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In class, we performed a lab that involved heating a solution of zinc sulfate with zinc strips at the bottom of the beaker. A penny was placed on top of the zinc strips and soon after the penny became covered in a layer of zinc. What process causes the zinc layer to develop?

The explanation was that the zinc metal transferred electrons to copper which in turn reduced the zinc ions in solution, but this does not seem to make any sense as the zinc is being oxidized to just be reduced again. Why does this happen. Is a thin zinc layer more stable?

Here are the instructions for the experiment

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  • $\begingroup$ Was any electricity involved in this plating? $\endgroup$ – March Ho Sep 5 '15 at 13:51
  • $\begingroup$ No the metals simply contacted each other $\endgroup$ – popgalop Jun 7 '16 at 23:45
  • $\begingroup$ I made a video on the process of electroplating copper (a penny): youtube.com/watch?v=JOeJpoVqBMQ Using Ni here, not Zn. Same principles though $\endgroup$ – khaverim Jun 8 '16 at 16:10
  • $\begingroup$ None of the answers to this question account for the electrochemical potential induced by the strain defects present from milling present in the zinc powder. $\endgroup$ – A.K. Dec 17 '16 at 19:57
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The system of $Zn$ metal in a solution of $\ce{Zn^{2+}(aq)}$ is an equilibrium. Which means that the rates of the reactions $\ce{Zn -> Zn^{2+} + 2e-}$ and $\ce{Zn^{2+} + 2e- -> Zn}$ are happening in exactly the same rate and this rate is not zero. It is critical no notice that this rate is non-zero.

By heating up, you are accelerating both reactions, probably at different rates so there will be a new equilibrium point.

The precipitation is happening on any surface that it can nucleate on and this surface might be the surface of the penny or some other zinc. This is immaterial.

The counter reaction doesn't really matter here since it just happens on whichever metal the other reaction is happening and it is just the reduction or oxidation of water.

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Electricity is involved: the copper (well, copper plated zinc in the US) penny is in direct contact with the zinc, forming a short-circuited connection. Forcing the plating action "backwards" (copper being more electronegative than zinc) requires current.

Try the same experiment but with a separator, such as a piece of filter paper, between zinc and copper; there should be no plating.

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    $\begingroup$ But i still dont understand why electrons would go from the zinc metal to the copper metal just to go back into the zinc $\endgroup$ – popgalop Sep 6 '15 at 2:43
  • $\begingroup$ That would make reaction have no net change as you start and end with zinc solution, copper metal, and zinc metal. $\endgroup$ – popgalop Jun 8 '16 at 15:02

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