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An Explanation of the Old Experiment and the Rational Design of the New and Simpler Demonstration
The demonstration referred to as "Striking It Rich" (I) and "Copper to Silver to Gold" (2), or more commonly as the "Golden Penny Experiment" is popular among general chemistry teachers and is described in the two widely used chemistry texts cited above. Both of these texts, however, present an insufficient or incorrect explanation for the major part of the experiment. This paper reports a series of electrochemical measurements that lead to a logical explanation for this demonstration and to a simplified design that makes it safer.
In the popular version of this experiment granular zinc is placed in 3M NaOH solution and heated until the liquid boils. A copper penny is then placed in the beaker and the heating continues. After a short time the copper coin becomes silvery (zinc deposits on copper). The silvery coin is subsequently heated in the cold part of a Bunsen burner flame and the coin turns golden (1, 2).
The experiment, as presented above, consists of three separate chemical processes:
Step A. Granular zinc dissolves in NaOH solution, forming a zincate anion, [Zn(OH)4]2-.
Step B. Zincate ion becomes reduced to metallic zinc on the surface of the copper penny.
Step C. Zinc and copper, when heated in the Bunsen burner flame, form brass.
Of the three chemical processes presented above, steps A and C are easy to explain (1, 2).
Step C of this experiment is a typical example of a hightemperature alloy formation and does not require ahy special explanation.
None of the cited references present, however, a satisfactory explanation for step B in the experiment. The question one needs to address is why does zincate anion become reduced to zinc (from +2 oxidation state to 0)? What is the reducing agent? It cannot be water, because it is shown above that water will actually oxidize zinc at the pH of the experiment. The same argument excludes hydrogen gas as a possible reducing agent. Metallic copper is also ruled out, because copper is a much poorer reducing agent than zinc at any pH. Besides, the solution remains colorless, indicating that no oxidation of copper takes place.
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