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Encyclopedia Britannica - Main :: ORC-PAI |
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OXIDE , in chemistry , a binary compound of oxygen and other elements. In general, oxides are the most important compounds with which the chemist has to deal, a study of their composition and properties permitting a valuable comparative
element
system
Five species of oxides may be distinguished: (I) basic oxides, (2) acidic oxides, (3) neutral oxides, (4) peroxides, (5) mixed anhydrides and salts. Basic oxides combine with acids or acidic oxides to form salts; similarly acidic oxides combine with basic oxides to form salts also. The former are more usually yielded by the metals (some metals, however, form oxides belonging to the other groups), whilst the latter are usually associated with the non-metals. An oxide may be both acidic and basic, i.e. combine with bases as well as acids; this is the case with elements occurring at the transition between basigenic and oxygenic elements in the periodic classification, e.g. aluminium and zinc. Neutral oxides combine neither with acids nor bases to give salts nor with water to give a base or acid. A typical member is nitric oxide; carbon monoxide and nitrous oxide may also be put in this class, but it must be remembered that these oxides may be regarded, in some measure at least, as the anhydrides of formic and hyponitrous acid, although, at the same time, it is impossible to obtain these acids by simple hydration of these oxides. Peroxides may in most cases be defined as oxides containing rnore oxygen than the typical oxide. The failure of this definition is seen in the case of lead dioxide, which is certainly a peroxide in properties, but it is also the typical oxide of Group IV. to which lead belongs. All peroxides have oxidizing properties. Peroxides may be basic or acidic. Some basic oxides yield hydrogen peroxide with acids, others yield oxygen (these also liberate chlorine from hydrochloric acid), and may combine with lower acidic oxides to form salts of the normal basic oxide with the higher acidic oxide. Examples are BaOz+H2SO4 = BaSO4+H2O2; 2MnO2+2H2SO4=2MnSO4+2H20+02; M nO2 +4HC1 = MnC12 + 2H20+C12i Pb02+S02=PbSO4 (i.e. PbO+S03). Two species of basic peroxides may be distinguished: (i) the superoxides or peroxidates, containing the oxygen atoms in a chain, e.g.Na O.O Na, O Ba.O, which yield hydrogen peroxide with acids ; and (2) the polyoxides, having the oxygen atoms doubly linked to the metallic atom, e.g. 0 : Mn: O, O : Pb : O, and giving oxygen with sulphuric acid, and chlorine with hydrochloric. L. Marino (Zeit. anorg. Chem., 1907, 56, p. 233) pointed out that manganese and lead dioxide behaved differently with sulphur
formulae: O: Mn :0, OPbi O, as explaining this difference. A simpler explanation is that the manganese dioxide first gives a normal sulphite which rearranges to dithionate, thus: MnO2+2502= Mn(SOi)2->MnS2Os, whilst the lead dioxide gives a basic sulphite which rearranges to sulphate, thus: PbO+SO2=PbOSO3-PbSO4. Acidic peroxides combine with basic oxides to form " per " salts, and by loss of oxygen yield the acidic oxide typical of the element
Oxidation and Reduction.In the narrow sense " oxidation " may be regarded as the combination of a substance with oxygen, and conversely, " reduction " as the abstraction of oxygen; in the wider sense oxidation includes not merely the addition of oxygen, but also of other electro-negative elements or groups, or the removal of hydrogen or an electro-positive element or group. In inorganic chemistry oxidation is associated in many cases with an increase in the active valency
Many substances undergo simultaneous oxidation and reduction when treated in a particular manner; this is known as self- or auto-oxidation. For example, on boiling an aqueous solution of a hypochlorite, a chlorate and a chloride results, part
original
part
The important oxidizing agents include: oxygen, ozone, per-oxides, the halogens chlorine and bromine, oxyacids such as nitric and those of chlorine, bromine and iodine, and also chromic and permanganic acid. The important reducing agents include hydrogen, hydrides such as those of iodine, sulphur
aluminium , magnesium, &c., salts of lower oxyacids, lower salts of metals and lower oxides.End of Article: OXIDE If you wish, you can link directly to this article.
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