Unique properties of the carbon atom. Physical and chemical properties of carbon

DEFINITION

Carbon- the sixth element of the Periodic table. Designation - C from the Latin "carboneum". Located in the second period, IVA group. Refers to non-metals. The nuclear charge is 6.

Carbon is found in nature both in the free state and in the form of numerous compounds. Free carbon occurs as diamond and graphite. In addition to fossil coal, there are large accumulations of oil in the bowels of the Earth. Huge amounts of carbonic acid salts, especially calcium carbonate, are found in the earth's crust. There is always carbon dioxide in the air. Finally, plant and animal organisms consist of substances in the formation of which carbon takes part. Thus, this element is one of the most common on Earth, although its total content in the earth's crust is only about 0.1% (wt.).

Atomic and molecular weight of carbon

The relative molecular weight of a substance (M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and the relative atomic mass of an element (Ar r) is how many times the average mass of atoms of a chemical element is greater than 1/12 the mass of a carbon atom.

Since in the free state carbon exists in the form of monatomic C molecules, the values ​​of its atomic and molecular masses are the same. They are equal to 12.0064.

Allotropy and allotropic modifications of carbon

In the free state, carbon exists in the form of diamond, which crystallizes in the cubic and hexagonal (lonsdaleite) systems, and graphite, which belongs to the hexagonal system (Fig. 1). Forms of carbon such as charcoal, coke or soot have a disordered structure. There are also allotropic modifications obtained synthetically - these are carbine and polycumulene - carbon varieties built from linear chain polymers of the -C= C- or = C = C= type.

Rice. 1. Allotropic modifications of carbon.

Allotropic modifications of carbon are also known, which have the following names: graphene, fullerene, nanotubes, nanofibers, astralene, glassy carbon, colossal nanotubes; amorphous carbon, carbon nanobuds and carbon nanofoam.

Isotopes of carbon

In nature, carbon exists in the form of two stable isotopes 12 C (98.98%) and 13 C (1.07%). Their mass numbers are 12 and 13, respectively. The nucleus of the 12 C carbon isotope contains six protons and six neutrons, and the 13 C isotope contains the same number of protons and five neutrons.

There is one artificial (radioactive) carbon isotope, 14 C, with a half-life of 5730 years.

carbon ions

At the outer energy level of the carbon atom, there are four electrons that are valence:

1s 2 2s 2 2p 2 .

As a result of chemical interaction, carbon can lose its valence electrons, i.e. be their donor, and turn into positively charged ions or accept electrons from another atom, i.e. be their acceptor, and turn into negatively charged ions:

C 0 -2e → C 2+;

C 0 -4e → C 4+;

C 0 +4e → C 4-.

Molecule and carbon atom

In the free state, carbon exists in the form of monatomic C molecules. Here are some properties that characterize the carbon atom and molecule:

Alloys of carbon

The best-known carbon alloys around the world are steel and cast iron. Steel is an alloy of iron and carbon, the carbon content of which does not exceed 2%. In cast iron (also an alloy of iron with carbon), the carbon content is higher - from 2 to 4%.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Carbon (from Latin: carbo "coal") is a chemical element with the symbol C and atomic number 6. Four electrons are available to form covalent chemical bonds. The substance is non-metallic and tetravalent. Three isotopes of carbon occur naturally, 12C and 13C are stable, and 14C is a decaying radioactive isotope with a half-life of about 5730 years. Carbon is one of the few elements known since antiquity. Carbon is the 15th most abundant element in the earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium and oxygen. The abundance of carbon, the unique diversity of its organic compounds, and its unusual ability to form polymers at temperatures commonly found on Earth allow this element to serve as a common element for all known life forms. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen. Carbon atoms can bind in different ways, while being called allotropes of carbon. The best known allotropes are graphite, diamond and amorphous carbon. The physical properties of carbon vary widely depending on the allotropic form. For example, graphite is opaque and black, while diamond is very transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek verb "γράφειν" meaning "to write"), while diamond is the hardest material known in nature. Graphite is a good electrical conductor, while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes and graphene have the highest thermal conductivity of any known material. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically stable and require high temperatures to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, and +2 in carboxyl complexes of carbon monoxide and transition metal. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant amounts come from organic deposits of coal, peat, oil and methane clathrates. Carbon forms an enormous number of compounds, more than any other element, with nearly ten million compounds described to date, and yet this number is only a fraction of the number theoretically possible under standard conditions. For this reason, carbon is often referred to as the "king of the elements."

Characteristics

Allotropes of carbon include graphite, one of the softest substances known, and diamond, the hardest natural substance. Carbon readily bonds to other small atoms, including other carbon atoms, and is capable of forming numerous stable covalent bonds with suitable multivalent atoms. Carbon is known to form nearly ten million different compounds, the vast majority of all chemical compounds. Carbon also has the highest sublimation point of any element. At atmospheric pressure, it has no melting point as its triple point is 10.8 ± 0.2 MPa and 4600 ± 300 K (~4330 °C or 7820 °F), so it sublimates at about 3900 K. Graphite is much more reactive than diamond under standard conditions despite being more thermodynamically stable as its delocalized pi system is much more vulnerable to attack. For example, graphite can be oxidized with hot concentrated nitric acid under standard conditions to C6(CO2H)6 mellitic acid, which retains the graphite's hexagonal units when the larger structure is destroyed. The carbon is sublimated in a carbon arc, which is about 5800 K (5,530 °C, 9,980 °F). Thus, regardless of its allotropic form, carbon remains solid at higher temperatures than the highest melting points such as tungsten or rhenium. Although carbon is thermodynamically prone to oxidation, it is more resistant to oxidation than elements such as iron and copper, which are weaker reducing agents at room temperature. Carbon is the sixth element with the ground state electron configuration 1s22s22p2, of which the four outer electrons are valence electrons. Its first four ionization energies of 1086.5, 2352.6, 4620.5 and 6222.7 kJ/mol, are much higher than those of the heavier group 14 elements. The electronegativity of carbon is 2.5, which is significantly higher than the heavier elements of group 14 (1.8-1.9), but is close to most neighboring non-metals, as well as to some transition metals of the second and third rows. The covalent radii of carbon are usually taken as 77.2 pm (C-C), 66.7 pm (C=C) and 60.3 pm (C≡C), although these can vary depending on the coordination number and what it is associated with. carbon. In general, the covalent radius decreases as the coordination number decreases and the bond order increases. Carbon compounds form the basis of all known life forms on Earth, and the carbon-nitrogen cycle provides some of the energy released by the Sun and other stars. Although carbon forms an extraordinary variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperatures and pressures, carbon will withstand all but the strongest oxidizers. It does not react with sulfuric acid, hydrochloric acid, chlorine or alkalis. At elevated temperatures, carbon reacts with oxygen to form carbon oxides and removes oxygen from metal oxides, leaving the elemental metal. This exothermic reaction is used in the steel industry to melt iron and control the carbon content of steel:

