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Suppose that I wanted to electroplate Al(s) onto a Cu(s) Cathode, (using an Aluminium Nitrate solution and Al(s) anode).

At standard conditions, Al(s) is the strongest reducing agent, which is good. However, H2O (l) is a stronger oxidising agent than Al3+, which should mean that no Al3+ can be reduced to form the coating on the cathode.

Despite this, I have seen several experiments achieve such an electroplating (with a solution, not molten).

How is this possible?

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    $\begingroup$ Can you link to those experiments because this is very unlikely, unless they have cracked the code of making a solution of Al3+ with very high activity. $\endgroup$ – Nisarg Bhavsar May 22 at 7:34
  • $\begingroup$ @NisargBhavsar core.ac.uk/download/pdf/11725439.pdf. Experiment F. I’m trying to replicate this experiment with oxidation state of 3+, and Al3+ is the only one that I can think of that can reduce directly to solid. Maybe there is another metal that could work $\endgroup$ – Simplex1 May 22 at 7:37
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No ! It is not possible to deposit aluminum by electrolysis of a water solution. It may be done in a solution of $\ce{AlCl3}$ in an organic solvent like toluene, but not in water. And even in toluene, the conductivity is low. Metallic aluminum reacts with water, when this metal is not protected by an outer layer of aluminum oxide $\ce{Al2O3}$ as it is as soon as an aluminum piece is in contact with air.

For those who doubt about this reaction, let me add a few words. The reaction ion Al + H2O is not really violent, but it is visible. The only way of seeing this reaction is to prevent the alumina layer to cover the surface of the metal. And the only way of preventing this phenomena, is to dip a piece of metallic aluminum into an aqueous solution of mercuric chloride $\ce{HgCl2}$. In this case, the ions $\ce{Hg^{2+}}$ are able to cross the alumina layer and react with aluminum according to : $$\ce{2 Al + 3 Hg^{2+} -> 3 Hg + 2 Al^{3+}}$$ This reaction produces the appearance of some atoms of metallic mercury which form an amalgam with aluminum. Here the alumina layer is immediately formed but it does not cover entirely the metallic surface of aluminum. So the layer of alumina is not air proof or waterproof. So the corrosion continues. If the wet piece of aluminum is removed from the $\ce{HgCl2}$ solution, dried and deposited somewhere on a table, the alumina is formed steadily making sorts of white filaments that are growing before one's very eye. In a couple of minutes these filaments look like a delicate beard and measure several centimeters long. This phenomena stops when the fist layer of aluminum atoms is oxidized, because the mercury atoms are also eliminated. And the next oxidation of the aluminum surface is entirely covered by alumina.

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  • $\begingroup$ I see. I’m currently doing research on oxidation states, and metals which ions of 3+ oxidation states seem few and far between. I suppose the report I read failed to mention any additional solutions used. $\endgroup$ – Simplex1 May 22 at 7:21
  • $\begingroup$ "Metallic aluminium reacts violently with water" I highly doubt that, I have seen aluminium chips from cutting aluminium pieces come into contact with water, and nothing has happened. At best, it will tarnish slowly. $\endgroup$ – S R Maiti May 22 at 9:29
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    $\begingroup$ You can buy gallium aluminum “amalgam” and use it to generate hydrogen gas by putting it into water. Very expensive hydrogen generation, but same principle as with mercury: breach the aluminum oxide coat and reactivity happens! I upvoted, by the way. $\endgroup$ – Ed V May 22 at 20:11
  • $\begingroup$ @EdV Thank you for for the information about the alloy Al-Ga, the I did not know. $\endgroup$ – Maurice May 23 at 9:47

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