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I'm watching this tin(II) chloride electrolysis video:
https://www.youtube.com/watch?v=coPGazAaVBE&ab_channel=FlinnScientific

Two phenomenon puzzled me, and I couldn't find a good explanation on google-

(1) Why is the tin metal dendritic when it's formed on the cathode? enter image description here(Left hand side)
I searched for tin, and I only found that its crystal structure is "​body-centered tetragonal" or " ​face-centered diamond-cubic", which I'm unable to link with the needle-like appearance. Is there a reason?

(2) Why does the unconnected paperclip act like an electrode? enter image description here(In the middle, it has dendritic metal on its right, and little amount of white precipitate on its left)
I thought Sn(II) ion would have to receive electrons to be reduced to tin metal. How can it be reduced in the middle of the petri dish, with no wire connected to it?

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    $\begingroup$ Regarding your second question, note that the tin chloride solution is a solution of an electrolyte, so it conducts electricity. It has resistance and conductance. So any point in the solution is at a potential intermediate between that of the cathode and anode. The little wire near the center has a potential that is negative with respect to the anode, but positive with respect to the cathode. So tin dendrites grow from the right (anode-facing) side of the little metal wire toward the anode. And the precipitate forms on the left (cathode-facing) side, etc. $\endgroup$
    – Ed V
    Oct 17, 2021 at 19:26
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    $\begingroup$ By the way, my hypothesis can be tested. Replace the 9 V battery by a DC power supply, adjustable from 0 V to 9 V or so. Start over with 0.5 V applied and observe that no tin dendrites occur. Increase the applied voltage in small increments, e.g., 0.2 V. Stop increasing the applied voltage when tin dendrites just start to appear slowly. Then put the little wire in the center. Nothing should happen: the potential is too low half-way between the cathode and anode. Now double the applied voltage or go a little higher. Dendrites should slowly grow on the little metal wire. $\endgroup$
    – Ed V
    Oct 17, 2021 at 20:46
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    $\begingroup$ The demo of electric field on a small wire is amazing (Q. no. 2). I never thought of this experiment. $\endgroup$
    – AChem
    Oct 18, 2021 at 1:47
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    $\begingroup$ Sure the ions carry current! I have never done this demo before, but I have a jar of granulated tin, hydrochloric acid (“muriatic acid”) is sold in hardware stores where I live, and I have adjustable DC power supplies, etc., so I will try the experiment next week (this week will be busy for me). As for your question about the paperclip, perhaps it is comproportionation? This will be a fun little experiment, however it turns out! Thanks for asking the question, which I upvoted first! $\endgroup$
    – Ed V
    Oct 18, 2021 at 11:56
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    $\begingroup$ @EdV The more pertinant point is that Tin(s) is a good conductor. Solution conductivity needs to be higher by an order of magnitude or so, in order to get uniform growth. $\endgroup$ Oct 19, 2021 at 5:28

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I have done this experiment plenty of times with my students. It is one of my favorites. The results are surprising, and often difficult to reproduce. It depends on the current. With a low current $(I < \pu{0.05 A}),$ long, bright and thin metallic needles are obtained starting from the cathode. They sometimes touch one another in the middle of the box, like in your experiment.

Anyway, after a couple of minutes, the needles grow so much that they join the electrodes. This must be avoided, because it stops the electrolysis process. In this case, the “bridge” should be broken by hand with a glass stick.

If the current increases $(I > \pu{0.1 A}),$ the needles are not so thin. On the contrary, they tend to be tightly packed. If the current is rather high $(I > \pu{0.2 A}),$ they even look like gray and dull moss… Deceiving!

On the anode, the $\ce{Sn^{2+}}$ ion is oxidized into $\ce{Sn^{4+}}$ according to

$$\ce{Sn^{2+} -> Sn^{4+} + 2 e-},\tag{R1}$$

but this ion is quickly hydrolyzed into weekly acidic solution, producing a white deposit of $\ce{SnO2·nH2O}$ according to

$$\ce{Sn^{4+} + $(2 + n)$ H2O -> SnO2·nH2O + 4 H+}.\tag{R2}$$

It is possible to prevent this white precipitate by dissolving $\ce{SnCl2}$ in a $\pu{2 mol/L}$ $\ce{HCl}$ solution, which reverses the last equation.

Something interesting can be observed at the end, when you think the experiment is finished. At this moment, be careful. Don't touch the solution, but disconnect the wires connecting the electrodes to the power supply. And quickly insert these plugs into a $\pu{50 mA}$ ammeter. You'll see that some current is produced for a couple of seconds. Your small tray is working like a galvanic cell!

Last but not least. At the end, when the tin has invaded the whole solution (except a tiny zone around the anode), try to save the tin from the tray with your hand (gloves!). Press it on a blotting paper or a filter paper. When dry put it in a tiny test tube, and heat it with a Bunsen burner. It quickly melts, and the liquid can be thrown on the lab bench: it makes a bright and shiny metallic stain or platelet. Looks like silver! Now try to bend this thin platelet with your hands near the ear. It makes a surprising noise, showing that bending the tin platelet breaks tiny tin crystals.

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    $\begingroup$ (+1) Nice! How did you learn about the short-lived galvanic operation? $\endgroup$
    – Ed V
    Oct 17, 2021 at 21:09
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    $\begingroup$ I did it so many times, for many years, that one of my students once asked me to try ! $\endgroup$
    – Maurice
    Oct 17, 2021 at 21:32
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    $\begingroup$ @Maurice, Since you have been teaching this experiment for ages, do you recall the second part of the experiment where the metal rod is placed in the center and then electrolysis occurs. It is a beautiful experiment. I am wondering if Ostwald ever noted it in his book on Electrochemistry: Theory and its history. $\endgroup$
    – AChem
    Oct 18, 2021 at 3:35
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    $\begingroup$ Nice observation on the different tin appearance at different current, and the galvanic cell! Do you know the theory on the appearance difference at different current? I would like to read that. $\endgroup$
    – Wang
    Oct 18, 2021 at 3:45
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    $\begingroup$ Is the sound at the end related to "tin cry"? $\endgroup$ Oct 18, 2021 at 14:24

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