Fe3O4 + 4 C (s) → 3 Fe (s) + 4 CO (g)

with sulfur to form carbon disulfide and with steam in the coal-gas reaction:

C(s) + H2O(g) → CO(g) + H2(g)

Carbon combines with some metals at high temperatures to form metal carbides, such as iron carbide cementite in steel and tungsten carbide, widely used as an abrasive and for making hard tips for cutting tools. The system of carbon allotropes covers a number of extremes:

Some types of graphite are used for thermal insulation (such as fire barriers and heat shields), but some other forms are good thermal conductors. Diamond is the best known natural thermal conductor. Graphite is opaque. Diamond is very transparent. Graphite crystallizes in the hexagonal system. Diamond crystallizes in the cubic system. Amorphous carbon is completely isotropic. Carbon nanotubes are among the best known anisotropic materials.

Allotropes of carbon

Atomic carbon is a very short-lived species and therefore carbon is stabilized in various polyatomic structures with various molecular configurations called allotropes. The three relatively well-known allotropes of carbon are amorphous carbon, graphite, and diamond. Previously considered exotic, fullerenes are now commonly synthesized and used in research; they include buckyballs, carbon nanotubes, carbon nanodots, and nanofibers. Several other exotic allotropes have also been discovered, such as lonsaletite, glassy carbon, carbon nanofaum, and linear acetylenic carbon (carbine). As of 2009, graphene is considered the strongest material ever tested. The process of separating it from graphite will require some further technological development before it becomes economical for industrial processes. If successful, graphene could be used to build space elevators. It can also be used to safely store hydrogen for use in hydrogen-based vehicles in vehicles. The amorphous form is a set of carbon atoms in a non-crystalline, irregular, glassy state, and not contained in a crystalline macrostructure. It is present in powder form and is the main component of substances such as charcoal, lamp soot (soot) and activated carbon. At normal pressures, carbon has the form of graphite, in which each atom is trigonally bonded by three other atoms in a plane composed of fused hexagonal rings, as in aromatic hydrocarbons. The resulting network is two-dimensional and the resulting flat sheets are folded and freely connected through weak van der Waals forces. This gives graphite its softness and splitting properties (sheets slide easily over each other). Due to the delocalization of one of the outer electrons of each atom to form a π cloud, graphite conducts electricity, but only in the plane of each covalently bonded sheet. This results in a lower electrical conductivity for carbon than for most metals. Delocalization also explains the energy stability of graphite over diamond at room temperature. At very high pressures, carbon forms a more compact allotrope, diamond, which has almost twice the density of graphite. Here, each atom is tetrahedrally connected to four others, forming a three-dimensional network of wrinkled six-membered rings of atoms. Diamond has the same cubic structure as silicon and germanium, and because of the strength of its carbon-carbon bonds, it is the hardest natural substance as measured by scratch resistance. Contrary to popular belief that "diamonds are forever", they are thermodynamically unstable under normal conditions and turn into graphite. Due to the high energy activation barrier, the transition to the graphite form is so slow at normal temperature that it is not noticeable. Under certain conditions, carbon crystallizes as a lonsaleite, a hexagonal crystal lattice with all atoms covalently bonded and properties similar to those of diamond. Fullerenes are a synthetic crystalline formation with a graphite-like structure, but instead of hexagons, fullerenes are composed of pentagons (or even heptagons) of carbon atoms. The missing (or extra) atoms deform the sheets into spheres, ellipses, or cylinders. The properties of fullerenes (divided into buckyballs, buckytubes, and nanobads) have not yet been fully analyzed and represent an intense area of ​​nanomaterials research. The names "fullerene" and "buckyball" are associated with the name of Richard Buckminster Fuller, who popularized geodesic domes that resemble the structure of fullerenes. Buckyballs are rather large molecules formed entirely of carbon bonds trigonally, forming spheroids (the most famous and simplest is C60 baksinisterfellerene with the shape of a soccer ball). Carbon nanotubes are structurally similar to buckyballs, except that each atom is trigonally bonded in a curved sheet that forms a hollow cylinder. Nanobads were first introduced in 2007 and are hybrid materials (buckyballs are covalently bonded to the outer wall of a nanotube) that combine the properties of both in a single structure. Of the other allotropes discovered, carbon nanofoam is a ferromagnetic allotrope discovered in 1997. It consists of a clustered assembly of low-density carbon atoms strung together in a loose three-dimensional network in which the atoms are trigonally linked in six- and seven-membered rings. It is among the lightest solids with a density of about 2 kg/m3. Similarly, glassy carbon contains a high proportion of closed porosity, but unlike regular graphite, the graphite layers are not stacked like pages in a book, but are more randomly arranged. Linear acetylenic carbon has the chemical structure - (C:::C) n-. The carbon in this modification is linear with sp orbital hybridization and is a polymer with alternating single and triple bonds. This carbine is of significant interest for nanotechnology because its Young's modulus is forty times greater than that of the hardest material, diamond. In 2015, a team at the University of North Carolina announced the development of another allotrope, which they called Q-carbon, created by a low-duration, high-energy laser pulse on amorphous carbon dust. Q-carbon is reported to exhibit ferromagnetism, fluorescence, and has a hardness superior to diamonds.

