into separate cells; thus, the probability of propagation of the common front of combustion and explosion is significantly reduced;
• allows you to increase the amount of acetylene in the cylinder, speed up the process of its dissolution when filling the cylinder and releasing it when taking gas - since when using a porous mass impregnated with acetone, a large surface of mutual contact between the gas and acetone is provided.

Activated carbon, pumice, and fibrous asbestos can be used as porous masses.

Table. Permissible gas pressure in the cylinder depending on temperature (at a nominal pressure of 1.9 MPa / +20°C) (GOST 5457-75)

Table. Residual gas pressure in the cylinder supplied from the consumer (GOST 5457-75)

40-liter cylinders with a maximum gas pressure of 1.9 MPa at a temperature of 20°C are usually filled with 5–5.8 kg of acetylene (4.6–5.3 m3 of gas at a temperature of 20°C and a pressure of 760 mm Hg) . The mass of acetylene in the cylinder is determined by the difference in the mass of the cylinder before and after filling with gas. The volume of acetylene is equal to the ratio of its mass and density. Thus, a volume of 5.5 kg of acetylene at a temperature of 20°C and a pressure of 760 mm Hg. Art. is 5.5/1.09 = 5.05 m3.

Table. Comparative characteristics of acetylene, propane and methyl acetylene-allen fraction (MAF)

Storage and transportation of acetylene

Acetylene is produced in accordance with GOST 5457 dissolved and gaseous. It is stored and transported in a dissolved state in special steel cylinders in accordance with GOST 949, filled with a porous mass impregnated with acetone. Acetylene dissolved in acetone is not prone to explosive decomposition.

The cylinders are painted gray with the inscription “ACETYLENE” in red letters on the top cylindrical part.

The maximum pressure of acetylene when filling the cylinder is 2.5 MPa (25 kgf/cm2); when the cylinder settles and cools to 20°C, it decreases to 1.9 MPa (19 kgf/cm2). At this pressure, a 40-liter cylinder holds 5-5.8 kg of C2H2 by mass (4.6-5.3 m3 of gas at 20°C and 760 mm Hg).

The pressure of acetylene in a fully filled cylinder changes with temperature as follows:

Other safety requirements can be found in the article on the hazard class and safety measures when working with acetylene

History of acetylene production

At a meeting of the British Association in Bristol in 1836, Edmund Davy, professor of chemistry at the Royal Society of Dublin and cousin of Humphry Davy, reported:

Davy obtained potassium carbide K2C2 and treated it with water.

In the article on the preparation of calcium carbide, we wrote that its “hydrogen bicarbonate” was first named acetylene by the French chemist Pierre Eugene Marcelin Berthelot in 1860. Only 60 years after Davy’s discovery, his predicted use of acetylene for lighting was the first impetus for its industrial production.

Acetylene reactions

Acetylene burns in concentrations in air from 2.5% to 80% (and almost up to 100% under certain conditions; at a concentration of 100% and the coincidence of certain conditions, acetylene can violently, explosively, self-decompose into carbon and hydrogen), with the formation of very hot, bright and smoky flame. The combustion temperature of acetylene in air or oxygen can reach 3300°C.

In reactions with metals such as copper, silver and mercury, as well as their alloys and salts, acetylene forms acetylenides. For example, silver nitrate reacts with acetylene to form silver acetylenide and nitric acid: 2AgNO3 + C2H2 → Ag2C2 + 2HNO3

Some acetylenides, including the aforementioned silver acetylenide Ag2C2, are strong and dangerous explosives that detonate when heated, as well as from impact. There are known cases where silver acetylide formed at the joints of pipes for transporting acetylene, which were soldered with silver solder.

German chemist Walter Reppe discovered that in the presence of metal catalysts, acetylene can react with many substances, forming industrially significant chemical compounds. These reactions are now called by his name, Reppe reactions.

Reactions of acetylene C2H2 with alcohols ROH, hydrocyanic acid HCN, hydrochloric acid HCl or carboxylic acids produce vinyl compounds. For example, acetylene and hydrochloric acid: C2H2 + HCl →

The reaction of ethylene with carbon monoxide produces acrylic acid or acrylic esters, used in the manufacture of organic glass: C2H2 + CO + H2O → CH2=CHCO2H

The cyclization reaction allows you to convert acetylene to benzene: 3C2H2 → C6H6

What is carbide used for?

Carbides

used in the production of cast iron and steel, ceramics, various alloys, as abrasive and grinding materials, as reducing agents, deoxidizers, catalysts, etc.

