kt5000
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'Silver' pennies
I the below project and figured I'd try it with the kids. Basically we prepare an NaOH solution, add zinc metal, apply heat and end up with
Na<sub>2</sub>ZnO<sub>2</sub> (aq). The solution is cooled, then a clean copper penny is added and the copper on the surface
of the penny is substituted with zinc. Sounds simple enough.
Zn (s) + 2NaOH (aq) --> Na<sub>2</sub>ZnO<sub>2</sub> (aq) + H<sub>2</sub> (g)
Cu (s) + Na<sub>2</sub>ZnO<sub>2</sub> (aq) --> Zn (s) + Na<sub>2</sub>CuO<sub>2</sub> (aq)
I started with 2.17 g Zn, obtained from filing the Cu plating off a post-1982 penny. That gives me 0.0332 mol Zn, and needing 0.0664 mol NaOH. I
measured out 2.66 g NaOH, added 50 ml distilled water, the zinc, and heated to about 80-85 degC. There was a very small amount of bubbles produced
and no noticible reduction in the size of the zinc.
It seems clear the first reaction didn't go very far to the right, and I'm scratching my head trying to understand why. As far as I know, the two
reagents were reasonably pure. Any ideas?
http://chemistry.about.com/od/chemistryhowtoguide/a/goldsilv...
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BromicAcid
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As I recall from doing the experiment in high school, we had an excess of zinc metal in the sodium hydroxide solution. At the time I was told that it
was necessary to have an excess because the copper penny had to touch the zinc metal to undergo a gravimetric reaction. I do not know if this is the
best explanation, but the pennies touching solid zinc did seem to coat better. In other words, do not worry that all of your zinc did not dissolve.
Give it a shot and it should work fine.
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kt5000
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I did toss a pre-1982 copper penny in the mix, thinking that some plating might occur. Nothing happened. I still have the mixture set aside, so I'll
go back and try it again with the penny touching the zinc. When you did it in HS, did you leave the solution heated up, or cool it first? Thanks
again.
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BromicAcid
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It was hot, may have even been boiling. I know it was bubbling but it could have just been from the zinc still being attacked at temperature.
It might help for you to look at different versions of the same experiment and find commonalities. The differences between this experiment and the
one you linked to seem small but might be significant.
http://www.nuffieldfoundation.org/practical-chemistry/turnin...
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DraconicAcid
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I've done that quite often- zinc sulphate solutions works better than sodium hydroxide solution, but the pennies have to be scrupulously clean. Dirty
pennies that I've scrubbed with acetone and hydrochloric acid never get clean enough to work as well as ones that start out shiny and new.
You don't actually need much of the zinc to dissolve- it's reacting reversibly, with zinc metal dissolving and giving its electrons to zinc ions at
the surface of the copper.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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kt5000
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I have sulfuric acid handy. That sounds like a simpler process.. Can I just prepare a solution of ZnSO<sub>4</sub> and drop a penny in
it?
I remember sulfuric acid doesn't dissolve copper, and CuSO<sub>4</sub> has to be prepared other ways. But if the zinc cation in solution
takes the place of the copper, I'd be left with a solution of copper ions and sulfate ions. (aqueous CuSO<sub>4</sub> And zinc being higher on the reactivity series, it doesn't seem like it would want to
exchange with copper in the first place. This is a lot of fun. I'm learning enough of the textbook stuff to start thinking some of these things
through. 
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DraconicAcid
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Quote: Originally posted by kt5000  | | But if the zinc cation in solution takes the place of the copper, I'd be left with a solution of copper ions and sulfate ions. (aqueous
CuSO<sub>4</sub> And zinc being higher on the reactivity series, it
doesn't seem like it would want to exchange with copper in the first place. |
Zinc doesn't exchange with copper- it exchanges with zinc that's in solution. Zinc that's attached to copper is more stable than pure zinc, so it
ends up plating out on the penny.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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kt5000
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Gotcha. But if the zinc cations stick to the copper surface, what balances to the sulfate anions left in solution? Do we leave some solid zinc in
there to compensate for that?