Prevalence

Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium and oxygen. Carbon is abundant in the Sun, stars, comets, and the atmospheres of most planets. Some meteorites contain microscopic diamonds that were formed when the solar system was still a protoplanetary disk. Microscopic diamonds can also form under intense pressure and high temperature at meteorite impact sites. In 2014, NASA announced an updated database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. More than 20% of the carbon in the universe can be associated with PAHs, complex compounds of carbon and hydrogen without oxygen. These compounds appear in the world PAH hypothesis, where they presumably play a role in abiogenesis and the formation of life. It appears that PAHs were formed "a couple of billion years" after the Big Bang, are widespread in the universe, and are associated with new stars and exoplanets. The hard shell of the earth is estimated to contain 730 ppm of carbon in total, with 2000 ppm in the core and 120 ppm in the combined mantle and crust. Since the mass of the earth is 5.9 x 72 x 1024 kg, this would mean 4360 million gigatonnes of carbon. This is much more than the amount of carbon in the oceans or the atmosphere (below). Combined with oxygen in carbon dioxide, carbon is found in the Earth's atmosphere (approximately 810 gigatons of carbon) and dissolved in all bodies of water (approximately 36,000 gigatons of carbon). There are about 1900 gigatons of carbon in the biosphere. Hydrocarbons (such as coal, oil, and natural gas) also contain carbon. Coal "reserves" (rather than "resources") are about 900 gigatons with perhaps 18,000 Gt of resources. Oil reserves are about 150 gigatons. Proven sources of natural gas are about 175,1012 cubic meters (containing about 105 gigatons of carbon), however studies estimate another 900,1012 cubic meters of "unconventional" deposits such as shale gas, which is about 540 gigatons of carbon. Carbon has also been found in methane hydrates in the polar regions and under the seas. According to various estimates, the amount of this carbon is 500, 2500 Gt, or 3000 Gt. In the past, the amount of hydrocarbons was greater. According to one source, between 1751 and 2008, about 347 gigatonnes of carbon were released into the atmosphere as carbon dioxide into the atmosphere from the burning of fossil fuels. Another source adds the amount added to the atmosphere between 1750 to 879 Gt, and the total in the atmosphere, sea and land (such as peat bogs) is almost 2000 Gt. Carbon is a component (12% by mass) of very large masses of carbonate rocks (limestone, dolomite, marble, etc.). Coal contains a very high amount of carbon (anthracite contains 92-98% carbon) and is the largest commercial source of mineral carbon, accounting for 4,000 gigatons or 80% of fossil fuels. In terms of individual carbon allotropes, graphite is found in large quantities in the United States (mainly New York and Texas), Russia, Mexico, Greenland, and India. Natural diamonds are found in rock kimberlite contained in ancient volcanic "necks" or "pipes". Most diamond deposits are found in Africa, especially in South Africa, Namibia, Botswana, the Republic of the Congo and Sierra Leone. Diamond deposits have also been found in Arkansas, Canada, the Russian Arctic, Brazil, and Northern and Western Australia. Now diamonds are also recovered from the ocean floor at the Cape of Good Hope. Diamonds occur naturally, but about 30% of all industrial diamonds used in the US are now produced. Carbon-14 is formed in the upper troposphere and stratosphere at altitudes of 9-15 km in a reaction that is deposited by cosmic rays. Thermal neutrons are produced that collide with nitrogen-14 nuclei to form carbon-14 and a proton. Thus, 1.2 × 1010% of atmospheric carbon dioxide contains carbon-14. Carbon-rich asteroids are relatively dominant in the outer parts of the asteroid belt in our solar system. These asteroids have not yet been directly explored by scientists. Asteroids could be used in hypothetical space-based coal mining, which may be possible in the future but is currently technologically impossible.

Isotopes of carbon

Isotopes of carbon are atomic nuclei that contain six protons plus a number of neutrons (from 2 to 16). Carbon has two stable naturally occurring isotopes. The isotope carbon-12 (12C) forms 98.93% of the carbon on Earth, and carbon-13 (13C) forms the remaining 1.07%. The concentration of 12C increases even more in biological materials because biochemical reactions discriminate against 13C. In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the isotopic carbon-12 as the basis for atomic weights. Identification of carbon in experiments with nuclear magnetic resonance (NMR) is carried out with the 13C isotope. Carbon-14 (14C) is a natural radioisotope created in the upper atmosphere (lower stratosphere and upper troposphere) by the interaction of nitrogen with cosmic rays. It is found in trace amounts on Earth at up to 1 part per trillion (0.0000000001%), primarily in the atmosphere and surface sediments, particularly peat and other organic materials. This isotope decays during 0.158 MeV β-emission. Due to the relatively short half-life of 5730 years, 14C is virtually absent from ancient rocks. In the atmosphere and in living organisms, the amount of 14C is almost constant, but decreases in organisms after death. This principle is used in radiocarbon dating, invented in 1949, which has been widely used to age carbonaceous materials up to 40,000 years old. There are 15 known isotopes of carbon and the shortest lifetime of them is 8C, which decays by proton emission and alpha decay and has a half-life of 1.98739 × 10-21 s. Exotic 19C exhibits a nuclear halo, meaning that its radius is significantly larger than what would be expected if the nucleus were a sphere of constant density.

Education in the stars

The formation of an atomic carbon nucleus requires an almost simultaneous triple collision of alpha particles (helium nuclei) inside the core of a giant or supergiant star, which is known as the triple alpha process, since the products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium -8 respectively, both of which are very unstable and decay almost instantly back into smaller nuclei. This occurs at temperatures over 100 megacalvins and helium concentrations, which is unacceptable in the conditions of the rapid expansion and cooling of the early universe, and therefore no significant amounts of carbon were created during the Big Bang. According to the modern theory of physical cosmology, carbon is formed inside stars in a horizontal branch by the collision and transformation of three helium nuclei. When these stars die in a supernova, the carbon is scattered into space as dust. This dust becomes the constituent material for the formation of second or third generation star systems with accreted planets. The solar system is one such star system with an abundance of carbon, allowing life as we know it to exist. The CNO cycle is an additional fusion mechanism that drives stars where carbon acts as a catalyst. Rotational transitions of various isotopic forms of carbon monoxide (for example, 12CO, 13CO, and 18CO) are detected in the submillimeter wavelength range and are used in the study of newly forming stars in molecular clouds.

carbon cycle

Under terrestrial conditions, the conversion of one element to another is a very rare phenomenon. Therefore, the amount of carbon on Earth is effectively constant. Thus, in processes that use carbon, it must be obtained from somewhere and disposed of elsewhere. The pathways of carbon in the environment form the carbon cycle. For example, photosynthetic plants extract carbon dioxide from the atmosphere (or sea water) and build it into biomass, as in the Calvin cycle, the process of carbon fixation. Some of this biomass is eaten by animals, while some of the carbon is exhaled by animals as carbon dioxide. The carbon cycle is much more complex than this short cycle; for example, some carbon dioxide is dissolved in the oceans; if bacteria do not absorb it, dead plant or animal matter can become oil or coal, which releases carbon when burned.

