Oxygen amphoteric oxides

The oxides are the compounds of any element with oxygen the oxides of non-metals are defined as acidic oxides, while the oxides of metals are denoted as basic oxides. Some oxides have both acidic and basic characters and are consequently defined as amphoteric [4,77]. 

Oxygen forms ionic oxides, such as Li20 and MgO, with active metals, and covalent oxides, such as P4OK) and SO3, with nonmetals. Oxides can also be classified according to their acid-base properties. Basic oxides are ionic, and acidic oxides are covalent. Amphoteric oxides, such as AI2O3, exhibit both acidic and basic properties. 

The sesquioxide, Cr Oa, containing trivalent chromium, is an amphoteric oxide. It yields chromic salts, such as chromic chloride, CrCla, and sulphate, Cr2(S04)a, which are very stable and show great similarity to the ferric salts and to salts of aluminium as, for example, in the formation of alums. Since, however, chromic oxide functions as a weaker base than chromous oxide, the latter having a lower oxygen content, the chromic salts are more liable to hydrolysis than the chromous salts. This is well marked in the case of the chlorides. Again, in spite of the stability of chromic salts, only a slight tendency to form simple Cr " ions is exhibited, whilst complex ions are formed much more readily, not only complex anions, as in the case of iron and aluminium, but also complex cations, as in the extensive chromammine series. In this respect chromium resembles cobalt and platinum. 

Aluminum is the third most abundant element in the earth s crust (after oxygen and silicon), accounting for 8.2% of the total mass. It occurs most commonly in association with silicon in the aluminosilicates of feldspars and micas and in clays, the products of weathering of these rocks. The most important ore for aluminum production is bauxite, a hydrated aluminum oxide that contains 50% to 60% AI2O3 1% to 20% FeiOs 1% to 10% silica minor concentrations of titanium, zirconium, vanadium, and other transition-metal oxides and the balance (20% to 30%) water. Bauxite is purified via the Bayer process, which takes advantage of the fact that the amphoteric oxide alumina is soluble in strong bases but iron(III) oxide is not. Crude bauxite is dissolved in sodium hydroxide... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

The well-characterized oxides of the three series of transition elements are shown in Table 7.7. The patterns of oxidation states of the oxides are similar to those of the fluorides of the same elements. The same reasons apply, with the added capacity in some cases for the elements to reach the oxidation state +8. This is mainly because of the removal of the steric factor in the oxides, in which an oxidation state of +8 may be achieved by only four ligand oxygen atoms. These are formally doubly bonded to the metals. The +2 oxides are ionic, as are those of the +3 metals. The oxides of the metals with oxidation states greater than +3 tend to be polymeric, with the highest oxidation state oxides having discrete covalent molecular structures. The lower oxides are basic, the highest ones are acidic, and there are some amphoteric oxides with intermediate oxidation states. 

Lead Monoxide. Lead monoxide (litharge), PbO, occurs as a reddish alpha form, which is stable up to 489°C where it transforms to a yellow beta form (massicot). The latter is stable at high temperatures. The solubihty of a-PbO ia water is 0.0504 g/L at 25°C the solubihty of the p-PbO is 0.1065 g/L at 25°C (40). Lead monoxide is amphoteric and dissolves ia both acids and alkahes. In alkahes, it forms the plumbite ion PbO - The monoxide is produced commercially by the reaction of molten lead with air or oxygen ia a furnace. Black or gray oxide is manufactured by the Barton process, by the oxidation of atomized molten lead ia air, as well as by the ball mill process, ia which metallic lead balls of high purity are tumbled ia the mill to form partially oxidized lead particles. 

Berzehus (19) further appHed and amplified the nomenclature introduced by Guyton de Morveau and Lavoisier. It was he who divided the elements into metalloids (nonmetals) and metals according to their electrochemical character, and the compounds of oxygen with positive elements (metals) into suboxides, oxides, and peroxides. His division of the acids according to degree of oxidation has been Httie altered. He introduced the terms anhydride and amphoteric and designated the chlorides in a manner similar to that used for the oxides. 

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

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