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DraconicAcid
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| Quote: Originally posted by kt5000 | | Gotcha. But if the zinc cations stick to the copper surface, what balances to the sulfate anions left in solution? Do we leave some solid zinc in
there to compensate for that? |
Yes, the solid zinc gives up its electrons and goes into solution a the same rate (and in the same quantity) as the zinc from the zinc sulphate leaves
solution. So the zinc sulphate is basically a catalyst for the migration of solid zinc onto the surface of the penny.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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MrHomeScientist
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Here's one of my older videos, where I perform this experiment: https://www.youtube.com/watch?v=qp_3gY5v2S4
In it, I simply make a solution of NaOH, sprinkle some Zn powder into it, drop a few pennies on top of the Zn, and heat to near boiling for several
minutes. This formed a very nice plating of silvery zinc metal! I then took a few of these and gently heated them with a torch, which causes the zinc
atoms to diffuse into the copper and form the alloy brass, giving the pennies a golden color.
Interestingly, several years later and the 'silver' pennies I made have now turned completely 'gold'! This was pretty surprising to me. Apparently,
the zinc slowly migrates into the copper on its own over a long time. Heating them as I did just speeds up this process.
[Edited on 1-21-2015 by MrHomeScientist]
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Brain&Force
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This raises an interesting question.
Modern pennies (since 1983) have copper plating on a zinc body. But none of those turn brassy themselves – they only become brassy when zinc-coated
though the processes mentioned here.
Maybe it has to do with the layering or the copper plating process?
[Edited on 21.1.2015 by Brain&Force]
At the end of the day, simulating atoms doesn't beat working with the real things...
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kt5000
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Thanks for the advice MrHomeScientist. I remember watching several of your videos on Youtube. Great stuff!
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kt5000
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| Quote: Originally posted by Brain&Force | This raises an interesting question.
Modern pennies (since 1983) have copper plating on a zinc body. But none of those turn brassy themselves – they only become brassy when zinc-coated
though the processes mentioned here.
Maybe it has to do with the layering or the copper plating process?
[Edited on 21.1.2015 by Brain&Force] |
I was wondering that too. Specifically, whether I could just heat up a post-1982 penny and get that brassy appearance. Seems like it should work.
But those pennies don't exhibit that slow mixing process like MrHomeScientist mentioned.
Maybe it has to do with the plating process. I'm under the impression that the Zinc/NaOH process does a substitution reaction, which would result in
an extremely thin plating. The zinc sulfate reaction DraconicAcid mentioned deposits zinc onto the surface without needing to remove a copper, so it
could create a much thicker plating. Just a theory..
[Edited on 23-1-2015 by kt5000]
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DraconicAcid
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| Quote: Originally posted by kt5000 |
Maybe it has to do with the plating process. I'm under the impression that the Zinc/NaOH process does a substitution reaction, which would result in
an extremely thin plating. The zinc sulfate reaction DraconicAcid mentioned deposits zinc onto the surface without needing to remove a copper, so it
could create a much thicker plating. Just a theory.. |
That's not a theory; that's a hypothesis. And a poor one- both the Zn/NaOH and the Zn/ZnSO4 plating reactions will layer zinc onto the
copper without replacing it.
I suspect the reason zinc-core pennies don't go brass by themselves is that the copper cladding is much thicker than the zinc layer put on by our
reactions. I suppose if you kept them at an elevated temperature for long enough (without melting the core, which I've done on a hot plate by
accident), the zinc might migrate to the surface.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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kt5000
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I shall add a disclaimer that my posts are the musings of a first year chem major.  At any rate, the clarification is much appreciated.
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gdflp
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| Quote: Originally posted by Brain&Force | This raises an interesting question.
Modern pennies (since 1983) have copper plating on a zinc body. But none of those turn brassy themselves – they only become brassy when zinc-coated
though the processes mentioned here.
Maybe it has to do with the layering or the copper plating process?
|
It must have to do with the plating process, or perhaps the zinc needs to be on top of the copper. I just tried heating a 1999 penny slowly in a
propane torch and eventually the zinc just melted and started seeping out the edge of the coin. No color change whatsoever.
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