Why does potassium dichromate burn?
Robert Cerpa
A friend of mine was going to do a "reaction" involving the
combustion of K2Cr2O7, potassium dichromate, which apparently
burns in air. Potassium dichromate is extremely toxic, and
ulcerates skin and mucous membranes, so fortunately she decided
not to do the experiment after all.
What I can't understand is what exactly is burning when the
potassium dichromate heats up. Chromium would have a formal
oxidation state of +6 in this compound, as high as it can go,
and K2Cr2O7 is a powerful oxidizing agent. So assuming that the
reaction is the reduction of chromium(VI), what exactly is getting
oxidized in this combustion? Or am I thinking about the problem
in the wrong way? None of my inorganic chemistry books discuss
this, and an inorganic chemist I consulted didn't know the answer
either.
Any ideas out there? Thanks!
Robert Cerpa
ce...@cgl.ucsf.edu
Graduate Group in Biophysics, Univ. of Calif., San Francisco
Matt Parker
writes:
> A friend of mine was going to do a "reaction" involving the
>combustion of K2Cr2O7, potassium dichromate, which apparently
>burns in air.
> What I can't understand is what exactly is burning when the
>potassium dichromate heats up.
Your friend was probably thinking of _ammonium_ dichromate, an orange solid
which when "ignited" decomposes according to the following reaction:
(NH4)2Cr2O7 ---> Cr2O3 + N2 + 4 H2O
Thus, two atoms of Cr(+6) are reduced to Cr(+3), and two atoms of N(-3) are
oxidized to elemental nitrogen(0).
I've never done this myself, but I've seen pictures of a small heap of the
stuff burning away, making a nice little "volcano" and leaving a pile of
green chromium (III) oxide. Relatively safe, I think; just don't get the
dichromate on your skin.
--Matt
Aaron Ray Clements
>--Matt
I've done this; the orange powder burns with an intense flame (cracked the
borosilicate beaker we were using, and permanently embedded chromium oxide
in the glass) and throws finely divided chromium oxide *everywhere*. It's
messy, but impressive.
aaron
a...@cco.caltech.edu
E. Spitzberg
ind...@mace.cc.purdue.edu (Matt Parker) writes:
And be careful, the chromium (III) oxide is toxic, too, and it can cause
cancer
Eddy