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Receipt

In the laboratory

In the laboratory, as well as in gas welding equipment, acetylene is produced by the action of water on calcium carbide (F. Wöhler, 1862),

CaC2+2H2O→Ca(OH)2+C2H2↑{\displaystyle {\mathsf {CaC_{2}+2H_{2}O\rightarrow Ca(OH)_{2}+C_{2}H_{2}\uparrow }}}

as well as during the dehydrogenation of two methane molecules at temperatures above 1400 °C:

2CH4→C2H2+3H2{\displaystyle {\mathsf {2CH_{4}\rightarrow C_{2}H_{2}+3H_{2}}}}

In industry

In industry, acetylene is produced by hydrolysis of calcium carbide and pyrolysis of hydrocarbon feedstocks - methane or propane with butane. In the latter case, acetylene is produced together with ethylene and impurities of other hydrocarbons. The carbide method produces very pure acetylene, but requires high power consumption. Pyrolysis is significantly less energy intensive, because To heat the reactor, combustion of the same working gas in an external loop is used, but in the gas stream of products the concentration of acetylene itself is low. Isolation and concentration of individual acetylene in this case is a difficult task. Economic evaluations of both methods are numerous but contradictory.

Preparation by pyrolysis

Electrocracking

Methane is converted into acetylene and hydrogen in electric arc furnaces (temperature 2000-3000 °C, voltage between electrodes 1000 V). Methane heats up to 1600 °C. Electricity consumption is about 13,000 kWh per 1 ton of acetylene, which is relatively high (approximately equal to the energy consumed by the carbide method) and therefore is a disadvantage of the process. The acetylene yield is 50%.

Regenerative pyrolysis

Another name is the Wolf process. First, the furnace head is heated by burning methane at 1350–1400 °C. Next, methane is passed through the heated nozzle. The residence time of methane in the reaction zone is very short and amounts to a fraction of a second. The process was implemented in industry, but economically it turned out to be not as promising as it was thought at the design stage.

Oxidative pyrolysis

Methane is mixed with oxygen. Part of the raw material is burned, and the resulting heat is used to heat the rest of the raw material to 1600 °C. The yield of acetylene is 30-32%. The method has the advantages of a continuous process and low energy consumption. In addition, synthesis gas is also formed with acetylene. This process (Sachse process or BASF process) has received the most widespread adoption.

Homogeneous pyrolysis

It is a type of oxidative pyrolysis. Part of the raw material is burned with oxygen in the furnace firebox, the gas is heated to 2000 °C. Then the remainder of the raw material, preheated to 600 °C, is introduced into the middle part of the furnace. Acetylene is formed. The method is characterized by greater safety and reliability of the furnace.

Pyrolysis in a low-temperature plasma jet

The process has been developed since the 1970s, but, despite its promise, has not yet been introduced into industry. The essence of the process is heating methane with ionized gas. The advantage of the method is its relatively low energy consumption (5000-7000 kWh) and high yields of acetylene (87% in argon plasma and 73% in hydrogen plasma).

Carbide method

This method has been known since the 19th century, but has not lost its importance to this day. First, calcium carbide is obtained by fusing calcium oxide and coke in electric furnaces at 2500–3000 °C:

CaO+3C→CaC2+CO{\displaystyle {\mathsf {CaO+3C\rightarrow CaC_{2}+CO}}}

Lime is obtained from calcium carbonate:

CaCO3→CaO+CO2{\displaystyle {\mathsf {CaCO_{3}\rightarrow CaO+CO_{2}}}}

Next, calcium carbide is treated with water:

CaC2+2H2O→C2H2+Ca(OH)2{\displaystyle {\mathsf {CaC_{2}+2H_{2}O\rightarrow C_{2}H_{2}+Ca(OH)_{2}}}}

The resulting acetylene has a high degree of purity of 99.9%. The main disadvantage of the process is the high energy consumption: 10,000-11,000 kWh per 1 ton of acetylene.

Receipt

In the laboratory

In the laboratory, acetylene is produced by the action of water on calcium carbide, see video of this process (F. Wöhler, 1862),

as well as during the dehydrogenation of two methane molecules at temperatures above 1400 °C:

In industry

In industry, acetylene is produced from calcium carbide and pyrolysis of hydrocarbon raw materials - methane or propane with butane. In the latter case, acetylene is produced together with ethylene. The carbide method allows one to obtain pure acetylene, but requires high power consumption. Pyrolysis is less energy intensive, but the resulting acetylene has a low concentration in the gas stream and requires separation. Economic evaluations of both methods are numerous but controversial.

Preparation by pyrolysis

Methane is converted into acetylene and hydrogen in electric arc furnaces (temperature 2000-3000°C, voltage between electrodes 1000 V). Methane heats up to 1600°C. Electricity consumption is about 13,000 kWh per 1 ton of acetylene, which is relatively high (approximately equal to the energy consumed by the carbide method) and therefore is a disadvantage of the process. The acetylene yield is 50%.

Another name is the Wolf process. First, the furnace head is heated by burning methane at 1350-1400°C. Next, methane is passed through the heated nozzle. The residence time of methane in the reaction zone is very short and amounts to a fraction of a second. The process was implemented in industry, but economically it turned out to be not as promising as it was thought at the design stage.

