CARBON

A. Characteristic

An : 6

N : 6

Am : 12.0107 g/mol

Group No : 14

Group Name : (none)

Block : p-block

Period : 2

State : solid at 298 K

Colour : graphite is black, diamond is colourless

Classification : Non-metallic

Boiling Point : 5100K (4827^oC)

Melting Point : 3773K (3500^oC)

Density : (graphite) 2.267g/cm³

Density : (diamond) 3.513g/cm³

B. Sources

Made by burning organic compounds with insufficient oxygen. Graphite deposits are found in Sri Lanka, Madagascar, Russia, South Korea, Mexico, Czech Republic and Italy. Diamonds are primarily found in South Africa, USA, Russia, Brazil, Zaire, Sierra Leone and Ghana.

C. Abundance

Universe : 5000 ppm

Sun : 3000 ppm

Atmosphere : 350 ppm

Earth’s Crust : 480 ppm

Seawater : Atlantic surface: 23 ppm; Atlantic deep: 26 ppm; Pacific surface: 23 ppm; Pacific deep: 28 ppm

Human : 2.3 x 10⁸ ppb by weight; 1.2 x 10⁸ ppb by atoms

D. Uses

As carbon’s major properties very widely depending upon its form, carbon’s uses also very greatly. Carbon-14 which is radioactive is used in "carbon dating" (telling how old something is by determining the amount of Carbon-14 present in the item being tested as compared to a standard value for a similar object which is new). Other uses include pencils, diamonds, steel, controls nuclear reactions, tire colourant, plastics, paint pigments, lubricants and much more.

E. Notes

Carbon has many allotropes each having very different physical properties from the other. Graphite (pencil lead) for instance is one of the softest forms of carbon, while diamonds are the hardest.

Carbon compounds are named according to the number of carbons present in the basic chain, the presence of single, double or triple bonds, whether or not the carbon chain forms a cyclic structure and the element or ions that substitute for hydrogens in the chain. A carbon compound with one carbon atom is a methyl-, two is an ethyl- , three is a propyl-, four butyl-, five penta, six hexa-, etc. Single a bonded hydrocarbon (hydrogen-carbon structure) is an alkane, double bond is an alkene and a triple bond is an alkyne.

 
With more than eighteen million compounds of carbon registered with the Chemical Abstract Registry (CAS), there is much to say about carbon. So much in fact that there is an entire field of chemistry called organic chemistry that is devoted to these compounds. One could get a Ph.D. in organic chemistry and still feel that one had barely gotten their feet wet.

F. Carbon Compounds

The abundance of carbon in the universe, along with the unusual polymer-forming ability of carbon-based compounds at the common temperatures encountered on Earth, make this element the basis of the chemistry of all known life. Carbon is the fourth most abundant chemical element in the universe by mass, after hydrogen, helium, and oxygen.

1. Carbon Dioxide CO₂

Carbon dioxide is a colourless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces a sour taste in the mouth and a stinging sensation in the nose and throat. This is a minor component of the Earth’s atmosphere, produced and used by living things, and a common volatile elsewhere.

2. Carbon Monoxide CO

Carbon monoxide, though thought of as a pollutant today, has always been present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the earth’s mantle. Carbon monoxide contents of volcanic gases vary from less than 0.01% to as much as 2% depending on the volcano. Carbon monoxide is a significantly toxic gas with poisoning being the most common type of fatal poisoning in many countries. Symptoms of mild poisoning include headaches and flu-like effects; larger exposures can lead to significant toxicity of the central nervous system and heart.

G. Allotropes of Carbon

1. Diamond [ C ]

The hardest known natural mineral. Each atom is bonded tetrahedrally to four others, making a 3-dimensional network of puckered six-membered rings of atoms.

2. Graphite [ C ]

Named by Abraham Gottlob Werner in 1789, from the Greek word "to draw/write", for its use in pencils) is one of the most common allotropes of carbon. Unlike diamond, graphite is a conductor, and can be used, for instance, as the material in the electrodes of an electrical arc lamp. Graphite holds the distinction of being the most stable form of solid carbon ever discovered.

Graphite is able to conduct electricity due to the unpaired fourth electron in each carbon atom. This unpaired 4th electron forms delocalised planes above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.

Graphite powder is used as a dry lubricant. Although it might be thought that this industrially important property is due entirely to the loose interlamellar coupling between sheets in the structure, in fact in a vacuum environment (such as in technologies for use in space), graphite was found to be a very poor lubricant. This fact lead to the discovery that graphite’s lubricity is due to adsorbed air and water between the layers, unlike other layered dry lubricants such as molybdenum disulfide. Recent studies suggest that an effect called superlubricity can also account for this effect.

When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon, a useful material in blood-contacting implants such as prosthetic heart valves. Natural and crystalline graphites are not often used in pure form as structural materials due to their shear-planes, brittleness and inconsistent mechanical properties. In its pure glassy (isotropic) synthetic forms, pyrolytic graphite and carbon fiber graphite is an extremely strong, heat-resistant (to 3000^oC) material, used in reentry shields for missile nosecones, solid rocket engines, high temperature reactors, brake shoes and electric motor brushes.

3. Amorphous Carbon [ C ]

Carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed, but there is no long-range pattern of atomic positions. Coal and soot are both informally called amorphous carbon. However, both are products of pyrolysis, which does not produce true amorphous carbon under normal conditions. The coal industry divides coal up into various grades depending on the amount of carbon present in the sample compared to the amount of impurities. The highest grade, anthracite, is about 90 percent carbon and 10% other elements. Bituminous coal is about 75-90 percent carbon, and lignite is the name for coal that is around 55 percent carbon.

