YT2095 - 10-8-2006 at 03:54
I`m wondering if anyone can explain my observations.
I took a small amount of KBr and added a few ml of 50% H2SO4 and let the crystals dissolve in the test tube.
I then took a 3cm length of 3mm copper wire and pushed the one end into the cork that fits the test tube.
I added an equal volume of NaOCl 5% to the now HBr and put the cork on the test tube.
Br2 was liberated and a small amount of heating took place, the Br2 gas level reached the copper wire.
I let it rest like that overnight expecting small Crystals to form on the copper wire.
there was a Blackish mass at the end of the copper wire, and greeny blue (like copper carbonate) patches dotted around the rest of the wire.
upon agitation of the soln bromine again was liberated and the green/blue patched turn into black droplets.
but left to stand, and the green/blue re-appears again???
anyone know why this is happening?
the black "Lump" at the end of the wire remains unchanged.
hodges - 10-8-2006 at 13:37
Well according to the CRC handbook, copper (II) bromide is black, and it absorbs moisture from the air which would explain the black liquid. You may
have a water solution of CuBr2.
The green may be copper sulfate, formed from the H2SO4 and NaOCl if any of it touched the wire.
Hodges
YT2095 - 11-8-2006 at 00:48
well another 24 hours later and the strangest thing, that lump of black material that formed on the tip of the copper wire is now Also
white/blue-green???
so copper bromide seems to form, and then decompose almost directly afterwards?
Nicodem - 11-8-2006 at 01:10
CuBr2 is in continous 2CuBr2 <=> Cu2Br2 + Br2 equilibrium (moving to the right as the temperature increases). Hence it is a moderately strong
oxidant and will oxidaze many things. It will also as all Cu(II) salts oxidaze copper to Cu2Br2 (which should be white to blue-green). You can't get
CuBr2 unless bromine is in excess (like it temporarily is on the surface of Cu). Otherwise you get Cu2Br2 from Br2 and Cu when copper is in excess.
YT2095 - 11-8-2006 at 01:47
that`s Fantastic, thanks Nicodem 
it explains what I`m seeing perfectly, and also goes some way to explain why they use Bromine reservoiurs in the Larger Copper Bromide lasers.
Cheers
woelen - 12-8-2006 at 09:38
Copper (II) ions also form a beautiful red/brown/purple complex with bromide ions at high concentration. At very high concentration that complex
almost looks black. That also may be an explanation of your observation, because I have strong doubts that such an extremely hygroscopic salt as CuBr2
remains black in the presence of water.
Just try to make 1 ml of very concentrated solution of KBr and add a very small pinch (just a few crystals of sub-millimeter size) of a copper salt
like CuSO4.5H2O. You'll see that the liquid does not turn blue, but kind of red/brown/purple. This is due to complex formation, CuBr4(2-).