Carbon compounds

Carbon can form very long chains of interlocking carbon-carbon bonds, a property called chain formation. Carbon-carbon bonds are stable. Through katanation (formation of chains), carbon forms an innumerable number of compounds. Evaluation of unique compounds shows that more of them contain carbon. A similar statement can be made for hydrogen because most organic compounds also contain hydrogen. The simplest form of an organic molecule is the hydrocarbon, a large family of organic molecules that are made up of hydrogen atoms bonded to a chain of carbon atoms. Chain length, side chains, and functional groups affect the properties of organic molecules. Carbon is found in every form of known organic life and is the basis of organic chemistry. When combined with hydrogen, carbon forms various hydrocarbons that are important to industry as refrigerants, lubricants, solvents, as chemical feedstocks for the production of plastics and petroleum products, and as fossil fuels. When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds, including sugars, lignans, chitins, alcohols, fats and aromatic esters, carotenoids, and terpenes. With nitrogen, carbon forms alkaloids, and with the addition of sulfur it also forms antibiotics, amino acids and rubber products. With the addition of phosphorus to these other elements, it forms DNA and RNA, the carriers of the chemical code of life, and adenosine triphosphate (ATP), the most important energy transport molecule in all living cells.

inorganic compounds

Typically, carbon-containing compounds that are associated with minerals or that do not contain hydrogen or fluorine are treated separately from classical organic compounds; this definition is not strict. Among them are simple oxides of carbon. The best known oxide is carbon dioxide (CO2). Once a major constituent of the paleoatmosphere, this matter is today a minor constituent of the Earth's atmosphere. When dissolved in water, this substance forms carbonic acid (H2CO3), but, like most compounds with several single-bonded oxygens on one carbon, it is unstable. However, resonant stabilized carbonate ions are formed through this intermediate. Some important minerals are carbonates, especially calcites. Carbon disulfide (CS2) is similar. Another common oxide is carbon monoxide (CO). It is formed during incomplete combustion and is a colorless, odorless gas. Each molecule contains a triple bond and is fairly polar, which results in it constantly binding to hemoglobin molecules, displacing oxygen, which has a lower binding affinity. Cyanide (CN-) has a similar structure but behaves like a halide ion (pseudohalogen). For example, it can form a cyanogen nitride (CN) 2 molecule similar to diatom halides. Other unusual oxides are carbon suboxide (C3O2), unstable carbon monoxide (C2O), carbon trioxide (CO3), cyclopentane peptone (C5O5), cyclohexanehexone (C6O6) and mellitic anhydride (C12O9). With reactive metals such as tungsten, carbon forms either carbides (C4-) or acetylides (C2-2) to form alloys with high melting points. These anions are also associated with methane and acetylene, both of which are very weak acids. At an electronegativity of 2.5, carbon prefers to form covalent bonds. Several carbides are covalent lattices, such as carborundum (SiC), which resembles diamond. However, even the most polar and salt-like carbides are not fully ionic compounds.

Organometallic compounds

Organometallic compounds, by definition, contain at least one carbon-metal bond. There is a wide range of such compounds; major classes include simple alkyl-metal compounds (eg tetraethyl elide), η2-alkene compounds (eg Zeise salt) and η3-allylic compounds (eg allylpalladium chloride dimer); metallocenes containing cyclopentadienyl ligands (eg ferrocene); and carbene complexes of transition metals. There are many metal carbonyls (for example, nickel tetracarbonyl); some workers believe that the carbon monoxide ligand is a purely inorganic, not organometallic, compound. While carbon is thought to exclusively form four bonds, an interesting compound has been reported containing an octahedral hexacoordinate carbon atom. The cation of this compound is 2+. This phenomenon is explained by the aurophilicity of gold ligands. In 2016, hexamethylbenzene was confirmed to contain a carbon atom with six bonds rather than the usual four.

History and etymology

The English name carbon (carbon) comes from the Latin carbo, meaning "charcoal" and "charcoal", hence the French word charbon, which means "charcoal". The German, Dutch, and Danish names for carbon are Kohlenstoff, koolstof, and kulstof, respectively, all literally meaning a coal substance. Carbon was discovered in prehistoric times and was known in the forms of soot and charcoal in the earliest human civilizations. Diamonds were known probably as early as 2500 BC. in China, and carbon in the form of charcoal was made in Roman times by the same chemistry as it is today, by heating wood in a clay-covered pyramid to exclude air. In 1722, René Antoine Ferhot de Réamour demonstrated that iron is converted into steel through the absorption of some substance now known as carbon. In 1772, Antoine Lavoisier showed that diamonds are a form of carbon; when he burned samples of charcoal and diamond and found that neither produced any water, and that both substances released an equal amount of carbon dioxide per gram. In 1779, Carl Wilhelm Scheele showed that graphite, thought to be a form of lead, was instead identical to charcoal but with a small amount of iron, and that it produced "air acid" (which is carbon dioxide) when oxidized with nitric acid. In 1786, French scientists Claude Louis Berthollet, Gaspard Monge, and C. A. Vandermonde confirmed that graphite was essentially carbon, by oxidizing it in oxygen in much the same way that Lavoisier did with diamond. Some iron remained again, which, according to French scientists, was necessary for the structure of graphite. In their publication, they proposed the name carbone (Latin for carbonum) for an element in graphite that was released as a gas when the graphite was burned. Antoine Lavoisier then listed carbon as an element in his 1789 textbook. A new allotrope of carbon, fullerene, which was discovered in 1985, includes nanostructured forms such as buckyballs and nanotubes. Their discoverers - Robert Curl, Harold Kroto and Richard Smalley - received the Nobel Prize in Chemistry in 1996. The resulting renewed interest in new forms leads to the discovery of additional exotic allotropes, including glassy carbon, and the realization that "amorphous carbon" is not strictly amorphous.

