Re: Question: Reaction between copper (II) and copper (0) in concentrated HCl

From: Muhammar (muhammar_at_hotmail.com)
Date: 08/22/04 

Date: 21 Aug 2004 17:21:06 -0700

Somebody found the explanation probably 2 centuries ago but I don't
know. I can guess: First, concentrated CuCl2 solutions or Cu solutions
in HCl or large excess of chloride are yellow-brown. But I don't think
this is your case - it would not explain the color change.

Cu(I) solutions react easily with oxygen, maybe you are getting some
peroxy complex of Cu(I) which turns into Cu(II) but is being
re-generated by the Cu metal and air. But I have seen peroxy Cu(I)
complexes only in basic conditions (like in presence of pyridine as a
ligand) so I don't think this is the case either.

My guess is that the color is caused by a mixed-valency complex,
containing both Cu(I) and Cu(II). And there is easy way to test the
mixed-valency hypothesis:

Do the experiment again in a closed tube (with a septum as a stopper)
until you get finaly a colorless solution. But save some of the
starting Cu(II) solution sepately. Then add some of the starting
Cu(II) solution dropwise into the colorless Cu(I) solution in a closed
tube using a injection syringe with a needle through the septum. It
should turn brown if the hypothesis is right.

The more rigorous way would be to remove dissolved oxygen from your
Cu(II) added solution, but amount of oxygen in few drops will be small
so you can neglect it.

photo@woelen.nl (Wilco Oelen) wrote in message news:<2e41c276.0408201310.3c288880@posting.google.com>...
> When metallic copper is added to a solution of a copper (II) salt in
> concentrated HCl, then a very dark compound is formed. What is this?
> A better description, with some images, is given at the following URL:
>
> http://www.woelen.nl/chem/experiment_with_cucl2.htm
>
> I expected lightening of the liquid after adding the copper, due to
> formation of copper (I), which forms a colorless complex in
> concentrated chloride solutions. When a lot of copper is added (excess
> amount, relative to the copper (II) salt), and the test tube is
> stoppered, such that no oxygen can reach the liquid, then after a few
> days, the liquid indeed becomes colorless, which can be explained by
> assuming that all copper (II) has been converted to copper (I). As
> soon as some air is allowed to reach the colorless liquid, it becomes
> dark brown again, starting from the surface of the liquid.
>
> I do not think the darkening is due to formation of more copper (II),
> the color is quite different and the liquid becomes dark already, even
> when only a tiny amount of copper has dissolved.
>
> If someone has a clue, I would appreciate to read about that.
>
> Thanks,
>
> Wilco

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