Inorganic compounds of carbon

Carbon dioxide isa colourless odourless gas, CO₂, soluble in water, ethanol and acetone. Density (d.) is 1.977 g / dm ³ (0 ^oC); m.p. is -56.6 ^oC; b.p. is -78.5 ^oC. It occurs in the atmosphere (0.04% by volume), but has a short residence time in this phase. It is both consumed by plants during photosynthesis and produced by respiration and combustion. It is readily prepared in the laboratory by the action of dilute acids on metal carbonates or the action of heat on heavy-metal carbonates. Carbon dioxide is a by-product from the manufacture of lime and from fermentation processes. Carbon dioxide has a small liquid range. Liquid carbon dioxide is produced only at high pressures. The molecule CO₂ is linear with each oxygen making a double bond to carbon. Chemically, it is not reactive and will not support combustion. It dissolves in water to give carbonic acid.

Large quantities of solid carbon dioxide (dry ice) are used in processes requiring large-scale refrigeration. It is also used in fire extinguishers as a desirable alternative to water for most fires and as a constituent of medical gases as it promotes exhalation. It is also used in carbonated drinks.

The level of carbon dioxide in the atmosphere has increased by some 12% in the last 100 years, mainly because of extensive burning of fossil fuels and the destruction of large areas of rain forest. This has been postulated as the main cause of the average increase of 0.5°C in global temperatures over the same period, through the greenhouse effect. Steps are now being taken to prevent further increases in atmospheric CO₂ concentration and subsequent global warming.

Carbon monoxide is a colourless odourless gas, CO, sparingly soluble in water and soluble in ethanol and benzene. Density is 1.25 g dm^-3 (0 ^o C); m.p. is -199^oC; b.p. is -191.5 ^o C. It is flammable and highly toxic. In the laboratory it can be made by the dehydration of methanoic acid (formic acid) using concentrated sulphuric acid. Industrially it is produced by the oxidation of natural gas (methane) or by the water-gas reaction. It is formed by the incomplete combustion of carbon and is present in car-exhaust gases.

It is a neutral oxide which burns in air to give carbon dioxide and is a good reducing agent, used in a number of metallurgical processes. It has the interesting chemical property of forming a range of transition metal carbonyls, e.g. Ni(CO)₄. Carbon monoxide is able to use vacant p-orbitals in bonding with metals. The stabilization of low oxidation states, including the zero state, is a consequence of this. This also accounts for its toxicity which is due to the binding of the CO to the iron in haemoglobin, thereby blocking the uptake of oxygen.

Carbonic acid is a dibasic acid, H₂CO₃, formed in solution when carbon dioxide is dissolved in water:

CO₂(aq) + H₂O(1) = H₂CO₃(aq).

The acid is in equilibrium with dissolved carbon dioxide and also dissociates as follows:

H₂ CO₃ = H⁺+ HCO^-₃

K_d = 4.5 x 10^-7 mol dm^-3

HCO₃^- = CO₃² ^-+ H⁺

K_d = 4.8 x 10 ^-11 mol dm ^-3.

The pure acid cannot be isolated, although it can be produced in ether solution at 30°C. Carbonic acid gives rise to two series of salts: the carbonates and the hydrogencarbonates.

Carbon disulphide (carbon bisulphide) is a colourless highly refractive liquid, CS₂, slightly soluble in water and soluble in ethanol and ether. Relative density (r.d.) is 1.261; m.p. is -110 ° C; b.p. is 46.3^oC. Pure carbon disulphide has an ethereal odour but the commercial product is contaminated with a variety of other sulphur compounds and has a very unpleasant smell. It was previously manufactured by heating a mixture of wood, sulphur and charcoal. Modern processes use natural gas and sulphur. Carbon disulphide is an excellent solvent for oils, waxes, rubber, sulphur and phosphorus, but its use is decreasing because of its high toxicity and its flammability. It is used for the preparation of xanthates in the manufacture of viscose yarns.

Carbide is any of various compounds of carbon with metals or other more electropositive elements. True carbides contain the ion C^4- as in A1₄C₃. These are saltlike compounds giving methane on hydrolysis. They were formerly called methanides. Compounds containing the ion C₂^2- are also saltlike and are known as dicarbides. They yield ethyne (acetylene) on hydrolysis. They were formerly called acetylides. The above types of compound are ionic but have partially covalent bond character, but boron and silicon form true covalent carbides, with giant molecular structures. In addition, the transition metals form a range of interstitial carbides in which the carbon atoms occupy interstitial positions in the metal lattice. These substances are generally hard materials with metallic conductivity. Some transition metals (e.g. Cr, Mn, Fe, Co and Ni) have atomic radii that are too small to allow individual carbon atoms in the interstitial holes. These form carbides in which the metal lattice is distorted and chains of carbon atoms exist (e.g. Cr₃C₂, Fe₃C, W₂C). Such compounds are intermediate in character between interstitial carbides and ionic carbides. They give mixtures of hydrocarbons on hydrolysis with water or acids.

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