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Today in class we did the "gold penny" experiment, where we put metallic zinc and a penny in a zinc chloride $\pu{1 M}$ solution at $\pu{100 °C}$, which caused the penny to turn silver as the zinc coated it. Then, we held the penny to a flame to form brass.

I know this is a redox reaction where the zimc is being reduced, but I am confused about the details. What is the exact reaction that goes on? I think the zinc is being reduced into metallic form, and the copper is oxidized. However, I am confused about the specifics - do the $\ce{Cl-}$ ions re-form $\ce{ZnCl2}$ with the metallic $\ce{Zn}$? Or do they stay with the copper after oxidizing it?

Sorry if this is simple or obvious - I am 14 years old and just started my first chemistry class.


I found this video on a similar reaction today:

https://youtu.be/EaGDdTKFTHY?si=111nRXewSfsPLwZM

It seems to state that a voltage of about 1.1V is created between the zinc and copper, which is enough to drive the electroplating reaction.

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    $\begingroup$ Was the metallic zinc piece connected to the penny ? Could they form a galvanic cell $\ce{Zn(s,zinc) + Zn^2+(aq) <-> Zn^2+(aq) + Zn(s, penny)}$? As Cu reducing Zn seems strange. $\endgroup$
    – Poutnik
    Aug 11, 2023 at 9:37
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    – Poutnik
    Aug 11, 2023 at 10:34
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    – Poutnik
    Aug 11, 2023 at 10:35
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    $\begingroup$ To the person attempting to edit this and replace the word "penny" with the word "cent": in the US, it's called the penny; you would use the word "cent" to refer to the decimal portion of a price of something less than $1 (which we would equate as 100 cents). Also, if you're going to edit, go all-in and fix all the things. $\endgroup$ Aug 11, 2023 at 14:51
  • $\begingroup$ @Poutnik The zinc was granular, and it was sitting at the bottom of the beaker along with the pennies. Therefore, the pennies & zinc definitely had contact. $\endgroup$ Aug 12, 2023 at 0:58

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Both the question and the other answer suggest reduction of zinc by metallic copper, which is impossible to any significant extent. Both the $\ce{Cu^{2+}/Cu}$ and $\ce{CuCl/Cu + Cl^-}$ couples have standard reduction potentials about 1 V greater than the $\ce{Zn^{2+}/Zn}$ couple, which would preclude any significant degree of reaction at equilibrium according to the Nernst Equation.

A more chemically plausible hypothesis is that a zinc(I) species is formed from the zinc metal and aqueous $\ce{ZnCl2}$. Wikipedia, citing Ref. [1], reports that molten zinc chloride dissolves metallic zinc and the solid formed by rapid cooling of this solution contains $\ce{Zn2^{2+}}$ ions:

$\ce{Zn^{2+} + Zn(s) <=> Zn2^{2+}}$

If such a reaction were to occur reversibly in aqueous solution, even to a minor extent, the copper could serve as a favored site for the reverse reaction with deposition of zinc. On this model the metallic zinc added to the solution as granules is effectively being redistributed onto the penny.

Once the zinc is deposited, the heating stage is simply an alloying of the zinc with the copper.

Modern-day US cents already contain zinc, so it may be possible to turn the surface to brass without the pre-deposition of a zinc coating from the solution.

Reference

  1. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 978-0-12-352651-9.
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    $\begingroup$ Thanks for the answer. On the tangent of the heating part, I was able to make a gold-ish penny just with a flame (albeit with colored bands of oxidization that weren’t present when melting the zinc onto the copper). $\endgroup$ Aug 12, 2023 at 0:59
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    $\begingroup$ And it won’t work with a penny minted before 1982. It will work with some pennies minted in 1982, but not all, and it works with all pennies (except very rare U.S. Mint planchette errors) minted after 1982. (I was the first upvote, BTW.) $\endgroup$
    – Ed V
    Aug 12, 2023 at 1:35
  • $\begingroup$ Modern US pennies are zinc with copper plating. $\endgroup$ Aug 12, 2023 at 22:45
  • $\begingroup$ Starting some time in 1982 and continuing on, U.S. pennies are copper plated planchettes, with the planchettes being 99.2% zinc and 0.2% copper. The overall composition is 97.5% zinc and 2.5% copper and their clean uncirculated weight is 2.50g. Pennies in, e.g., 1975, were 95% copper and 5% zinc and their clean uncirculated weight was 3.11g. $\endgroup$
    – Ed V
    Aug 13, 2023 at 12:50

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