Methane is mixed with oxygen. Part of the raw material is burned, and the resulting heat is used to heat the rest of the raw material to 1600°C. The acetylene yield is 30-32%. The method has the advantages of a continuous process and low energy consumption. In addition, synthesis gas is also formed with acetylene. This process (Sachse process or BASF process) has received the most widespread adoption.

It is a type of oxidative pyrolysis. Part of the raw material is burned with oxygen in the furnace firebox, the gas is heated to 2000°C. Then the remainder of the raw material, preheated to 600°C, is introduced into the middle part of the furnace. Acetylene is formed. The method is characterized by greater safety and reliability of the furnace.

Pyrolysis in a low-temperature plasma jet

The process has been developed since the 1970s, but, despite its promise, has not yet been introduced into industry. The essence of the process is heating methane with ionized gas. The advantage of the method is its relatively low energy consumption (5000-7000 kWh) and high yields of acetylene (87% in argon plasma and 73% in hydrogen plasma).

Carbide method

This method has been known since the 19th century, but has not lost its importance to this day. First, calcium carbide is obtained by fusing calcium oxide and coke in electric furnaces at 2500-3000°C:

Lime is obtained from calcium carbonate:

Next, calcium carbide is treated with water:

The resulting acetylene has a high degree of purity of 99.9%. The main disadvantage of the process is the high energy consumption: 10,000-11,000 kWh per 1 ton of acetylene.

Safety precautions when working with acetylene

Acetylene is a colorless flammable gas C2H2 with an atomic mass of 26.04, slightly lighter than air. Has a pungent odor.

Acetylene spontaneously ignites at a temperature of 335°C, a mixture of acetylene with oxygen ignites at a temperature of 297–306°C, and a mixture of acetylene with air ignites at a temperature of 305–470°C.

Acetylene is explosive under the following conditions:

• with an increase in temperature over 450–500° C and pressure over 1.5–2 at (about 150–200 kPa);
• at atmospheric pressure, an acetylene-oxygen mixture containing acetylene from 2.3 to 93% explodes from a spark, flame, strong local heating, etc.;
• under similar conditions, a mixture of acetylene with air explodes when the acetylene content in it is from 2.2 to 80.7%;
• As a result of prolonged contact of acetylene with silver or copper, explosive acetylene silver or copper is formed, which explodes when the temperature rises or when impacted.

An acetylene explosion can cause significant destruction and serious accidents: the explosion of 1 kg of acetylene releases approximately twice as much heat as the explosion of 1 kg of TNT and approximately 1.5 times more than the explosion of 1 kg of nitroglycerin.

Rules

• the acetylene content in the air of the working area must be continuously monitored by automatic devices that signal if the permissible explosion-proof concentration of acetylene in the air is exceeded, equal to 0.46%;
• when working with acetylene cylinders, there should be no open flame or heating system nearby; It is prohibited to work with cylinders that are in a horizontal position, with loose cylinders, or with faulty cylinders; it is necessary to use non-sparking tools, lighting and electrical equipment only in explosion-proof versions;
• if an acetylene leak is detected from the cylinder (by smell and sound), it is necessary to close the cylinder valve as quickly as possible with a special non-sparking key;
• when heated, an acetylene cylinder can explode with extremely destructive consequences; in the event of a fire, it is necessary, if possible, to remove cold acetylene cylinders from the danger zone; the remaining cylinders should be constantly cooled with water or special compounds until they cool completely; when the acetylene coming out of the cylinder catches fire, it is necessary to quickly close the cylinder valve with a special non-sparking key and pour water over the cylinder until it cools completely; In case of a strong fire, fire extinguishing must be done from a safe distance; When extinguishing fires, it is recommended to use fire extinguishers containing a phlegmatizing concentration of nitrogen 70% by volume, carbon dioxide 57% by volume, water jets, sand, compressed nitrogen, asbestos sheet, electrically sprayed foam and water; when extinguishing a strong fire, fireproof suits, gas masks, etc. are used.

Storage and safety precautions

Acetylene should be stored indoors away from oxygen and flammable gases. Do not allow the gas to come into contact with silver or copper. In a storage room, not only the gas content in the air must be controlled. It is advisable to always keep an eye on the pressure and temperature. When the pressure rises to a critical level, acetylene explodes. The same thing happens when the temperature rises.

Where acetylene is stored and used, open flame sources are prohibited. The premises must always be maintained at an optimal temperature. Fires are especially dangerous in such places. If a fire suddenly occurs, acetylene cylinders should be cooled and removed. In some cases, acetylene leaks occur. To avoid an explosion, you need to use a special non-sparking key and close the cylinder.

When extinguishing, it is better to use asbestos cloth, sand, fire extinguishers containing nitrogen and carbon dioxide. In case of severe fires, it is necessary to extinguish the fire from a distance and be sure to use a special protective suit.

To prevent accidents, gas cylinders must be checked regularly. They must be sealed. You can recognize a gas leak by smell and characteristic noise.

