In the Classroom edited by
JCE DigiDemos: Extensions
New Observations on the Copper-to-Silver-toGold Demonstration Dorin Bejan, Jeff Hastie, and Nigel J. Bunce* Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1; *
[email protected] In the Golden Penny demonstration a copper penny or token is converted first to “silver” then to “gold”. The original demonstration involves dissolution of powdered or (preferably as it is safer) granular zinc in hot NaOH to give a solution of sodium zincate. Upon immersion of the copper token, its surface almost immediately becomes coated “silver”. When the token is removed, dried, and heated in a flame, its color changes to “gold”. The first stage of the reaction involves the formation of the silver-colored alloy γ-brass (copper content less than 55%) on the surface of the copper (1): Zn(s) Cu(s) H-Zn/Cu(s) The reaction requires particles of zinc to be in contact with the surface of the copper. The reaction can also be driven electrolytically from a zinc anode. Upon heating, the surface layer of zinc diffuses further into the crystal structure of the copper, giving the goldencolored alloy α-brass (copper content greater than 55%). The present version of the “copper to silver” part of the experiment replaces the hazardous hot NaOH with a zinc salt such as ZnSO4. A further improvement allows the deposition of γ-brass to take place under electrolytic conditions at room temperature using a mixed electrolyte of ZnSO4 and Na2SO4, thereby significantly reducing the use of chemical reagents.
Ed Vitz
Kutztown University Kutztown, PA 19530
Procedure 1 ZnSO4⋅7H2O (7 g) is dissolved in 25 mL water in a 100 mL beaker and heated to near boiling. A small quantity (
