Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium bromate : KBrO3
sulphuric acid : H2SO4
sodium bisulfite : NaHSO3
potassium permanganate : KMnO4
malonic acid : HOOC CH2 COOH
Class:
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elem=Br;Mn
redox
Summary:
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When bromate, manganese (II), malonic acid and sulphuric acid are mixed at
certain conditions, then a very convincing oscillating reaction occurs,
which persists for few minutes, going through a cycle many times, before
a final state is reached.
Description:
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Prepare a solution, containing a fairly high concentration of Mn2+ ions
from potassium permanganate, dissolved in warm dilute H2SO4 (appr. 2 mol/l),
adding solid sodium metabisulfite, until all permanganate has reacted.
After the reaction, the liquid is boiled for some time, in order to expell
an excess amount of sulphur dioxide. This liquid is called MN2.
Sequence 1:
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Dissolve some malonic acid in a small volume of MN2.
Dissolve approximately the same volume of solid KBrO3 in some dilute H2SO4
(1 mol/l), the volume of dilute H2SO4 being approximately the same as the
volume of MN2 used for dissolving the malonic acid.
Keep a few crystals of KBrO3 undissolved.
Mix both liquids with each other and keep the solid KBrO3 at the bottom:
As soon as the liquids mix, the mixture becomes red/brown and remains clear.
The crystals of KBrO3 become black (or dark brown) and a gas is evolved at
them. After a while, the liquid becomes lighter and at a certain point, it
is almost colorless. Further changes do not occur or only very slowly.
Heat the mixture to appr. 60C: The liquid becomes brown again, starts bubbling
and becomes colorless again. This cycle is repeated many times. From the
solid particles, there is a constant bubbling. The liquid does not become
brown again homogeneously, but the coloration starts at certain spots and
quickly spreads throughout the liquid.
When the liquid is shaken now and then, then the cycle is affected. The
shaking may sometimes lead to making the liquid colorless, sometimes it may
trigger the change to red/brown. In all cases, shaking causes the bubbling
to be intensified.
When the liquid is left alone, then it nicely cycles through the different
phases.
Remark: Sequence 1 is repeated two times, one time with excess malonic acid
and the other time with excess KBrO3: With excess malonic acid, the final
state is a clear and colorless liquid. With excess KBrO3, the final state
is a dark brown turbid liquid (apparently, this is hydrous MnO2).
Sequence 2:
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This is the same as sequence 1, but now no solid KBrO3 is left, so there are
no solid particles, which are bubbling constantly. The temperature was
raised to appr. 70C.
Under this condition, the oscillation between the colorless and red/brown
state and the bubbling is less pronounced. The same number of cycles can be
achieved, but some 'help' is needed. When the liquid is brown, then it
becomes colorless again, but going from colorless to brown does not occur
spontaneously. A little shake, however, quickly makes the liquid brown. This
color remains for a few seconds and then fades again.
Remarks:
The brown color observed is a bright reddish brown, not the dark brown of
quadrivalent manganese. Probably this bright reddish brown is the color of
trivalent manganese.
The liquid has a smell of brominated organic compounds. A certain sweetish
smell. The smell of bromine can also be observed, although not very strong.
There is no brown vapour of bromine above the liquid.