Acetylene combustion

Acetylene combustion occurs according to the reaction: C2H2 + 2.5O2 = 2CO2 + H2O + Q1

For complete combustion of 1 m3 of acetylene according to the above reaction, 2.5 m3 of oxygen or = 11.905 m3 of air is theoretically required. In this case, heat is released Q1? 312 kcal/mol. Higher calorific value 1 m3 C2H2 at 0°C and 760 mm Hg. Art., determined in a gas calorimeter, is QB = 14000 kcal/m3 (58660 kJ/m3), which corresponds to the calculated:

312?1.1709?1000/26.036 = 14000 kcal/m3

The lower calorific value under the same conditions can be taken as QH = 13500 kcal/m3 (55890 kJ/m3).

In practice, for combustion in burners with a reducing flame, not 2.5 m3 of oxygen per 1 m3 of acetylene is supplied to the burner, but only from 1 to 1.2 m3, which approximately corresponds to incomplete combustion according to the reaction:

С2H2 + О2 = 2СО + H2 + Q2

where is Q2? 60 kcal/mol or 2300 kcal/kgC2H2. The remaining 1.5-1.3 m3 of oxygen enters the flame from the surrounding air, resulting in the following reaction occurring in the outer shell of the flame:

2CO + H2 + 1.5O2 = 2CO2 + H2O + Q3

The reaction of incomplete combustion of acetylene occurs on the outer shell of the luminous inner cone of flame, and under the influence of high temperature on the inner surface of the cone, C2H2 decomposes into its components according to the reaction:

С2H2 = 2С + H2 + Q4

where Q4?54 kcal/mol or 2070 kcal/kg C2H2.

Thus, the total useful heat output of the flame in relation to welding processes is the sum of the heat released during the decomposition of C2H2 and the heat released during incomplete combustion, which is Q4 + Q2 = 2070 + 2300 = 4370 kcal/kg or 4370? 1.1709 ? 5120 kcal/m3.

When the C2H2 content in the mixture is about 45% (i.e., with an oxygen to acetylene ratio of approximately 1.25), the maximum acetylene combustion temperature is reached, which is 3200°C.

With a content of 27% C2H2, the maximum ignition speed of the acetylene-oxygen mixture is achieved, which is 13.5 m/sec.

Data on the dependences of the ignition speed and flame temperature and on the acetylene content in it are presented in the table below.

It is necessary to understand that complete combustion of the acetylene-air mixture is achieved when it contains no more than 1?100/(1+11.905)=7.75% acetylene (the so-called stoichiometric mixture). In this case, the reaction products are only carbon dioxide (CO2) and water (H2O). When the acetylene content is more than 17.37%, free carbon is released in the form of soot.

With an increase in the percentage of acetyl, the emission of soot also increases (smoking flame), and at 81% C2H2, the combustion process stops or does not occur.

C2H2 acetylene. Properties of acetylene. Preparation of acetylene.

Properties of acetylene.

The most widely used flammable gas for oxy-fuel cutting is acetylene, which is a chemical compound of carbon and hydrogen ( C2H2 ). At normal temperature and pressure, acetylene is in a gaseous state. Chemically pure acetylene is a colorless gas with a weak ethereal odor and a slightly sweet taste. Technical acetylene, used for oxygen cutting, due to the presence of certain impurities in it (hydrogen sulfide, ammonia, hydrogen phosphorous, etc.), has a strong unpleasant odor and is harmful to the human body.

At a pressure above 2 kg/cm2, acetylene in large volumes acquires the properties of an explosive gas and, upon contact with a heated surface or from an electric spark, explodes, with the temperature reaching 3000°C and the pressure increasing by more than 10 times.

With oxygen and air, acetylene forms explosive mixtures that explode when exposed to fire. Acetylene - air mixture is explosive in the presence of from 2.3 to 81% acetylene in the air. Even more dangerous is the acetylene-oxygen mixture, which explodes from fire when the oxygen content is from 2.3 to 93% acetylene, while the speed of propagation of the blast wave reaches 3000 m/sec, so you need to especially carefully monitor the density of all acetylene devices and pipelines etc.

Also read: What are the properties of oxygen. Weight of 1 m3 of oxygen. Weight of liquid oxygen. Methods for obtaining oxygen.

The explosiveness of acetylene is greatly reduced when placed in capillary (very thin) channels. This property of acetylene is used when filling cylinders under pressure - acetylene cylinders are filled with a special porous mass.

Reaction of acetylene with acetone.

Acetylene is highly soluble in many liquids, especially acetone . In one volume of acetone at 15°C and normal pressure, 23 volumes of acetylene are dissolved, and at elevated pressure - proportionally more. This property of acetylene is also used when filling cylinders. The solubility of acetylene in acetone largely depends on temperature: with increasing temperature, the solubility decreases, and with decreasing temperature it increases sharply.

The ability of dissolved acetylene to explode is reduced.

Acetylene combustion reaction.

Acetylene, when burned in a mixture with pure oxygen, produces a flame with a temperature of 3050 - 3150 ° C, and when burned in a mixture with air, it produces a flame with a temperature of 2350 ° C.

