Common chemicals -- strange reaction

[woelen]

Recently I have done quite some experimenting with copper ions and also with iron ions. Now I decided to combine the two metals and a whole bunch of new and special phenomena pops up .

 

Many of you probably know the FeCl3.6H2O in the form of small pea-sized globules, used for etching copper from printed circuit boards. Especially when combined with some HCl it works quite well.

 

If you add some copper to a solution of FeCl3.6H2O in conc. HCl, then the copper dissolves and the solution darkens. Most people think that the following reaction occurs:

 

Cu + 2Fe(3+) --> Cu(2+) + 2Fe(2+)

 

Well, I found that the real reaction is MUCH more complex!

 

What do you expect to happen, when CuCl2 (containing Cu(2+) ion) is added to a solution of FeCl2 in HCl? If you look at the above equation, then you would not expect a reaction at all. In reality the solution turns black or very dark brown.

 

If someone has access to FeCl2 (or FeSO4) and also has access to CuCl2 (or CuSO4), then dissolve some of the iron (II) salt in conc. HCl (appr. 30%) and also dissolve some of the copper (II) salt in conc. HCl. Then add the two solutions to each other. You'll be surprised that the liquid turns almost black. So, there definitely is a reaction. So, apparently Cu(2+) oxidizes Fe(2+) to Fe(3+), itself being reduced to Cu(+).

 

Another experiment: Add a small amount of copper wire to a concentrated solution of FeCl3.6H2O (from an electronics parts store) in conc. HCl. The liquid becomes amazingly dark, almost black, the copper dissolves. Even with a large excess amount of FeCl3 the liquid still becomes black. So even a large excess of FeCl3 does not oxidize all copper to Cu(2+), but part of the copper remains in solution as Cu(+).

 

Finally, add a small amount of CuCl2 (or CuSO4) to a large excess of a solution of FeCl2 (or FeSO4) in conc. HCl. Again, the solution becomes almost black.

 

Conclusion:

Excess Fe(3+) ---> Not all copper (I) is oxidized to copper (II)

 

Excess Fe(2+) ---> Not all copper (II) is reduced to copper (I).

 

This seems a contradiction, so there must be a copper (A) species in solution with 1 < A < 2, in other words a fractional oxidation state of copper or a mixed valency complex of copper. This seemingly very simple reaction raises a lot of questions and a lot of interesting things to research. If anybody has ideas or is willing to repeat the experiment and play around with it, you're welcome .

[RyanJ]

OOO, Interesting! Thanks for sharing woelen. As soon as I get sometime free I'll he shure to try it out

 

Cheers,

 

Ryan Jones

[jdurg]

Don't disregard the ability of the H+ ion to act as an electron carrier in this instance. Since these reactions do not occur without the presence of a concentrated HCl solution, the hydrogen ions must be playing a role in this reaction.

[YT2095]

it might be an idea to try fractional crystalisation with some of the product also, just to see what you get, once or twice this has helped me identify "stuff" in the past