I conducted an experiment to find out the difference in products/ the final solution whilst using different electrodes. On using copper I obtained a teal blue soluble salt I believe to be $\ce{CuCl2}$ yet upon using aluminum I obtained a silvery sludge of a solution. The aluminum anode was definitely oxidized by chlorine ions in the solution. Also there was notable effervescence at the cathode of what can only be hydrogen gas.

Could someone please tell me what aluminum compound remains in the Liquid and shed some light on the anode and cathode reactions. I'm an amateur so I had some difficulty understanding the outcome.

  • $\begingroup$ You are getting Al(OH)3, and "solution" is a wrong word to describe it. $\endgroup$ – Ivan Neretin Feb 15 at 8:36
  • $\begingroup$ Yes sorry. I forgot to mention it is insoluble $\endgroup$ – Ab sans Feb 15 at 10:05

Aluminum pieces are always covered by a thin, waterproof and nearly monomolecular layer of alumina, or aluminum oxide $\ce{Al2O3}$. This layer prevents the aluminum metal to get in touch with water. Because, pure and non-protected aluminum would immediately react with water according to : $$\ce{2 Al + 6H2O -> Al(OH)3 (s) + 3 H2 (g)}$$ This reaction can only occur when the alumina layer has been removed. And electrolysis is a way of destroying this layer. There are also other ways, like adding $\ce{CuCl2}$ or $\ce{HgCl2}$ in the solution. But rubbing or scratching aluminum is not such a way, because removing alumina mechanically will allow oxygen from the air to immediately reoxidize the metal and regenerate the alumina layer.

So it is difficult to understand what is happening in the electrolysis of an aqueous solution with aluminum electrodes, because there are two sorts of chemical reactions happening simultaneously : the chemical reaction of aluminum on the solution without electricity, and the electrolysis of the solution.


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