Complete combustion of acetylene.

For complete combustion of 1 m3 of acetylene, 2.5 m3 of oxygen or 12.5 m3 of air is required.

Acetylene is lighter than oxygen and air. Specific gravity of acetylene at 0°C and 760 mm Hg. Art. 1.17 kg/m3.

With prolonged interaction with red copper and silver, acetylene produces compounds that explode when heated or upon strong impact. Therefore, in the manufacture of acetylene equipment and fittings, the use of silver solders and red copper in their pure form is prohibited. Only technical copper alloys containing no more than 70% copper are allowed.

Preparation of acetylene.

The main industrial method for producing acetylene is the decomposition of calcium carbide with water. Calcium carbide is a chemical compound of calcium with carbon ( CaC2 ) and is a dark gray or grey-colored solid.

Technical calcium carbide is obtained by fusing quicklime with coal (coke or anthracite). The charge, consisting of a mixture of coal and lime in a certain proportion, is immersed in an electric arc furnace, where, under the influence of the heat of the electric arc, it melts to form calcium carbide.

Molten carbide is poured from the furnace into molds, in which it solidifies. The solidified calcium carbide is then crushed, sorted into pieces of certain sizes and packaged in special drums for storage and transportation. Drums for packaging calcium carbide are made of sheet iron with a thickness of at least 0.5-0.6 mm with a hermetically sealed lid. Corrugations are rolled onto the cylindrical surface of the drums to increase their strength. The drum holds 50-130 kg of calcium carbide.

Carbide drums should be opened very carefully, since for one reason or another moisture may penetrate inside the drum and an explosive acetylene-air mixture may form. It is best to cover the drum cover with a layer of grease and remove it with a special knife . Under no circumstances should you resort to hammer blows.

When uncorking carbide drums, it is strictly forbidden to use a flame or cut out the drum cover with a chisel.

Acetylene explosion

Since acetylene combustion and explosion are possible in the absence of oxidizing agents, including oxygen, it, like hydrogen, is the most explosive gas. The similarity with hydrogen is that they have the lowest ignition energy, but more on that later.

The explosiveness of flammable gases and vapors is characterized by an indicator - the amount of ignition energy. A substance with a low value is more explosive. Ignition energy values ​​(MJ) for stoichiometric gas mixtures at atmospheric pressure and temperature 20°C are given in the table below.

From the data in the table it can be seen that the ignition energy of mixtures with air is approximately 100 times greater than that of mixtures with oxygen.

Pure acetylene is capable of exploding upon rapid heating to 450-500°C and excess pressure over 1.5 kgf/cm2. The most explosive mixtures with air are those containing from 7 to 13% C2H2. If C2H2 is present in a mixture with air from 2.2 to 81% by volume, the mixture explodes at atmospheric pressure.

Also, at atmospheric pressure, a mixture of oxygen and acetylene from 2.8 to 93% by volume is explosive. The most explosive mixture with oxygen contains 30% C2H2. Therefore, strong local heating, a flame, and even a spark can cause an explosion of a mixture of acetylene with oxygen or air.

The explosiveness of pure acetylene is determined by pressure, temperature, and also depends on its purity, moisture content, the presence of catalysts, the nature of the explosion agent, the size and shape of the vessel, heat removal conditions and a number of other reasons. As the pressure increases, the molecules of acetylene gas come closer together, which facilitates the spread of decomposition throughout the entire mass of gas. This is confirmed, on the one hand, by the fact that liquid acetylene, in which the proximity of molecules is especially great, is a highly explosive substance even at ordinary temperatures. On the other hand, compressed acetylene loses its explosiveness if its molecules are in any way separated from each other. This is achieved by mixing acetylene with nitrogen or inert gases that do not interact with it, as well as by absorbing acetylene with acetone or other solvent in the presence of a porous substance. For example, wet acetylene is less explosive than dry acetylene. A mixture containing 1.15 volumes of C2H2 per volume of water vapor is not capable of explosive decomposition. This can be explained similarly to what was said above by the separation of C2H2 molecules by water vapor.

The explosion of acetylene or its mixture with oxygen and air is accompanied by the release of heat, resulting in an increase in pressure and temperature. Consequently, such explosions can cause destruction and accidents. Therefore, handling acetylene requires strict adherence to safety measures.

The pressure generated during an explosion depends on the initial parameters and nature of the explosion, and increases by approximately 10-15 times compared to the initial pressure

When acetylene is dissolved in liquids, its explosiveness decreases. It dissolves best in acetone, but more about this in the article on polymerization and dissolution of acetylene

With prolonged contact of C2H2 with copper, silver and mercury, explosive compounds are formed - acetylenoids. Acetylenoids explode on impact or when heated above 100°C. Therefore, for the manufacture of equipment in contact with acetylene, alloys with a copper content of no more than 70% are used.

That is why all equipment for transporting and storing acetylene is made of steel and has a specific design that excludes the possibility of connecting equipment for other gases.

