Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
ammonia : NH3
perchloric acid : HClO4
potassium dichromate : K2Cr2O7
DL-malic acid : HOOCCH(OH)CH2COOH
Class:
------
elem=C,Cr
coordination
redox
Summary:
--------
Dichromate/chromic acid is reduced by malic acid and in this reaction an
intensely colored purple complex of chromium(III) is formed. If the reaction is
performed in the presence of an excess amount of strong non-coordinating acid,
then the reaction proceeds faster, and also a purple complex is formed, but the
color is much less intense.
Without the extra acid, there really is a new complex with chromium(III). This
complex reacts with ammonia, giving a nice green solution.
Description:
------------
Dissolve a few hundreds of mg of malic acid in appr. 4 ml of water. Keep this
in test tube A. This is a colorless solution.
Dissolve the same amount of malic acid in appr. 4 ml of around 10% of
perchloric acid. Keep this in test tube B. This also is a colorless solution.
Dissolve 100 mg or so of potassium dichromate in 3 ml of water: Add half of
this orange liquid to test tube A and add the other half of this liquid to test
tube B.
The liquid in test tube A remains orange. The liquid in test tube B almost
immediately loses its bright orange color and becomes more brown/orange and
darkens.
Gently heat both test tubes A and B to 60 C or so (just too hot to touch it for
more than a second):
- The liquid in test tube B fairly quickly (in a minute or so) turns very dark
and then it becomes lighter again and ends with a nice purple color. During the
reaction, there is production of a non-flammable, colorless gas. There also is
a weak smell, a little bit like over-ripe fruit.
- The liquid in test tube A first does not seem to change, but after a few
minutes, it becomes darker and more brown. On stronger heating (near boiling),
the liquid in test tube A also becomes purple. The intensity of the color,
however, is much stronger. The liquid becomes very dark purple. On strong
dilution it has a beautiful deep purple color. Also during this reaction, there
is production of a non-flammable, colorless gas. Again, there also is a weak
smell, a little bit like over-ripe fruit.
Add 15 ml of 5% ammonia to both test tubes: The liquid in test tube A turns
green and remains clear. Apparently, a new complex is formed, with malate ion
and with ammonia as ligands. The liquid in test tube B becomes turbid and a
grey/green flocculent precipitate is formed. This precipitate most likely
simply is Cr(OH)3.
-------------------------------------------------------------
Dichromate, acting as oxidizer, uses up a lot of acid. This explains why the
reaction with extra acid added (in test tube B) proceeds much faster. Malic
acid only is a weak acid, so without additional added acid, the concentration
of free hydrated H(+) ions is low. Most of it is bound in the malic acid
molecules.
When the reaction proceeds in test tube A, then all acid, consumed is from the
malic acid, and hence, malate ion is formed (probably only one time
deprotonated). This malate ion can form complexes with chromium(III). The
oxidation product of malic acid also may form anionic species, which can
coordinate to chromium(III).
In test tube B, the liquid is strongly acidic and no malate ion will be
present, nor any anionic reaction products. The only free anionic species will
be perchlorate. That ion does not form complexes in aquous solution. So, in
test tube B, most chromium(III) will be present as aqueous hydrated Cr(3+),
only a small part will be coordinated to malate or some oxidized species.
With ammonia, hydrated chromium(III) forms Cr(OH)3. Formation of an
ammine-complex with chromium(III) is very slow (hours, days). Hence the
formation of a precipitate in test tube B.
In test tube A, all chromium(III) is present in some complex. This complex
apparently can react with ammonia, coordinating one or more molecules of this,
forming a green soluble complex.