tailieunhanh - Silver

The Chemistry of Silver Copper, silver, and gold form group 1B of the periodic table, and together they are known as the ‘coinage metals’. Silver, like gold and the six platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium), is a noble metal. Silver has the highest electrical conductivity and the highest thermal conductivity of any metal, as well as the lowest electrical contact resistance (Table 1). | Silver DS Cameron The Interact Consultancy Reading UK 2009 Elsevier . All rights reserved. The Chemistry of Silver Copper silver and gold form group 1B of the periodic table and together they are known as the coinage metals . Silver like gold and the six platinum group metals platinum palladium rhodium ruthenium iridium and osmium is a noble metal. Silver has the highest electrical conductivity and the highest thermal conductivity of any metal as well as the lowest electrical contact resistance Table 1 . Silver is second only to gold in terms of malleability and ductility and is mainly used in the form of alloys. Silver is highly permeable to oxygen which dissolves and migrates in the atomic state the rate of diffusion increasing with temperature. However silver is second only to gold in having the weakest chemical interaction with oxygen. Silver comprises about 2 x 10 5 of the lithosphere and is found as argentite Ag2S horn silver AgCl and pyrogyrite Ag3SbS3 and is also recovered from the electrolytic refining of copper lead and nickel separating out as anode sludge. Silver and gold are extracted by aqueous sodium cyanide which reduces the oxidation potential of the metals so that after oxidation by atmospheric oxygen they form soluble cyano complexes 4Ag 8NaCN 2H2O O2 -4NaAg CN 2 4NaOH I Ag2S 4NaCN-2NaAg CN 2 Na2S II The sodium sulfide Na2S is largely oxidized to sodium sulfate Na2SO4 by oxygen in the air which impedes the back reaction. Silver is then precipitated from cyano ions in solution by the addition of zinc 2NaAg CN 2 Zn-2Ag Na2Zn CN 4 III Silver I oxide is made by heating finely divided silver at 573 K under 1500 kPa of oxygen 4Ag IV Pure silver sulfide conducts electricity like a metal of high specific resistance yet has a zero temperature coefficient. This may result from a few silver ions existing in the divalent state providing free electrons to transport current. For high-temperature fuel cells that may incorporate internal reforming of