[Shadow3299]

I hope this is apropiate for this forum...  I was in my class, doing some chemical equations that looked like this:

2AgNO₃ + Cu ? 2Ag + Cu(NO₃)₂
2AgNO₃ + Mg ? 2Ag + Mg(NO₃)₂


just part of a silver extraction experiment.  But my teacher took the copper nitrate and the magnesium nitrate, and poured them into the same beaker.  The copper nitrate was light blue, the Mg nitrate was clear.  It turned a lime green color.  I don't know what he did with the solution, and i've been trying to figure out why this color change happend.

So far, I think it might be somthing with patina.  I think that Iron nitrate might be green, but i'm looking at that.  I hear that if you take copper nitrate and soak it on a stick, then burn it on a buncen burner it will turn emerald green.

If anyone can help explain this to me, that'd be great.

[wereworm73]

It looks like the additional nitrate ions from your Mg(NO₃)₂ solution replaced the water ligands on the copper ions.  The nitratocopper(II) complex is green while hexaaquocopper(II) complexes are light blue.

[Borek]

my teacher took the copper nitrate and the magnesium nitrate, and poured them into the same beaker

Was it an empty beaker, or did it contained something already?

wereworm answer looks plauisble, but I wonder if the excess of nitrates was high enough. Do you know what were the conentrations of both nitrates solutions?

[Shadow3299]

I think he poured one of the solutions into the beaker with the other... I'm pretty sure there wasn't anything in it already.

As for the concentration, i know that the silver nitrate was .5m/L, but I don't know about the others.

And I forgot to say that we used water to filter the silver, so each solution has water (I don't know how much) in it.

[Shadow3299]

It looks like the additional nitrate ions from your Mg(NO₃)₂ solution replaced the water ligands on the copper ions.  The nitratocopper(II) complex is green while hexaaquocopper(II) complexes are light blue.

I'm not too famillar with these, can you explain the chemical equation? Cu(H₂O)₆ is hexaquocopper, right?  so the equation would look like this?: [Cu(H₂O)₆](NO₃)₂ + (NO₃)₂ ? [Cu(NO₃)₂](NO₃)₂ + xH₂O  ...I really have no idea...

[Borek]

Replace (NO₃)₂ with NO₃^-.

Complex charge depends on number of nitrate ions, it is multi stage equilibrium - up to six NO₃^- can replace water particles.

Write only net ionic equation.

[wereworm73]

I would say the lime-green complex was [Cu(H₂O)_6-x(NO₃)_x]^2-x, with x probably being 1 or 2 (assuming your Mg(NO₃)₂ solution was roughly the same concentration as your copper one).  I'm not 100% sure on this, but I think only one nitrate ligand is enough to make the complex green.


So, you could write the equation as:

[Cu(H₂O)₆]²⁺  + x NO₃^- <==> [Cu(H₂O)_6-x(NO₃)_x]^2-x + x H₂O