Description of experiment
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Needed compounds:
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ethanol : CH3CH2OH
hydrochloric acid : HCl
methylamine : CH3NH2
anhydrous chromic acid : CrO3
Class:
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elem=Cr,N,C
redox
Summary:
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It is not possible to make methylammonium chromate or methylammonium
dichromate by mixing solutions of CrO3 and CH3NH2 in water and letting
the solution evaporate. Apparently, chromic acid is not sufficiently
acidic in aqueous solution to protonate the methylamine to a salt.
This is in strong contract with perchloric acid, which, when mixed
with excess methylamine, gives a nice dry solid CH3NH3ClO4 on evaporation.
This also is in strong contrast with ammonia, which gives a mix of
(NH4)2CrO4 and (NH4)2Cr2O7 when excess ammonia and CrO3 are mixed and
allowed to evaporate to dryness.
Description:
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Dissolve some CrO3 in water. Assure that the solution is not too strong,
e.g. 30% CrO3 by weight is suitable.
Prepare a 20% solution of methylamine in water and slowly mix this with
the solution of CrO3 and assure that an excess amount of methylamine is
used.
The resulting solution is dark red/brown. The solution is not orange or
yellow due to formation of dichromate ion or chromate ion.
Allow the solution to stand for a few days in a dry place, free of dust:
The liquid becomes more concentrated, most water evaporates, but no
solid is formed. The final product is a syruppy dark brown/red liquid
which has a faint smell of methylamine.
Take some of the syruppy liquid in a test tube and heat the test tube:
The liquid can be brought to boiling, but this drives off a lot of
methylamine. The methylamine even can be ignited by keeping a flame near
the open end of the test tube. On prolonged heating, the liquid
violently decomposes, accompanied with firy sparks and a lot of fizzling.
The final result is a dark green solid, which is not soluble in water.
The methylamine is oxidized and the hexavalent chromium is reduced to
trivalent chromium.
This experiment clearly shows that it is not possible to create a
solid chromate or dichromate of methylammonium by evaporating an aqueous
solution of excess methylamine and CrO3. It might be possible with drying
over P4O10 under vacuum, but simply air drying in a warm and dry place
only allows a thick syruppy liquid to be formed.
Add this syruppy liquid to ethanol: The liquid does not dissolve, it
sinks to the bottom. On shaking only a small amount of CrO3 dissolves,
the dark red/brown liquid sinks to the bottom again.
Add some concentred HCl and shake: A very violent reaction starts. The
ethanol is oxidized by the CrO3, a strong smell of aceetaldehyde is
produced and the liquid starts foaming violently and almost is ejected
from the test tube. The final liquid is dark green.