New Observations on the Copper-to-Silver-to-Gold Demonstration.

• Autores: Dorin Bejan, Jeff Hastie, Nigel J. Bunce
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 85, Nº 10, 2008, pág. 1381
• Idioma: inglés
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• Resumen

  • The demonstration in which a copper penny is converted first to "silver" and then to "gold" involves, as the first step, the deposition of zinc onto copper in hot NaOH solution. The reaction involves an electrochemical cell, with the deposition of zinc in the form of a surface layer of the silver-colored alloy ?-brass, accompanied by evolution of hydrogen at the copper cathode. The cathode potential falls progressively for a series of cells having Zn anode and cathodes varying in composition from Cu to brasses with Cu content down to less than 55% Cu, but H2 production is an independent process which appears to be unaffected. The silvery deposit can also be obtained conveniently by electrolytic deposition from a cell comprising zinc anode, copper cathode, and an electrolyte containing millimolar concentrations of both zinc sulfate and sodium sulfate.