Allotropes of carbon
Allotropy is the property of some chemical elements to exist in two or more different forms, or allotropes, when found in nature. There are several allotropes of carbon.
Diamond is probably the most well known carbon allotrope. The carbon atoms are arranged in a lattice, which is a variation of the face-centered cubic crystal structure. It has superlative physical qualities, most of which originate from the strong covalent bonding between its atoms. Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedrons together form a three-dimensional network of six-membered carbon rings in the chair conformation, allowing for zero bond-angle strain. This stable network of covalent bonds and hexagonal rings is the reason that diamond is so incredibly strong as a substance.
As a result, diamond exhibits the highest hardness and thermal conductivity of any bulk material. In addition, its rigid lattice prevents contamination by many elements. The surface of diamond is lipophillic and hydrophobic, which means it cannot get wet by water but can be in oil. Diamonds do not generally react with any chemical reagents, including strong acids and bases. Uses of diamond include cutting, drilling, and grinding; jewelry; and in the semi-conductor industry.
Graphite is another allotrope of carbon; unlike diamond, it is an electrical conductor and a semi-metal. Graphite is the most stable form of carbon under standard conditions and is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds.
Graphite has a layered, planar structure. In each layer, the carbon atoms are arranged in a hexagonal lattice with separation of 0.142 nm, and the distance between planes (layers) is 0.335 nm. The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral), have very similar physical properties (except that the layers stack slightly differently). The hexagonal graphite may be either flat or buckled. The alpha form can be converted to the beta form through mechanical treatment, and the beta form reverts to the alpha form when it is heated above 1300 °C. Graphite can conduct electricity due to the vast electron delocalization within the carbon layers; as the electrons are free to move, electricity moves through the plane of the layers. Graphite also has self-lubricating and dry lubricating properties. Graphite has applications in prosthetic blood-containing materials and heat-resistant materials as it can resist temperatures up to 3000 °C.
Fullerenes and Nanotubes
Carbon nanomaterials make up another class of carbon allotropes. Fullerenes (also called buckyballs) are molecules of varying sizes composed entirely of carbon that take on the form of hollow spheres, ellipsoids, or tubes. Buckyballs and buckytubes have been the subject of intense research, both because of their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology. Carbon nanotubes are cylindrical carbon molecules that exhibit extraordinary strength and unique electrical properties and are efficient conductors of heat. Carbon nanobuds are newly discovered allotropes in which fullerene-like “buds” are covalently attached to the outer side walls of a carbon nanotube. Nanobuds therefore exhibit properties of both nanotubes and fullerenes.
Glassy or vitreous carbon is a class of carbon widely used as an electrode material in electrochemistry as well as in prosthetic devices and high-temperature crucibles. Its most important properties are high temperature resistance, hardness, low density, low electrical resistance, low friction, low thermal resistance, extreme resistance to chemical attack, and impermeability to gases and liquids.
Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions.
Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semimetallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.
In general, catalytic action is a chemical reaction between the catalyst and a reactant, forming chemical intermediates that are able to react more readily with each other or with another reactant, to form the desired end product. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid–base reactions, oxidation–reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants; bifunctional catalysts are used extensively for reforming reactions in the petroleum industry.
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