Production

Graphite

Commercially viable natural graphite deposits occur in many parts of the world, but the most economically important sources are in China, India, Brazil, and North Korea. Graphite deposits are metamorphic in origin, found in association with quartz, mica, and feldspars in shales, gneisses, and metamorphosed sandstones and limestones in the form of lenses or veins, sometimes several meters or more thick. Graphite stocks at Borrowdale, Cumberland, England were at the beginning of sufficient size and purity that until the 19th century pencils were made simply by sawing blocks of natural graphite into strips before pasting the strips into wood. Today, smaller graphite deposits are obtained by crushing the parent rock and floating the lighter graphite on water. There are three types of natural graphite - amorphous, flake or crystalline. Amorphous graphite is of the lowest quality and is the most common. In contrast to science, in industry "amorphous" refers to a very small crystal size rather than a complete lack of crystalline structure. The word "amorphous" is used to refer to products with a low amount of graphite and is the cheapest graphite. Large deposits of amorphous graphite are found in China, Europe, Mexico and the USA. Planar graphite is less common and of higher quality than amorphous; it looks like separate plates that crystallize in metamorphic rocks. The price of granular graphite can be four times the price of amorphous. Good quality flake graphite can be processed into expandable graphite for many applications such as flame retardants. Primary graphite deposits are found in Austria, Brazil, Canada, China, Germany and Madagascar. Liquid or lump graphite is the rarest, most valuable and highest quality type of natural graphite. It is found in veins along intrusive contacts in hard lumps and is only commercially mined in Sri Lanka. According to the USGS, global production of natural graphite in 2010 was 1.1 million tons, with China producing 800,000 tons, India 130,000 tons, Brazil 76,000 tons, North Korea 30,000 tons, and Canada, 25,000 tons. No natural graphite was mined in the United States, but 118,000 tons of synthetic graphite was mined in 2009 at an estimated cost of $998 million.

Diamond

The supply of diamonds is controlled by a limited number of businesses and is also highly concentrated in a small number of locations around the world. Only a very small proportion of diamond ore is made up of real diamonds. The ore is crushed, during which care must be taken to prevent the destruction of large diamonds in this process, and then the particles are sorted by density. Today, diamonds are mined in the diamond-rich fraction using X-ray fluorescence, after which the final sorting steps are performed manually. Prior to the spread of the use of X-rays, the separation was carried out using lubricating tapes; it is known that diamonds have only been found in alluvial deposits in southern India. It is known that diamonds are more likely to stick to the mass than other minerals in the ore. India was the leader in the production of diamonds from their discovery around the 9th century BC until the middle of the 18th century AD, but the commercial potential of these sources was exhausted by the end of the 18th century, by which time India was swamped by Brazil, where the first diamonds were found. in 1725. Diamond production of primary deposits (kimberlites and lamproites) began only in the 1870s, after the discovery of diamond deposits in South Africa. Diamond production has increased over time, with only 4.5 billion carats accumulated since that date. About 20% of this amount has been mined in the last 5 years alone, and during the last ten years, 9 new deposits have started production, and 4 more are waiting to be discovered soon. Most of these deposits are located in Canada, Zimbabwe, Angola and one in Russia. In the United States, diamonds have been discovered in Arkansas, Colorado, and Montana. In 2004, a startling discovery of a microscopic diamond in the United States led to the release in January 2008 of a mass sampling of kimberlite pipes in a remote part of Montana. Today, the majority of commercially viable diamond deposits are in Russia, Botswana, Australia and the Democratic Republic of the Congo. In 2005, Russia produced nearly one-fifth of the world's diamond supply, according to the British Geological Survey. In Australia, the richest diamonded pipe reached peak production levels of 42 metric tons (41 tons, 46 short tons) per year in the 1990s. There are also commercial deposits, which are actively mined in the Northwest Territories of Canada, Siberia (mainly in Yakutia, for example, in the Mir Pipe and the Udachnaya Pipe), in Brazil, as well as in Northern and Western Australia.

Applications

Carbon is essential for all known living systems. Without it, life as we know it cannot exist. The main economic uses of carbon other than food and wood are hydrocarbons, primarily fossil fuels methane gas and crude oil. Crude oil is processed by refineries to produce gasoline, kerosene and other products. Cellulose is a naturally occurring carbon-containing polymer produced by plants in the form of wood, cotton, flax and hemp. Cellulose is mainly used to maintain the structure of plants. Commercially valuable animal-based carbon polymers include wool, cashmere, and silk. Plastics are made from synthetic carbon polymers, often with oxygen and nitrogen atoms incorporated at regular intervals into the polymer backbone. The raw material for many of these synthetics comes from crude oil. The use of carbon and its compounds is extremely diverse. Carbon can form alloys with iron, the most common of which is carbon steel. Graphite combines with clays to form the "lead" used in pencils used for writing and drawing. It is also used as a lubricant and pigment as a molding material in glass manufacture, in electrodes for dry batteries and electroplating and electroforming, in brushes for electric motors, and as a neutron moderator in nuclear reactors. Charcoal is used as a material for making art, as a barbecue grill, for smelting iron, and for many other uses. Wood, coal and oil are used as fuel for energy production and for heating. High quality diamonds are used in jewelry making, while industrial diamonds are used for drilling, cutting and polishing metal and stone working tools. Plastics are made from fossil hydrocarbons, and carbon fiber, made from the pyrolysis of synthetic polyester fibers, is used to reinforce plastics into advanced, lightweight composite materials. Carbon fiber is made by pyrolyzing extruded and stretched filaments of polyacrylonitrile (PAN) and other organic materials. The crystal structure and mechanical properties of the fiber depend on the type of starting material and subsequent processing. Carbon fibers made from PAN have a structure resembling narrow filaments of graphite, but heat treatment can reorder the structure into a continuous sheet. As a result, the fibers have a higher specific tensile strength than steel. Carbon black is used as a black pigment in printing inks, artists' oil paint and watercolors, carbon paper, automotive trim, inks, and laser printers. Carbon black is also used as a filler in rubber products such as tires and in plastic compounds. Activated carbon is used as an absorbent and adsorbent in filter media in applications as diverse as gas masks, water treatment, and cooker hoods, and in medicine to absorb toxins, poisons, or gases from the digestive system. Carbon is used in chemical reduction at high temperatures. Coke is used to reduce iron ore in iron (smelting). Solidification of steel is achieved by heating finished steel components in carbon powder. Silicon, tungsten, boron and titanium carbides are among the hardest materials and are used as cutting and grinding abrasives. Carbon compounds make up most of the materials used in clothing, such as natural and synthetic textiles and leather, and almost all interior surfaces in environments other than glass, stone, and metal.

