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Name: Zach
Status: student
Grade: 9-12
Country: USA
Date: Summer 2009


Question:
Hi,

I recently performed the "gold" penny experiment. In this experiment copper pennies were coated with zinc and then heated to combine the zinc coating with the copper surface of the penny, forming brass. I noticed very strange patterns on my pennies - there were patches that would not plate (or were a copperish color I have not determined if these are un-plated copper portions). I was very careful to wash the pennies in a solution of 20% agricultural vinegar and salt.

In addition I polished them with steel wool. This suggests to me that the lack of plating was not due to oils or corrosion on the pennies. In another note, the spots only appeared after the pennies dried - they appeared completely coated when I removed them from solution (I made sure to wash them off in tap water and dry them after I removed them). If someone could explain this to me I would be very appreciative. Also, I want to understand the chemistry of this experiment better. I know we are forming a zincate ion here and that the H gas is produced when water is turned into OH-. However, I do not understand why a metal high on the activity series would be reduced by a metal lower on the activity series (it should be vice versa -- and it is in acid solution). Can this experiment be performed with metals like aluminum? What is the ideal concentration of NaOH and why?

P.S. Why do paper towels with dried Zn and NaOH sometimes spontaneously combust (this happened to me and I found a website mentioning this)



Replies:
Zach, Thanks for bringing up my all time favorite experiment! You already have a good understanding of what is happening here, let me summarize it: 1) Zn + 2H2O +2OH- -> [Zn(OH)4]2- + H2 (formation of zincate and H2, zincate is often also represented as ZnO2(2-) 2) [Zn(OH)4]2- + 2e- -> Zn + 4OH- (zinc is reduced on copper surface) The exact species that is present a solution of sodium zincate is still debated, you can find an array of formulae to represent what it is, but most sources suggest there is a mixture of various complex ions with hydroxide ligands (as I show). The bit that really interests me is the reduction of the zincate to elemental zinc on the surface of the coin. Where do these electrons come from? You suggest they come from the Cu itself. Thermodynamically this is unlikely due to the reduction potentials of [Zn(OH)4]2- /Zn (-1.23V)and Cu2+/Cu (+0.34V) which would not give a spontaneous conversion of zincate to zinc on the copper surface with the Cu supplying the electrons. Also, what would happen to the Cu2+ ions that form? You would see a precipitation of Cu (or, kinetically Cu(OH)2) as Zn reacted with the Cu2+ ions. So where do the electrons come from? Another suggestion is that the H2 produced in equation 1 could reduce the zincate. This could happen, as the reverse of the zincate formation but why would it happen on the Cu? And would the H2 really be involved in this reaction if it was formed as a gas in equation 1 (H2 has a low solubility in water). Also, what is driving this reaction? What else could be reducing the zincate? There is a famous paper on this subject by Szczepankiewicz, Bieron and Kozik (Journal of Chemical Education 1995, 72, 386 - 388). They propose that the zincate is reduced at the Cu because it is not actually being reduced to Zn at all, but to an alloy of Zn and Cu that forms on the surface of the Cu. This low temperature alloying (mixing of Cu and Zn) gives a silvery brass alloy, so that it looks like pure Zn on the surface. The formation of the silvery brass from the zincate makes the reduction potential more positive than the reduction potential of zincate to Zn. This gives a setup for a spontaneous change from zinc to zincate to silver brass alloy. Overall this process happens because the alloy is more stable than the separate Zn and Cu reactants (it is all down to entropy, as everything is, in the end :) The net effect of all this is that the electrons come from more Zn, giving a catalytic cycle with a zincate intermediate. When you heat the initial silver coin the thermal energy allows the silver alloy layer to mix further with the Cu of the coin, giving the more Cu rich gold alloy. This answer is far from complete, if you are interested in this subject I believe there is lots of room for further research. Start with the paper I mentioned above and look for unanswered questions (there are plenty). As for your other questions, here are some things to think about. The initial Cu layer is thin on the coin, so rubbing the coin with steel wool could rub through the Cu to the core of the coin (depending on the year of the penny, this varies, but later coins have a Zn core). Any impurities on the surface of the penny (even after acid treatment) will produce problems with the deposition of Zn and alloying processes. Also temperature fluctuations and impurities in the flame can affect the alloy formed. I have also done this experiment with tin, forming a stannate intermediate ion with sodium hydroxide. The coating of tin on the coin looks the same as the zinc but heating gives a bronze colour, not gold. This takes longer than the Zn coating, but does work. Aluminium is also an amphoteric metal, like Zn and Sn, so it could work. But it has a more negative reduction potential from its 3+ ion. I wonder if the driving force would be enough for the Al to work, am also not so sure about its propensity to alloy with Cu. Please be careful with this reaction too, Al can react vigourously with NaOH, producing a lot of H2 at high temperatures. Best wishes,

Tom Collins



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