[Stinkytick]

First of all, I'm truly sorry if I have any mistake, because I'm not English and I'm not used to chemistry specific terms.

So last week, we were working on copper recycling in the school lab, and in the end of the first experience, which involved adding NHO3, Na2O, HSO4, solid Zn and HCl (in this order) to 0.3g of Cu. However, we ended up having a yield of 105%. Suspecting it was because of some Zn that hadn't reacted we added more HCl to the copper.

Then we left it there for 5 days, reacting and allowing the particles to reach the bottom of the beaker. But when we came back to the lab we noticed that the solution was an intense green, instead of the usual blue we were supposed to have. So we added some Zn for it to react with whatever was the intense green component, which led us to a solution that no color at all with plenty of what looked like a dark salt in the bottom.

We actually thought it was some kind of copper oxide, so we added some HSO4 which led to a "boiling" reaction that didn't have any noticeable changes to the naked eye.

So we know that the dark substance isn't copper oxide, and we suppose that it's some kind of copper salt, but we have no idea of what it specifically is, and how are going to have solid and pure copper again.

Hope you can help me...

[Stinkytick]

I don't know if bumping is against the rules...

Anyone out there who can help me?:/

[Grundalizer]

What was the point of this reaction?  Why did you add the sodium oxide?  Why did you add zinc and HCl?

I've done something similar to this but it used electrolysis to recover the copper.  Adding Zn and HCl would just create a bunch of hydrogen gas and aqueous zinc chloride

[Stinkytick]

Which reaction are you referring to? The first one or the one that we tried in order do reduce the yield?

[Grundalizer]

 which involved adding NHO3, Na2O, HSO4, solid Zn and HCl (in this order) to 0.3g of Cu.

[Stinkytick]

Oh we were trying to have this cycle:

Cu -> Cu(NO3)2 -> Cu(OH)2 -> CuO -> CuSO4 -> Cu

Here are the reactions:

Cu (s) + 4 HNO3 (aq) -> Cu(NO3)3 (aq) + 2 H2O (l) + 2 NO2 (g)

Cu(NO3)2 (aq) + 2 NaOH (aq) -> Cu(OH)2 (s) + 2 NaNO3 (aq)

Cu(OH2) (s) -> CuO (s) + H2O (l)  (by heating)

CuO (s) + H2SO4 (aq) -> CuSO4 (aq) + H2O (l)

CuSO4 (aq) + Zn (s) -> Cu (s) + ZnSO4 (s)

[Grundalizer]

Were you adding excess reagents in the next steps of the reactions?  Was everything just added to one single beaker?

I could see the sodium hydroxide in step two reacting with the nitric acid to neutralize it, but if it was not added in excess your next step wouldn't take place

[Zerm]

It seems likely to me that the green solution obtained after adding more HCl is caused by presence of copper chlorides.  This could be caused by contamination with copper salts in your end product if it were not properly washed which could account for your unexpected extra yield. 

Copper chloride can still form in HCl from pure copper, although slowly, by the slow oxidation of copper from dissolved oxygen.  The resulting chlorides can catalyze the further decomposition of the copper, speeding the process as Copper (II) Chloride is reduced to Copper (I) Chloride by the copper metal and the HCl oxidizes the Copper (I) back to Copper (II).

I read an interesting article that was linked from science madness:
http://woelen.homescience.net/science/chem/riddles/copperI+copperII/index.html

The article proposes the formation of copper chloride complexes which can have a very dark color.  The site has experiments which demonstrate the phenomena.  In the experiment demonstrating this supposed dark colored complex, copper metal is used to reduce some Copper (II) Chloride to Copper (I) Chloride which supposedly forms a dark colored complex. 

I'm not sure how this complex works but it seems a possible explanation for the dark color seen at the end of your experiment.  Of course, more information is needed about your procedure to more accurately determine what could have gone wrong.  I love copper chemistry though, so many beautiful colors.

[ardbeg]

if you have CuCl2 in solution and keep adding HCl you will get [CuCl4]2- ions which are yellow, might help.