Plating zinc onto a penny with sodium hydroxide

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From: Geoff Ruth (gruth@leadershiphigh.org)
Date: Tue Dec 14 2004 - 07:25:00 PST 

Message-Id: <p06110409bde36909fc95@[10.100.1.247]>
Date: Tue, 14 Dec 2004 07:25:00 -0800
From: Geoff Ruth <gruth@leadershiphigh.org>
Subject: Plating zinc onto a penny with sodium hydroxide

Some people at Saturday's Chemapolooza were curious about the
reactions when you heat up 6M sodium hydroxide + Zn to plate zinc
onto a penny.

First, the "zincate" ion is created: [Zn(OH)4]-2

This ion is then reduced to zinc metal at the surface of the penny.
The reducing agent is incorrectly reported on many web sites as being
copper or water, which doesn't make sense since pH is constant
throughout and since there is no color change (with Cu2+ being
formed).

According to the following interesting article in J Chemical
Education, the oxidation of zinc (at the Zn metal) itself provides
the e- needed to reduce the zinc back at the penny:

Volume 72 Issue 5 / May 1995
Szczepankiewicz, Steven H.; Bieron, Joseph F.; Koz
The "Golden Penny" Demonstration: An Explanation of the Old
Experiment and the Rational Design of the New and Simpler
Demonstration, pp. 386-388

If anyone wants, I can e-mail or mail the actual JCE article.

- Geoff Ruth

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