Chemical Compounds - Oxides of carbon.

Carbon forms two well-known oxides, carbon monoxide, CO, and carbon dioxide, CO2. In addition, it also forms carbon suboxide, C3O2.

• Carbon monoxide.

Carbon monoxide is produced when graphite (one of the naturally occurring forms of elemental carbon) is heated or burned in a limited amount of oxygen. The reaction of steam with red-hot coke also produces carbon monoxide along with hydrogen gas (H2). (Coke is the impure carbon residue resulting from the burning of coal.) This mixture of CO and H2 is called water gas and is used as an industrial fuel. In the laboratory, carbon monoxide is prepared by heating formic acid, HCOOH, or oxalic acid, H2C2O4, with concentrated sulfuric acid, H2SO4. The sulfuric acid removes the elements of water (i.e., H2O) from the formic or oxalic acid and absorbs the water produced. Because carbon monoxide burns readily in oxygen to produce carbon dioxide,

2CO + O2 ® 2CO2,

it is useful as a gaseous fuel. It is also useful as a metallurgical reducing agent because at high temperatures it reduces many metal oxides to the elemental metal. For example, copper(II) oxide, CuO, and iron(III) oxide, Fe2O3, are both reduced to the metal by carbon monoxide.

Carbon monoxide is an extremely dangerous poison. Because it is an odourless and tasteless gas, it gives no warning of its presence. It binds to the hemoglobin in blood to form a compound that is so stable that it cannot be broken down by body processes. When the hemoglobin is combined with carbon monoxide, it cannot combine with oxygen; this destroys the ability of hemoglobin to carry essential oxygen to all parts of the body. Suffocation can occur if sufficient amounts of carbon
monoxide are present to form complexes with the hemoglobin.

•Carbon dioxide.

Carbon dioxide is produced when any form of carbon or almost any carbon compound is burned in an excess of oxygen. Many metal carbonates liberate CO2 when they are heated. For example, calcium carbonate (CaCO3) produces carbon dioxide and calcium oxide (CaO).

CaCO3 + heat ® CO2 + CaO

The fermentation of glucose (a sugar) during the preparation of ethanol, the alcohol found in beverages such as beer and wine, produces large quantities of CO2 as a by-product.

C6H12O6 ® 2C2H5OH+ 2CO2

glucoseethanol

In the laboratory CO2 can be prepared by adding a metal carbonate to an aqueous acid; e.g.,

CaCO3 + 2H3O+ ® Ca²+ + 3H2O+ CO2.

Carbon dioxide is a colourless and essentially odourless gas that is 1.5 times as dense as air. It is not toxic, although a large concentration could result in suffocation simply by causing a lack of oxygen in the body. All carbonated beverages contain dissolved CO2; hence the name carbonated. One litre (1.06 quarts) of water at 20° C dissolves 0.9 litre of CO2 at one atmosphere, forming carbonic acid (H2CO3, discussed below in the section Oxyacids and their salts), which has a mildly acidic (sour) taste. Solid CO2 sublimes at normal atmospheric pressure. Thus, solid CO2, called dry ice, is a valuable refrigerant that is always free of the liquid form. Carbon dioxide is also used as a fire extinguisher, because most substances do not burn in it, and it is readily available and
inexpensive. Air containing as little as 2.5 percent CO2 extinguishes a flame.

The Earth's atmosphere contains approximately 0.04 percent carbon dioxide by volume and serves as a huge reservoir of this compound. The carbon dioxide content of the atmosphere has signif-icantly increased in the last several years largely because of the burning of fossil fuels. A so-called greenhouse effect can result from increased carbon dioxide and water vapour in the atmosphere. These gases allow visible light from the Sun to penetrate to the Earth's surface, where it is absorbed and reradiated as infrared radiation. This longer-wavelength radiation is absorbed by the carbon dioxide and water and cannot escape back into space. There is increasing concern that the resulting increased heat in the atmosphere could cause the Earth's average temperature to increase 2° to 3° C over a period of time. This change would have a serious impact on the environment, affecting climate, ocean levels, and agriculture. The solubility of carbon dioxide in water makes oceans and lakes significant sources of this gas. Atmospheric carbon dioxide is in dynamic equilibrium with that dissolved in water and with that bound primarily as carbonate in the Earth's crust. With sunlight and chlorophyll serving as a catalyst (i.e., a compound that increases the rate of a reaction without being
consumed itself), green plants convert carbon dioxide and water into sugar and oxygen. This pro-cess is called photosynthesis and uses the energy of light.

6CO2 + 6H2O ® C6H12O6+ 6O2

Conversely, carbon dioxide is a product of respiration and is returned to the air by plants and animals.

•Carbon suboxide.

Carbon suboxide, C3O2, is a foul-smelling, lacrimatory gas produced by the dehydration of malonic acid, CH2(COOH)2, with P4O10 in a vacuum at 140° to 150° C. Carbon suboxide is a linear, symmetrical molecule whose structure can be represented as O=C=C=C=O. At 25° C the compound is unstable and polymerizes to highly coloured solid products, but it is a stable molecule at -78° C. Under the influence of ultraviolet light (in the process known as photolysis), C3O2 de-composes to the very reactive molecule ketene, C2O. Since carbon suboxide is the acid anhydride of malonic acid, it reacts slowly with water to produce that acid.