Hello everyone. Recently I made a solution of CuCl2 with some copper, HCl and H2O2. I used the following formula:
Cu + H2O2 + 2HCl -> CuCl2 + 2H2O
I used about 1g of copper wire, 16ml of 3% H2O2 and 3ml of 33% HCl, and unless I got my math wrong I should have respected the ratio given by the formula.
The thing is, the solution I got is a green solution. This indicates the presence of [CuCl4]2- complex ions. I studied that complex ions only form when an excess of a particular substance, in this case HCl, is added. But here I didn't add anything in excess, I followed the proportions as best as I could.
I even kept into account the side reaction of oxidation of HCl due to H2O2:
2HCl + H2O2 -> 2H2O + Cl2(g)
And I tried to reduce this side reaction by using very thin wires that react faster to leave less chlorine free to react with the peroxide, and I also added the peroxide one drop at a time, to not cause a buildup that would react with the acid.
But still I got a green solution. I can't explain this in any way. Can you help me? Maybe some concepts of complex ions are still not clear.
Another thing I studied is that when adding water, the H2O replaces the other ions around the metal, in this case it would replace Cl- around copper. This means that [CuCl4]2- goes back to CuCl2 and that should have a blue colour due to the hydration of copper. Anyway, if I let the solution evaporate, it's like reversing the addition of water, so indeed CuCl2 would become again [CuCl4]2- without adding Cl- in excess. Can you confirm this?
If so, it would have to be reformulated into something like "Complex ions form when there's a high concentration of ions per liter of solution" and not "They form when you add x susbtance in excess". Even if they're both right, the second definition is more confusing and less intuitive.
And to add to this (sorry for the long and articulated post, but I have so many questions) if I wanted to crystallize CuCl2, I would always get green crystals because before crystallizing, it would come to a very saturated solution, so high Cl- concentration per liter of solution.
And last part. When we have a concentrated solution of CuCl2, or any other salt that can form complex ions, does the [CuCl4]2- combine with a Cu2+ (possible but I think it wouldn't even exist if the Cl- is so concentrated to form complex ions) or would it combine with H+ instead, making something similar to chloroauric acid, but with copper?
Thank you very much for the help!!!
