YT2095 - 15-2-2007 at 06:38
I`ve just seen/done something that totaly defies any explaination I`m capable of!
Cr2O3 (made from burned Ammonium dichromate and washed in plain water until clear).
1 heaped spatula of it in a test tube and then 3 ml of Ammonia soln (about 10%).
there will be a slight color change but nothing drastic, then add the same amount again of H2O2 (9% 30vols).
gas will be liberated and very slight warming will be noticed, it will also make little "underwater volcanos" for half hour or so, quite interesting
to watch actualy.
there will Also be a Blackish PPT in there amongst the green Cr2O3.
now get a NIB magnet next to the test tube, all these black particles will slowly move towards the magnet, not like Iron would (instantly) but very
slowly as the liquid fizzes a little, remove the magnet and they drop again.
Chromium is Not a Magnetic material???
what`s going on?
Unch - 15-2-2007 at 08:38
Pardon my ingenuity, Did you use a metal spatula?
When did you do that?
Thanks for the anshwer. Lovely.
[Edited on 15-2-2007 by Unch]
12AX7 - 15-2-2007 at 08:46
Ever heard of CrO2 casette tapes? 
Burnt Cr2O3 being what it is, it's most likely unburnt Cr(VI) being reduced by the H2O2, as:
CrO4(2-) + H2O2 = CrO2(s) + 2OH- + O2
Erm, does that work? Guess so.. charge is conserved and everything adds up... 
http://en.wikipedia.org/wiki/Chromium(IV)_oxide
Not a mentioned synthesis, but CrO2 fits the observation.
Tim
[Edited on 2-15-2007 by 12AX7]
YT2095 - 15-2-2007 at 08:56
Tim, You`re a Genius! Thanks!
Mystery Solved
YT2095 - 15-2-2007 at 09:00
about 2 and a bit hours ago.
not_important - 15-2-2007 at 10:08
Yeah, heating Cr2O3 in air generates some higher oxidatation states, if you do the dichromate volcanoe the darker colour on the oxide is from those
higher oxides.
This is a problem in industry where Cr2O3 or Cr(III) containing catalysts are use for oxidations. The chromium is slowly oxidised, when replacing the
catalyst care must be taken because of the higher oxides present as a fine dust. The higher oxides are also slightly volatile, and migrate about in
the reaction stream, leaving little piles in the system.
In fireing pottery, if a Cr(III) containing glaze has been used, it is not uncommon to traces of green, yellow, or pink on pieces further 'downwind'
in the flue gas flow, formed by traces of the higher Cr oxides.
YT2095 - 15-2-2007 at 10:23
I`ll try again in the morning, but this time only with the washed Cr2O3 and H2O2, I`m not sure the Ammonia soln played a big roll in any of this.
I know what I was thinking when I did this experiment and won`t risk embarasing myself by stating my intent, but MnO2 and acid catalysed by H2O2
played a part in my thinking, and so a stronger Base will be the other experiment for the morning.
woelen - 15-2-2007 at 10:32
Unch Unch .... we have another troll.
<<<<
YT2095, the decomposition of ammonium dichromate always is incomplete, even when the remaining ashes are calcined for some time (assuring that all
chromium (VI) is destroyed and all ammonia is expelled). I have some pure Cr2O3 (prepared from a chromium (III) compound and calcining) and that looks
really green. I also made CrO2 (by mixing a solution of potassium dichromate and a solution of chrome alum and leaving this in contact for a few
weeks). The CrO2 is fairly dark brown.
Pictures of pure Cr2O3 and of CrO2:
http://woelen.scheikunde.net/science/chem/compounds/chromium...
http://woelen.scheikunde.net/science/chem/solutions/crIVO2.j...
My 'ashes' from lab reagent grade ammonium chromate always are dark green, almost black. The 'ashes' from ammonium dichromate are the same.
YT2095 - 15-2-2007 at 12:41
I`m not entirely sure How this works (or even if it does) but CrO2 being ferromagnetic and having a currie point (temp) at 298k workable seemed a
little strange. so looking at the PTOE, if you remove all from La onwards and fit the F Block in, you`ll see that Neodymium fits in exactly the same
group as Cr.
Ce/Ti, Pr/V, Nd/Cr.
I could be talking out my ass, but it seems very coincidental, when you consider that Neodymium is one of the 3 elements in a NIB magnet.
of Course doing the same Samarium Cobalt magnets (also very strong) don`t line up as perfectly it`s offset by One and should be Fe/Sm.
but it makes you wonder
well it does Me at least!
12AX7 - 15-2-2007 at 14:40
Doesn't work that way though. Just coincidence. The lanthanides and actinides actually belong between the alkaline earths and the rare transition
elements Y, Lu, Lr, much the same as the transition metal (d-block) is placed between them and B, Al, etc. (p-block). And yes, for really really big
atoms (Z > 120, if they're even detectably stable), we'll get a g-block.
Almost all the lanthanides have their electrons hiding under the s orbital, so unlike the rich transition metals, the rare earths are almost
completely boringly uniform. Exceptions are La, Ac and Th at the left end (just one electron (or, for Th, two electrons) prefers to put itself in the
d shell) and Gd and Cm in the middle (apparently, half-filled shells are akward: see also Cr and Mo, but not W).
If you don't know what orbitals and shells are, it will suffice to imagine the atoms' electrons buzzing around the nucleus in specific "shells". A
number of electrons can occupy each shell, in a number of states. The higher shells (in order: s, p, d, f, g, ...) can contain more electrons,
therefore the (s, p, d, f, g..)-blocks get progressively wider down the periodic table. (Note that the rows count 2, 8, 18, ...) When a shell is
full, electrons pile into the next, and so on. Shells repeat, which is why the s- and p-blocks are so tall. Occasionally an electron finds its way
into an unexpected shell (as with Cr, Cu, Gd, etc.) because...no reason really, it just finds its lowest energy state there.
Tim