diamonds

The diamond industry is divided into two categories, one is high quality diamonds (gems) and the other is industrial grade diamonds. While there is a lot of trading in both types of diamonds, the two markets operate quite differently. Unlike precious metals such as gold or platinum, gemstone diamonds are not traded as a commodity: there is a substantial markup on the sale of diamonds and the resale market for diamonds is not very active. Industrial diamonds are valued mainly for their hardness and thermal conductivity, while the gemological qualities of clarity and color are largely irrelevant. About 80% of mined diamonds (equal to about 100 million carats or 20 tons per year) are unusable and are used in industry (diamond scrap). Synthetic diamonds, invented in the 1950s, found industrial applications almost immediately; 3 billion carats (600 tons) of synthetic diamonds are produced annually. The dominant industrial use of diamond is cutting, drilling, grinding and polishing. Most of these applications do not require large diamonds; in fact, most gem-quality diamonds, with the exception of small-sized diamonds, can be used in industry. Diamonds are inserted into drill tips or saw blades, or ground into a powder for use in grinding and polishing. Specialized applications include use in laboratories as storage for high pressure experiments, high performance bearings, and limited use in specialized windows. Thanks to advances in the production of synthetic diamonds, new applications are becoming feasible. Much attention has been given to the possible use of diamond as a semiconductor suitable for microchips and because of its exceptional thermal conductivity as a heat sink in electronics.

Structure of a diamond (A) and graphite (b)

Carbon(Latin carboneum) - C, a chemical element of the IV group of the periodic system of Mendeleev, atomic number 6, atomic mass 12.011. It occurs in nature in the form of crystals of diamond, graphite or fullerene and other forms and is part of organic (coal, oil, animal and plant organisms, etc.) and inorganic substances (limestone, baking soda, etc.). Carbon is widespread, but its content in the earth's crust is only 0.19%.

Carbon is widely used in the form of simple substances. In addition to precious diamonds, which are the subject of jewelry, industrial diamonds are of great importance - for the manufacture of grinding and cutting tools. Charcoal and other amorphous forms of carbon are used for decolorization, purification, adsorption of gases, in areas of technology where adsorbents with a developed surface are required. Carbides, compounds of carbon with metals, as well as with boron and silicon (for example, Al 4 C 3, SiC, B 4 C) are highly hard and are used to make abrasive and cutting tools. Carbon is present in steels and alloys in the elemental state and in the form of carbides. Saturation of the surface of steel castings with carbon at high temperature (carburizing) significantly increases the surface hardness and wear resistance.

Historical reference

Graphite, diamond and amorphous carbon have been known since antiquity. It has long been known that other material can be marked with graphite, and the very name "graphite", which comes from the Greek word meaning "to write", was proposed by A. Werner in 1789. However, the history of graphite is confused, often substances with similar external physical properties were mistaken for it. , such as molybdenite (molybdenum sulfide), at one time considered graphite. Among other names of graphite, "black lead", "iron carbide", "silver lead" are known.

In 1779, K. Scheele found that graphite can be oxidized with air to form carbon dioxide. For the first time, diamonds found use in India, and in Brazil, precious stones acquired commercial importance in 1725; deposits in South Africa were discovered in 1867.

In the 20th century The main diamond producers are South Africa, Zaire, Botswana, Namibia, Angola, Sierra Leone, Tanzania and Russia. Artificial diamonds, the technology of which was created in 1970, are produced for industrial purposes.

Properties

Four crystalline modifications of carbon are known:

• graphite,
• diamond,
• carbine,
• lonsdaleite.

Graphite- gray-black, opaque, oily to the touch, scaly, very soft mass with a metallic sheen. At room temperature and normal pressure (0.1 MN/m2, or 1 kgf/cm2), graphite is thermodynamically stable.

Diamond- very solid, crystalline substance. Crystals have a cubic face-centered lattice. At room temperature and normal pressure, diamond is metastable. A noticeable transformation of diamond into graphite is observed at temperatures above 1400°C in vacuum or in an inert atmosphere. At atmospheric pressure and a temperature of about 3700 ° C, graphite sublimates.

Liquid carbon can be obtained at pressures above 10.5 MN/m2 (105 kgf/cm2) and temperatures above 3700°C. Solid carbon (coke, soot, charcoal) is also characterized by a state with a disordered structure - the so-called "amorphous" carbon, which is not an independent modification; its structure is based on the structure of fine-grained graphite. Heating some varieties of "amorphous" carbon above 1500-1600 ° C without air causes their transformation into graphite.

The physical properties of "amorphous" carbon depend very strongly on the dispersion of particles and the presence of impurities. Density, heat capacity, thermal conductivity and electrical conductivity of "amorphous" carbon is always higher than graphite.

Carbine obtained artificially. It is a finely crystalline powder of black color (density 1.9-2 g / cm 3). Built from long chains of atoms WITH laid parallel to each other.

Lonsdaleite found in meteorites and obtained artificially; its structure and properties have not been finally established.

Alloys

Steel

Coke is used in metallurgy as a reducing agent. Charcoal - in forges, to obtain gunpowder (75% KNO 3 + 13% C + 12% S), to absorb gases (adsorption), as well as in everyday life. Soot is used as a rubber filler, for the manufacture of black paints - printing ink and ink, as well as in dry galvanic cells. Glassy carbon is used for the manufacture of equipment for highly aggressive environments, as well as in aviation and astronautics.

Activated charcoal absorbs harmful substances from gases and liquids: they fill gas masks, purification systems, it is used in medicine for poisoning.

Carbon is the basis of all organic substances. Every living organism is made up largely of carbon. Carbon is the basis of life. The source of carbon for living organisms is usually CO 2 from the atmosphere or water. As a result of photosynthesis, it enters biological food chains in which living things eat each other or the remains of each other and thereby extract carbon to build their own body. The biological cycle of carbon ends either with oxidation and return to the atmosphere, or with disposal in the form of coal or oil.