When chlorine reacts with acetylene containing even a small amount of air, an explosion occurs. Pure acetylene, when in contact with chlorine, explodes under intense light.

Combustion of acetylene water gas

Acetylene is used in industry as a fuel for gas welding and metal cutting, and also as a raw material for various chemical industries.
Acetylene is a chemical compound of carbon and hydrogen. Technical acetylene is a colorless gas with a sharp, characteristic odor. Inhaling it for a long time causes dizziness, nausea and can lead to poisoning. Acetylene is lighter than air and dissolves well in various liquids. It dissolves especially well in acetone. Acetylene, when burned in a mixture with pure oxygen, produces a flame with a temperature of 3050-3150 ° C. It is an explosive gas.

Acetylene explodes under the following conditions:

1) with an increase in temperature above 500 ° C and pressure above 1.5 at\

2) a mixture of acetylene and oxygen with a content of acetylene from 2.8 to 93% explodes at atmospheric pressure from a spark, flame, strong local heating, etc.;

3) under the same conditions, the acetylene-air mixture explodes when it contains from 2.8 to 80.7% acetylene;

4) with prolonged contact of acetylene with copper or silver, explosive acetylene copper or acetylene silver is formed, which explode upon impact or increased temperature.

An acetylene explosion is accompanied by a sharp increase in pressure and temperature and can cause serious accidents and significant damage.

When acetylene is placed in narrow channels, its ability to explode with increasing pressure is significantly reduced. In industry, acetylene is obtained by decomposing calcium carbide with water in special devices - acetylene generators. The technical acetylene obtained in this way usually contains harmful impurities: hydrogen sulfide, ammonia, hydrogen phosphide, silicon hydrogen, which give acetylene a pungent odor and impair the quality of welding. Impurities are removed from acetylene by washing in water and chemical cleaning with special cleaning agents. In addition, acetylene may contain water vapor and mechanical particles (lime and coal dust). To remove moisture, acetylene is dried. Dust is removed using a fabric filter. For welding, acetylene can be taken from an acetylene pipeline coming from an acetylene generator station, or directly from a single-station generator. Acetylene can also be supplied in cylinders under 16 at pressure, dissolved in acetone.

In addition to acetylene, other flammable gases or vapors of flammable liquids can be used when welding and cutting metals: hydrogen, petroleum gas, gasoline vapors, kerosene, etc.

Hydrogen is a flammable, colorless and odorless gas. Hydrogen is one of the lightest gases. The flame temperature during combustion in oxygen is 2300° C. Hydrogen easily ignites and in a certain mixture with oxygen or air produces an explosive mixture, which is called detonating gas. Therefore, when carrying out welding and cutting work with hydrogen, it is necessary to strictly follow safety regulations to avoid explosions. Hydrogen is produced by decomposing water with electric current. It is stored and transported in steel cylinders in gaseous form under a pressure of 150 atm.

Propane-butane mixture is obtained during the extraction and processing of natural petroleum gases and oil. The flame temperature during combustion of the mixture in oxygen reaches 2100° C.

At low pressure, mixtures of propane and butane liquefy. They are stored and transported in steel cylinders with a capacity of 33 and 45 kg under a pressure of up to 16 atm, filled with a liquid mixture to half the volume, since when the cylinder is heated, the pressure can increase significantly, which can lead to an explosion of the cylinder. The mixture is used for cutting, soldering, hardening, welding lead, aluminum and thin steel.

Petroleum gas is a mixture of flammable gases, has an unpleasant odor, and is colorless. Obtained from the processing of oil and petroleum products. The flame temperature during combustion in oxygen is 2300° C. It is stored and transported in a gaseous state in cylinders under a pressure of 150 at. At this pressure it partially liquefies. An oil gas cutting and welding machine requires an evaporator. Used for cutting, soldering, hardening, welding steel up to 2-3 mm thick, welding brass, lead, aluminum.

Coke oven gas is a gaseous mixture of flammable products obtained at coke plants when coke is produced from coal. The flame temperature during combustion in oxygen is about 2000° C.

Delivered to the welding site via a gas pipeline or in cylinders under a pressure of 150 at. Coke oven gases are contaminated with cyanide compounds, which can lead to poisoning. Therefore, they are thoroughly cleaned before use. Used for cutting, soldering and welding low-melting metals.

Methane at normal temperature and pressure is a colorless gas. Methane is found in large quantities in natural gases, where its content reaches 95-98%, the flame temperature during combustion in oxygen is 1850 ° C for Dashavsky and 2000 ° C for Saratov gas.

Natural gases are usually supplied to places of consumption through pipelines and are relatively rarely transported in a gaseous state in cylinders under a pressure of 150 atm. Used for welding low-melting metals, cutting and soldering.

City gas (Moscow) is a mixture of coke oven, oil and natural gases. It is obtained by gasification of solid fuel. The flame temperature during combustion in oxygen is about 2000° C.

Low-melting metals are supplied to places of consumption for cutting and welding through gas pipelines or in compressed form in cylinders under a pressure of 150 at.