4. Fullerenes [ eg Buckminsterfullerene C60 ]

The fullerenes are recently-discovered allotropes of carbon named after the scientist and architect Richard Buckminster Fuller, but were discovered in 1985 by a team of scientists from Rice University and the University of Sussex, three of whom were awarded the 1996 Nobel Prize in Chemistry. They are molecules composed entirely of carbon, which take the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are sometimes called buckyballs, while cylindrical fullerenes are called buckytubes or nanotubes.

As of the early twenty-first century, the chemical and physical properties of fullerenes are still under heavy study, in both pure and applied research labs. In April 2003, fullerenes were under study for potential medicinal use - binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma.

Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar. Spherical fullerenes are often refered to as buckyballs. The smallest fullerene is the dodecahedron--the unique C20.

Buckminsterfullerene (C60) was named after Richard Buckminster Fuller, a noted architect who popularized the geodesic dome.

5. Nanotubes (buckytube) [ C ]

Nanotubes are cylindrical carbon molecules with novel properties that make them potentially useful in a wide variety of applications (e.g., nano-electronics, optics, materials applications, etc.). They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

A nanotube (also known as a buckytube) is a member of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

6. Aggregated diamond nanorods [ C ]

Aggregated diamond nanorods, or ADNRs, are an allotrope of carbon believed to be the least compressible material known to humankind. They are also 0.3% denser than diamonds.

7. Carbon nanofoam [ C ]

Carbon nanofoam was discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a low-density cluster-assembly of carbon atoms strung together in a loose three-dimensional web.

Each cluster is about 6 nanometers wide and consists of about 4000 carbon atoms linked in graphite-like sheets that are given negative curvature by the inclusion of heptagons among the regular hexagonal pattern. This is the opposite of what happens in the case of buckminsterfullerenes, in which carbon sheets are given positive curvature by the inclusion of pentagons.

The large-scale structure of carbon nanofoam is similar to that of an aerogel, but with 1% of the density of previously produced carbon aerogels - only a few times the density of air at sea level. Unlike carbon aerogels, carbon nanofoam is a poor electrical conductor.

8. Glassy Carbon [ C ]

Glassy carbon is a class of non-graphitizing carbon which is widely used as an electrode material in electrochemistry, as well as for high temperature crucibles and as a component of some prosthetic devices. It was first produced by workers at the laboratories of The General Electric Company, UK, in the early 1960s, using cellulose as the starting material. A short time later, Japanese workers produced a similar material from phenolic resin. The preparation of glassy carbon involves subjecting the organic precursors to a series of heat treatments at temperatures up to 3000^oC. Unlike many non-graphitizing carbons, they are impermeable to gases and are chemically extremely inert, especially those which have been prepared at very high temperatures. It has been demonstrated that the rates of oxidation of certain glassy carbons in oxygen, carbon dioxide or water vapour are lower than those of any other carbon. They are also highly resistant to attack by acids. Thus, while normal graphite is reduced to a powder by a mixture of concentrated sulphuric and nitric acids at room temperature, glassy carbon is unaffected by such treatment, even after several months.

9. Lonsdaleite [ C ]

Lonsdaleite is a hexagonal allotrope of the carbon allotrope diamond, believed to form when meteoric graphite falls to Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite’s hexagonal crystal lattice. Lonsdaleite was first identified from the Canyon Diablo meteorite at Barringer Crater (also known as Meteor Crater) in Arizona. It was first discovered in 1967. Lonsdaleite occurs as microscopic crystals associated with diamond in the Canyon Diablo meteorite; Kenna meteorite, New Mexico; and Allan Hills (ALH) 77283, Victoria Land, Antarctica meteorite. It has also been reported from the Tunguska impact site, Russia.

10. Chaoite [ C ]

Chaoite is a mineral believed to have been formed in meteorite impacts. It has been described as slightly harder than graphite with a reflection colour of grey to white.

H. Reactions of Carbon

1. Reactions with water

Carbon, either as graphite or diamond does not react with water under normal conditions. Under more forceful conditions, the reaction becomes important. In industry, water is blown through hot coke. The resulting gas is called water gas and is a mixture of hydrogen (H₂, 50%), carbon monoxide (CO, 40%), carbon dioxide (CO₂, 5%), nitrogen and methane (N₂ + CH₄, 5%). It is an important feedstock gas for the chemical industry.

C + H₂O --> CO + H₂

This reaction is endothermic which means that the coke cools down during the reaction. To counteract this, the steam flow is replaced by air to reheat the coke allowing further reaction.

2. Reactions with air

Carbon, as graphite, burns in oxygen to form gaseous carbon(IV) dioxide. Carbon, as diamond, also burns in air when heated to 600-800’C to also form carbon(IV) oxide.

C_(s) + O_2(g) --> CO_2(g)

When the air or oxygen is restricted then incomplete combustion to carbon monoxide (CO) occurs.

2C_(s) + O_2(g) --> CO_(g)

3. Reactions with halogens

Graphite reacts with fluorine (but none of the other halogens) at high temperatures to make a mixture of carbon tetrafluoride, CF₄, together with some C₂F₆ and C₅F₁₂.

C_(s) + excess F_2(g) --> CF_4(g) + C₂F₆ + C₅F₁₂

4. Reactions with acids

Graphite reacts with hot concentrated nitric acid to form mellitic acid, C₆(CO₂H)₆.

I. Isotopes of Carbon

₁₂C [6 neutrons]

Abundance : 98.9%

Stable with 6 neutrons

₁₃C [7 neutrons]

Abundance : 1.1%

Stable with 7 neutrons

₁₄C [8 neutrons]

Abundance : trace

Half life: 5730 years [ beta- ]

Decay Energy: 0.156MeV

Decays to ₁₄N.

Bagikan Artikel ini