I’m researching how the classic “alchemy” demonstration works in which zinc is plated onto a copper penny in a solution of sodium hydroxide. Most online resources refer to the electroplating of copper with zinc, which is not what I’m interested in. The best article I’ve been able to find on the electroless plating of zinc onto copper is from the Royal Chemistry Society. However, I have several questions. The three reactions this article describes are:

$\ce{Zn(s) + 2NaOH(aq) + 2H2O(l) -> Na2Zn(OH)4(aq) + H2(g)}$, which is the “formation of sodium zincate”

$\ce{Zn(s) -> Zn^2+(aq) + 2e- }$, which is the “electrode reaction” at the “zinc electrode” and is “followed by the complexing of the zinc ions as $\ce{Zn(OH)4^2- (aq)}$”

$\ce{Zn(OH)4^2- (aq) + 2e- -> Zn(s) + 4OH- (aq)}$, which is the “electrode reaction” at the “copper electrode”

I really don’t understand how these three reactions come together to describe the plating of the zinc onto the copper. Specifically, I don’t understand:

1) What is the role of zincate, $\ce{Zn(OH)4^2- (aq)}$, in the reaction?

2) On the molecular level, how are the zinc and copper actually attached together?

3) How is this an electrochemical cell (i.e. how is electricity involved)? Where can I learn more about electrochemical cells?


1 Answer 1


An explanation is provided in this reference. The idea is that the electrode reactions are

$\ce{Zn(s) + 4OH^- -> [Zn(OH)_4]^{2-} + 2e-}$ and

$\ce{[Zn(OH)_4]^{2-} + 2e- -> Zn(alloy) + 4OH^-}$

So the zinc powder remaining will go into solution and the zincate is reduced on the copper surface to form a form of zinc rich brass. Brass is an alloy and the atoms are held together via metallic bonding.


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