Tin - chemistry.

Tin - chemistry. I INTRODUCTION Tin, symbol Sn, metallic element that has been used by people since ancient times. Tin is in group 14 (or IVa) of the periodic table (see Periodic Law). The atomic number of tin is 50. Tin has been found in the tombs of ancient Egyptians and was exported to Europe in large quantities from Cornwall, England, during the Roman period. The ancient Egyptians considered tin and lead different forms of the same metal. Tin was the Anglo-Saxon name for the metal. II PROPERTIES AND OCCURRENCE Tin is highly ductile and malleable at a temperature of 100°C (212°F). It is attacked by strong acids. Ordinarily a silver-white metal sometimes called white tin, at temperatures below 13°C (55°F) it often changes into an allotropic (distinctly different) form known as gray tin. Gray tin is an amorphous, grayish powder with a specific gravity of 5.75. Because of the mottled appearance of tin objects undergoing this decomposition, the action is commonly referred to as tin disease or tin pest. Ordinary bar tin, when bent, issues a crackling sound called tin cry, caused by the friction of the tin crystals. Tin ranks about 49th in abundance of the elements in Earth's crust. Ordinary tin melts at about 232°C (about 450°F), boils at about 2602°C (about 4716°F), and has a specific gravity of 7.31. The atomic weight of tin is 118.71. The principal ore of tin is the mineral cassiterite (or tinstone), SnO2, found abundantly in Cornwall, England and in Germany, the Malay Peninsula, Bolivia, Brazil, and Australia. In the United States workable deposits of tin have been found only on Alaska's Seward Peninsula. In the extraction of tin, the ore is first ground and washed to remove all impurities and then roasted to oxidize the sulfides of iron and copper. After a second washing, the ore is reduced by carbon in a reverberatory furnace; the molten tin that collects on the bottom is drawn off and molded into blocks known as block tin. In this form, the tin is resmelted at low temperatures; the impurities form an insoluble mass. Tin may also be purified by electrolysis. III COMPOUNDS Tin forms stannic acid, H2SnO4, when heated in air or oxygen at high temperatures. It dissolves in hydrochloric acid to form stannous chloride, SnCl2, and in aqua regia to form stannic chloride, SnCl4, and it reacts with sodium hydroxide solution to form sodium stannite and hydrogen gas. In cold and very dilute nitric acid, tin dissolves to form stannous nitrate and ammonium nitrate; in concentrated nitric acid, it produces metastannic acid, H2SnO3. Stannous sulfide, SnS, is yielded as a dark brown precipitate by the action of hydrogen sulfide on a solution of stannous chloride. Stannic sulfide, SnS2, is produced by passing hydrogen sulfide through a solution of stannic salt. The two hydroxides of tin, Sn(OH)2 and Sn(OH)4, are produced by adding a soluble hydroxide to solutions of stannous and stannic salts. Stannous oxide, SnO, a black insoluble powder, is obtained by heating stannous oxalate in the absence of air. In the presence of air, stannous oxide burns to form the dioxide, or stannic oxide, SnO2, a white insoluble solid. The dioxide may also be prepared by heating stannic acid or by heating tin metal in air at high temperatures. IV USES Tin is a widely sought metal and is used in hundreds of industrial processes throughout the world. In the form of tinplate, it is used as a protective coating for copper vessels, various metals used in the manufacture of tin cans, and similar articles. Tin is important in the production of the common alloys bronze (tin and copper), solder (tin and lead), and type metal (tin, lead, and antimony) (see Metalwork). It is also used as an alloy with titanium in the aerospace industry and as an ingredient in some insecticides. Stannic sulfide, known also as mosaic gold, is used in powdered form for bronzing articles made of plaster of paris or wood. The United States imports more than one-fifth of the average annual world production of tin. Most of the world's tin is produced by Malaysia, Brazil, Indonesia, Thailand, Bolivia, and Australia. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.