Gasoline is a transparent liquid that evaporates easily. Gasoline vapors, when burned in oxygen, give a temperature of 2400° C. Gasoline is obtained by refining oil. Stored and transported in liquid form in vessels at atmospheric pressure. Special equipment is used for welding and cutting. Gasoline is more often used for cutting than for welding.

Kerosene for gas-flame processing is used, like gasoline, in the form of vapor. For this purpose, special burners and cutters equipped with evaporators are used. Kerosene-oxygen flame has a lower temperature (2700°C) than gasoline-oxygen flame. Nevertheless, kerosene is widely used in gas cutting.

It should be borne in mind that all the gases discussed, as well as gasoline vapors, are explosive.

Source

Physical properties

Fig.1. Pi bonds in the acetylene molecule

Under normal conditions, it is a colorless gas, lighter than air. Pure 100% acetylene is odorless. Technical acetylene is stored in cylinders with porous filler impregnated with acetone (since pure acetylene explodes when compressed), and may contain other impurities that give it a pungent odor. Slightly soluble in water, soluble in acetone. Boiling point −83.6 °C. Triple point −80.55 °C at a pressure of 961.5 mmHg. Art., critical point 35.18 °C at a pressure of 61.1 atm.

Acetylene requires great care when handling. May explode on impact, when heated to 500 °C, or when compressed above 0.2 MPa at room temperature

A stream of acetylene released into the open air can ignite from the slightest spark, including from a discharge of static electricity from a finger. To store acetylene, special cylinders filled with porous material impregnated with acetone are used.

Acetylene has been discovered on Uranus and Neptune.

Addition reactions

A triple bond consists of a σ bond and two π bonds. Let’s compare the characteristics of a C–C single bond, a C≡C triple bond, and a C–H bond:

Thus, the C≡C triple bond is shorter than the C–C single bond, so the π-electrons of the triple bond are held more tightly by the nuclei of carbon atoms and have less polarizability and mobility. Triple bond addition reactions to alkynes are more complex than double bond addition reactions to alkenes.

Alkynes are characterized by addition reactions at the C≡C triple bond with the breaking of π bonds.

1.1. Hydrogenation

The hydrogenation of alkynes occurs in the presence of catalysts (Ni, Pt) with the formation of alkenes, and then immediately alkanes.

When using a less active catalyst (Pd, CaCO3, Pb(CH3COO)2), hydrogenation stops at the stage of formation of alkenes.

1.2. Halogenation of alkynes

The addition of halogens to alkynes occurs even at room temperature in solution (solvents - water, CCl4).

Alkynes react similarly with chlorine, but chlorine water does not become discolored, because chlorine water is already colorless)

Reactions occur in the presence of polar solvents according to an ionic (electrophilic) mechanism.

1.3. Hydrohalogenation of alkynes

Alkynes add hydrogen halides. The reaction proceeds by the mechanism of electrophilic addition with the formation of a halogenated alkene or dihaloalkane.

When hydrogen halides and other polar molecules add to symmetrical alkynes, as a rule, one reaction product is formed, where both halogens are located at the same C atom.

When polar molecules add to unsymmetrical alkynes, a mixture of isomers is formed. In this case, Markovnikov's rule is satisfied.

1.4. Alkyne hydration

Hydration (addition of water) of alkynes occurs in the presence of an acid and a catalyst (mercury salt II).

First, an unstable alkene alcohol is formed, which then isomerizes to an aldehyde or ketone.

The hydration of alkynes occurs via an ionic (electrophilic) mechanism.

For unsymmetrical alkenes, the addition of water predominantly follows the Markovnikov rule.

1.5. Dimerization, trimerization and polymerization

The addition of one acetylene molecule to another (dimerization) occurs under the influence of an ammonia solution of copper (I) chloride. This produces vinyl acetylene:

Trimerization of acetylene (the addition of three molecules to each other) occurs under the influence of temperature, pressure and in the presence of activated carbon with the formation of benzene (Zelinsky reaction):

Alkynes also undergo polymerization reactions - the process of repeatedly combining molecules of a low molecular weight substance (monomer) with each other to form a high molecular weight substance (polymer).

nM → Mn (M is a monomer molecule)

… –CH=CH–CH=CH–CH=CH–…

Acetylene

Acetylene was first discovered in 1836 by Davy.

, acting on calcium carbide with water. The French chemist P.E. Berthelot in 1860

year.

Acetylene, or ethylene, is the first representative of unsaturated hydrocarbons with a triple bond

. Its molecular formula is C2H2. Starting with acetylene, we can construct a homologous series

, in which each subsequent hydrocarbon differs from the previous one by one CH2 group.

C3H4 is propyne

, C4H6 – butine

.
Therefore, the general formula of the homologous series of unsaturated hydrocarbons with one triple bond is C n H n -2
.
The common name for this class is alkynes
.

The names of acetylene and its homologues are formed by changing the suffix – an to the suffix – in.

