Aluminium — страница 5

  • Просмотров 7374
  • Скачиваний 138
  • Размер файла 35

produces aluminium billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia) which spontaneously ignites on contact with air;[26] contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, however, the waste has found use as a filler in asphalt and concrete.[27] 7. Chemistry 7.1 Oxidation state +1 AlH is produced when aluminium is heated in an atmosphere of hydrogen. Al2O is made by heating the normal oxide, Al2O3, with silicon at 1800 °C in a vacuum.[28] Al2S can be made by heating Al2S3 with aluminium shavings at 1300 °C in a vacuum.[28] It quickly disproportionates to the starting

materials. The selenide is made in a parallel manner. AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium. Aluminium halides usually exist in the form AlX3, where X is F, Cl, Br, or I.[28] 7.2 Oxidation state +2 Aluminium monoxide, AlO, has been detected in the gas phase after explosion[29] and in stellar absorption spectra.[30] 7.3 Oxidation state +3 Fajans' rules show that the simple trivalent cation Al3+ is not expected to be found in anhydrous salts or binary compounds such as Al2O3. The hydroxide is a weak base and aluminium salts of weak acids, such as carbonate, cannot be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.

Aluminium hydride, (AlH4)n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent. Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li[AlH5]. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry, particularly as a reducing agent. The aluminohalides have a similar structure. Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and

forming aluminates with alkalis. It exists in various crystalline forms. Aluminium carbide, Al4C3 is made by heating a mixture of the elements above 1000 °C. The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al2(C2)3, is made by passing acetylene over heated aluminium. Aluminium nitride, AlN, can be made from the elements at 800 °C. It is hydrolysed by water to form ammonia and aluminium hydroxide. Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine. Aluminium oxide, Al2O3, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride, and

carborundum. It is almost insoluble in water. Aluminium sulfide, Al2S3, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic. Aluminium iodide, AlI3, is a dimer with applications in organic synthesis. Aluminium fluoride, AlF3, is made by treating the hydroxide with HF, or can be made from the elements. It consists of a giant molecule which sublimes without melting at 1291 °C. It is very inert. The other trihalides are dimeric, having a bridge-like structure. When aluminium and fluoride are together in aqueous solution, they readily form complex ions such as [AlF(H3O)5]2+, AlF3(H3O)3, and [AlF6]3−. Of these, [AlF6]3− is the most stable. This is explained by the fact that aluminium and fluoride, which are both very compact ions, fit together

just right to form the octahedral aluminium hexafluoride complex. When aluminium and fluoride are together in water in a 1:6 molar ratio, [AlF6]3− is the most common form, even in rather low concentrations. Organometallic compounds of empirical formula AlR3 exist and, if not also polymers, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium. 7.4 Analysis The presence of aluminium can be detected in qualitative analysis using aluminon. 8. Applications 8.1 General use Aluminium is the most widely used non-ferrous metal.[31] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes).[32] Relatively pure aluminium is encountered only when corrosion