The use of the radioactive isotope 14 C contributed to the success of molecular biology in studying the mechanisms of protein biosynthesis and the transmission of hereditary information. Determination of the specific activity of 14 C in carbonaceous organic remains makes it possible to judge their age, which is used in paleontology and archeology.

Sources

Carbon (chemical symbol - C) is a chemical element of the 4th group of the main subgroup of the 2nd period of the periodic system of Mendeleev, serial number 6, the atomic mass of the natural mixture of isotopes is 12.0107 g / mol.

At ordinary temperatures, carbon is chemically inert, at sufficiently high temperatures it combines with many elements, and exhibits strong reducing properties. The chemical activity of different forms of carbon decreases in the following order: amorphous carbon, graphite, diamond, in air they ignite at temperatures above 300-500 °C, 600-700 °C and 850-1000 °C, respectively.

Isotopes:
Natural carbon consists of two stable isotopes - 12C (98.892%) and 13C (1.108%) and one radioactive isotope 14C (β-emitter, T½ = 5730 years) concentrated in the atmosphere and the upper part of the earth's crust. It is constantly formed in the lower layers of the stratosphere as a result of the action of cosmic radiation neutrons on nitrogen nuclei by the reaction: 14N (n, p) 14C, and also, since the mid-1950s, as a man-made product of nuclear power plants and as a result of testing hydrogen bombs.
The formation and decay of 14C is the basis of the radiocarbon dating method, which is widely used in Quaternary geology and archeology.

Allotropy:
The electron orbitals of a carbon atom can have different geometries, depending on the degree of hybridization of its electron orbitals. There are three basic geometries of the carbon atom.

Tetrahedral, formed by mixing one s- and three p-electrons (sp3 hybridization). The carbon atom is located in the center of the tetrahedron, connected by four equivalent σ-bonds to carbon atoms or others at the vertices of the tetrahedron. This geometry of the carbon atom corresponds to the allotropic modifications of carbon diamond and lonsdaleite. Carbon has such hybridization, for example, in methane and other hydrocarbons.

Trigonal, formed by mixing one s- and two p-electron orbitals (sp² hybridization). The carbon atom has three equivalent σ-bonds located in the same plane at an angle of 120° to each other. The p-orbital, which is not involved in hybridization and is located perpendicular to the plane of σ-bonds, is used to form π-bonds with other atoms. This geometry of carbon is typical for graphite, phenol, etc.
- digonal, formed by mixing one s- and one p-electrons (sp-hybridization). In this case, two electron clouds are elongated along the same direction and look like asymmetric dumbbells. The other two p-electrons form π-bonds. Carbon with such a geometry of the atom forms a special allotropic modification - carbine.

Oxidation states +4, −4, rarely +2 (CO, metal carbides), +3 (C2N2, halocyanates); electron affinity 1.27 eV; the ionization energy in the successive transition from C0 to C4+ is 11.2604, 24.383, 47.871, and 64.19 eV, respectively.

Chemical properties of carbon
Interaction with fluorine
Carbon has a low reactivity; of the halogens, it reacts only with fluorine:

C + 2F2 = CF4.

Interaction with oxygen
When heated, it interacts with oxygen:

2C + O2 = 2CO,

C + O2 = CO2,

forming oxides of CO and CO2.

Interaction with other nonmetals
Reacts with sulfur:

does not interact with nitrogen and phosphorus.

Reacts with hydrogen in the presence of a nickel catalyst to form methane:

Interaction with metals
Able to interact with metals, forming carbides:
Ca + 2C = CaC2.

Interaction with water
When water vapor is passed through hot coal, carbon monoxide (II) and hydrogen are formed:
C + H2O = CO + H2.

Restorative properties
Carbon is capable of reducing many metals from their oxides:
2ZnO + C = 2Zn + CO2.

Concentrated sulfuric and nitric acids, when heated, oxidize carbon to carbon monoxide (IV):

C + 2H2SO4 = CO2 + 2SO2 + 2H2O;
C + 4HNO3 = CO2 + 4NO2 + 2H2O.

MOU "Nikiforovskaya secondary school No. 1"

Carbon and its main inorganic compounds

Essay

Completed by: student of class 9B

Sidorov Alexander

Teacher: Sakharova L.N.

Dmitrievka 2009

Introduction

Chapter I. All About Carbon

1.1. carbon in nature

1.2. Allotropic modifications of carbon

1.3. Chemical properties of carbon

1.4. Application of carbon

Chapter II. Inorganic carbon compounds

Conclusion

Literature

Introduction

Carbon (lat. Carboneum) C is a chemical element of Group IV of the Mendeleev periodic system: atomic number 6, atomic mass 12.011(1). Consider the structure of the carbon atom. There are four electrons in the outer energy level of the carbon atom. Let's graph it:

Carbon has been known since ancient times, and the name of the discoverer of this element is unknown.

At the end of the XVII century. Florentine scientists Averani and Targioni tried to fuse several small diamonds into one large one and heated them with the help of burning glass with the sun's rays. The diamonds disappeared after burning in the air. In 1772, the French chemist A. Lavoisier showed that CO 2 is formed during the combustion of diamond. Only in 1797, the English scientist S. Tennant proved the identity of the nature of graphite and coal. After burning equal amounts of coal and diamond, the volumes of carbon monoxide (IV) turned out to be the same.

The variety of carbon compounds, which is explained by the ability of its atoms to combine with each other and with atoms of other elements in various ways, determines the special position of carbon among other elements.

Chapter I . All about carbon

1.1. carbon in nature

Carbon is found in nature both in the free state and in the form of compounds.

Free carbon occurs as diamond, graphite, and carbine.

Diamonds are very rare. The largest known diamond - "Cullinan" was found in 1905 in South Africa, weighed 621.2 g and measured 10 × 6.5 × 5 cm. The Diamond Fund in Moscow holds one of the largest and most beautiful diamonds in world - "Orlov" (37.92 g).

The diamond got its name from the Greek. "adamas" - invincible, indestructible. The most significant diamond deposits are located in South Africa, Brazil, and Yakutia.