Unlike ethylene, which has a planar structure, acetylene has a linear structure

: all four atoms of the acetylene molecule lie on the same straight line with bond angles equal to 1800. Each carbon atom forms three bonds with carbon atoms and one with hydrogen atoms.

Acetylene under normal conditions

is a colorless gas, a mixture of which with air explodes from a spark. Acetylene is poorly soluble in water, but soluble in acetone: twenty-five volumes of acetylene dissolve in one volume of acetone under normal conditions.

Since acetylene contains multiple bonds, it is characterized by addition reactions.

For example, when one mole of hydrogen is added to one mole of acetylene in the presence of a catalyst, ethylene

.

In this reaction, only one covalent bond in the acetylene molecule is broken.

Ethylene, in turn, can also react with hydrogen in the presence of a catalyst to form ethane

. Therefore, we can write the overall equation for the addition of two moles of hydrogen to one mole of acetylene with the breaking of two covalent bonds and the formation of ethane.

Acetylene also undergoes halogen addition reactions.

For example, when chlorine in an amount of 1 mole is added to acetylene in an amount of one mole, one covalent bond is broken and 1,2-dichloroethene

, if two moles of chlorine are added to acetylene at once, then two covalent bonds are broken and the compound 1,1,2,2-tetrachloroethane

.

In addition to halogens, acetylene undergoes addition reactions with hydrogen halides.

For example, when hydrogen chloride in an amount of one mole is added to acetylene in an amount of one mole, only one covalent bond is broken and chloroethene

.
If two moles of hydrogen chloride are added to acetylene in an amount of one mole, then two bonds are broken and 1,1-dichloroethane
.

When acetylene burns, it produces carbon dioxide and water.

Acetylene is produced in the laboratory

the action of water on calcium carbide CaC2. As a result of this interaction, acetylene and calcium hydroxide are formed.

For example, if you apply water to calcium carbide and then ignite the resulting gas, it burns with a smoky flame. This burns acetylene.

Acetylene is widely used mixed with oxygen

for cutting and welding metals. Acetylene is used in the production of solvents, acetic acid, PVA glue, polyvinyl chloride, and it is also used as fuel.

Let's solve the problem. Calculate what volume of acetylene, under normal conditions, is formed as a result of the interaction with water of 100 g of technical calcium carbide containing 4% impurities.

According to the conditions of the problem, we are given a mass of calcium carbide of 100 g and a mass fraction of impurities in this technical calcium carbide of 4%. You need to find the volume of acetylene. According to the equation, this reaction produces acetylene gas and calcium hydroxide. Let's find the mass fraction of pure calcium carbide; to do this, subtract 4 from 100 and get 96%, therefore the mass of pure calcium carbide is equal to the product of the mass of technical calcium carbide by the mass fraction of pure calcium carbide. It turns out we need to multiply 100 by 0.96%, we get 96 g. We need to find the amount of calcium carbide substance, for this we need to divide its mass by its molar mass. To find the molar mass of calcium carbide, you need to multiply the atomic mass of calcium by one and add it to the product of the atomic mass of carbon multiplied by two, you get 64 g/mol. We find the amount of calcium carbide substance; to do this, divide the mass by the molar mass, substitute the values: 96 g divided by 64 g/mol, we get 1.5 mol. Using the reaction equation, we find the amount of acetylene substance; to do this, multiply 1.5 by 1 and divide by 1, it will be 1.5 mol. Now you can find the volume of acetylene; to do this, you need to multiply the amount of substance by the molar volume. It turns out 1.5 mol multiplied by 22.4 l/mol, it will be 33.6 l. Therefore, as a result of this reaction, 33.6 liters of acetylene are formed.

Thus, unsaturated hydrocarbons with one triple bond (alkynes) form a homologous series with the general formula C n H n -2 . The names of acetylene and its homologues are formed by changing the suffix –an to the suffix –in. Acetylene has a linear structure and forms a bond angle of 1800. The characteristic properties of acetylene are addition reactions.

Acetylene: application in construction and industry

Autogenous and welding works accompany almost all stages of construction. It is in these types of work that acetylene is used. In a special device called a burner, gases are mixed and the combustion reaction itself occurs. The highest temperature of this reaction is achieved when the acetylene content is 45% of the total volume of the cylinder.

Cylinders with this gas are marked as follows: they are painted white and the inscription “Acetylene” is written in large red letters.

Construction work is carried out mainly outdoors. The use of acetylene and its homologues under these conditions should not be exposed to direct sunlight. Short breaks should be accompanied by closing the valves on the burner, and long breaks should be accompanied by closing the valves on the cylinders themselves.

Acetylene is in great demand in the chemical industry. Its application lies in the use of this substance in the process of obtaining organic synthesis products. These are synthetic rubber, plastics, solvents, acetic acid, etc.

Acetylene, being a universal fuel, is often used in processes involving gas-flame processing

It is important that the use of acetylene in industry is possible only if safety measures are observed, since it is an explosive gas