Large deposits of graphite are located in Germany, in Sri Lanka, in Siberia, in Altai.

The main carbon-bearing minerals are: magnesite MgCO 3, calcite (lime spar, limestone, marble, chalk) CaCO 3, dolomite CaMg (CO 3) 2, etc.

All fossil fuels - oil, gas, peat, hard and brown coal, shale - are built on a carbon basis. Close in composition to carbon are some fossil coals containing up to 99% C.

Carbon accounts for 0.1% of the earth's crust.

In the form of carbon monoxide (IV) CO 2 carbon is part of the atmosphere. A large amount of CO 2 is dissolved in the hydrosphere.

1.2. Allotropic modifications of carbon

Elemental carbon forms three allotropic modifications: diamond, graphite, carbine.

1. Diamond is a colorless, transparent crystalline substance that refracts light rays extremely strongly. Carbon atoms in diamond are in a state of sp 3 hybridization. In the excited state, the valence electrons in the carbon atoms are depaired and four unpaired electrons are formed. When chemical bonds are formed, electron clouds acquire the same elongated shape and are located in space so that their axes are directed towards the vertices of the tetrahedron. When the tops of these clouds overlap with clouds of other carbon atoms, covalent bonds appear at an angle of 109°28", and an atomic crystal lattice is formed, which is characteristic of diamond.

Each carbon atom in a diamond is surrounded by four others located from it in directions from the center of the tetrahedra to the vertices. The distance between atoms in tetrahedra is 0.154 nm. The strength of all bonds is the same. Thus, the atoms in a diamond are "packed" very tightly. At 20°C, the density of diamond is 3.515 g/cm 3 . This explains its exceptional hardness. Diamond is a poor conductor of electricity.

In 1961, the industrial production of synthetic diamonds from graphite began in the Soviet Union.

In the industrial synthesis of diamonds, pressures of thousands of MPa and temperatures from 1500 to 3000°C are used. The process is carried out in the presence of catalysts, which can be some metals, such as Ni. The bulk of the formed diamonds are small crystals and diamond dust.

Diamond, when heated without access to air above 1000 ° C, turns into graphite. At 1750°C, the transformation of diamond into graphite occurs rapidly.

Structure of a diamond

2. Graphite is a gray-black crystalline substance with a metallic sheen, greasy to the touch, inferior in hardness even to paper.

Carbon atoms in graphite crystals are in a state of sp 2 hybridization: each of them forms three covalent σ bonds with neighboring atoms. The angles between the bond directions are 120°. The result is a grid composed of regular hexagons. The distance between adjacent nuclei of carbon atoms within the layer is 0.142 nm. The fourth electron of the outer layer of each carbon atom in graphite occupies a p-orbital, which is not involved in hybridization.

Non-hybrid electron clouds of carbon atoms are oriented perpendicular to the plane of the layer, and overlapping with each other, form delocalized σ-bonds. Neighboring layers in a graphite crystal are located at a distance of 0.335 nm from each other and are weakly interconnected, mainly by van der Waals forces. Therefore, graphite has low mechanical strength and is easily split into flakes, which are very strong in themselves. The bond between the layers of carbon atoms in graphite is partially metallic. This explains the fact that graphite conducts electricity well, but still not as well as metals.

graphite structure

Physical properties in graphite differ greatly in directions - perpendicular and parallel to the layers of carbon atoms.

When heated without access to air, graphite does not undergo any changes up to 3700°C. At this temperature, it sublimates without melting.

Artificial graphite is obtained from the best grades of hard coal at 3000°C in electric furnaces without air access.

Graphite is thermodynamically stable over a wide range of temperatures and pressures, so it is accepted as the standard state of carbon. The density of graphite is 2.265 g/cm 3 .

3. Carbin - fine-grained black powder. In its crystal structure, carbon atoms are connected by alternating single and triple bonds into linear chains:

−С≡С−С≡С−С≡С−

This substance was first obtained by V.V. Korshak, A.M. Sladkov, V.I. Kasatochkin, Yu.P. Kudryavtsev in the early 1960s.

Subsequently, it was shown that carbine can exist in different forms and contains both polyacetylene and polycumulene chains in which carbon atoms are linked by double bonds:

C=C=C=C=C=C=

Later, carbine was found in nature - in meteorite matter.

Carbyne has semiconductor properties; under the action of light, its conductivity increases greatly. Due to the existence of different types of bonds and different ways of stacking chains of carbon atoms in the crystal lattice, the physical properties of carbine can vary over a wide range. When heated without access to air above 2000°C, carbine is stable; at temperatures of about 2300°C, its transition to graphite is observed.

Natural carbon is made up of two isotopes

Previously, it was believed that charcoal, soot and coke are close in composition to pure carbon and differ in properties from diamond and graphite, represent an independent allotropic modification of carbon (“amorphous carbon”). However, it was found that these substances consist of the smallest crystalline particles in which carbon atoms are connected in the same way as in graphite.

4. Coal - finely divided graphite. It is formed during the thermal decomposition of carbon-containing compounds without air access. Coals differ significantly in properties depending on the substance from which they are obtained and the method of preparation. They always contain impurities that affect their properties. The most important grades of coal are coke, charcoal, and soot.

Coke is obtained by heating coal in the absence of air.

Charcoal is formed when wood is heated in the absence of air.

Soot is a very fine graphite crystalline powder. It is formed during the combustion of hydrocarbons (natural gas, acetylene, turpentine, etc.) with limited air access.

Activated carbons are porous industrial adsorbents consisting mainly of carbon. Adsorption is the absorption by the surface of solids of gases and dissolved substances. Active carbons are obtained from solid fuels (peat, brown and hard coal, anthracite), wood and its products (charcoal, sawdust, paper production waste), leather industry waste, animal materials, such as bones. Coals, characterized by high mechanical strength, are produced from the shells of coconuts and other nuts, from the seeds of fruits. The structure of coals is represented by pores of all sizes, however, the adsorption capacity and adsorption rate are determined by the content of micropores per unit mass or volume of granules. In the production of active carbon, the raw material is first subjected to heat treatment without air access, as a result of which moisture and partially resins are removed from it. In this case, a large-pore structure of coal is formed. To obtain a microporous structure, activation is carried out either by oxidation with gas or steam, or by treatment with chemical reagents.