Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium bromide : NaBr
potassium bromide : KBr
sodium hydroxide : NaOH
hydrogen peroxide : H2O2
sulphuric acid : H2SO4
potassium bromate : KBrO3
potassium ferricyanide : K3 [Fe(CN)6]
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
Class:
------
elem=Fe,Br
coordination
redox
Summary:
--------
Ferrocyanide in acidic environments reacts with bromine in a very peculiar
way. An extremely dark compound is formed. This reaction does not occur
in neutral environments and this cannot be observed with ferricyanides.
What is the dark compound? Is it a coordination complex or a condensation
product of many ferric/ferrous ions, close to formation of solid particles?
Description:
------------
Sequence 1:
-----------
Dissolve some potassium ferrocyanide in dilute H2SO4 (appr. 1 mol/l): The
liquid becomes pale yellow and clear.
Add some solid KBrO3: The KBrO3 dissolves. While it dissolves, the liquid
becomes more yellow (pale yellow ferrocyanide is converted to intense yellow
ferricyanide?), but from a certain point in time the liquid becomes green.
Around the crystals of KBrO3 a dark green/blue compound appears, dissolved
in the liquid. After a while, the liquid is dark green. The smell of Br2
can be observed.
Add more KBrO3: The KBrO3 partly dissolves and the liquid becomes dark green/
brown and the air above the liquid clearly is colored by gaseous Br2.
Heat and boil gently for a while: Liquid becomes very dark grey, when viewed
under TL-light, dark purple/red when viewed under tungsten light. It also
looks turbid, as if there is a very finely divided dark (almost black) solid
in the liquid. However, it is hard to see, whether the liquid is dark and
clear or dark and turbid with a very finely divided solid. This is due to the
extreme darkness of the liquid. The air above the liquid becomes colorless
again. The bromine vapors are driven off by the boiling and lateron no new
bromine is produced. If the heat source is removed, then the liquid keeps
on bubbling for a while. Is this boiling or is a colorless gas produced?
Sequence 2:
-----------
Dissolve some potassium ferrocyanide in dilute H2SO4 (1 mol/l) and add some
H2O2 (appr. 10% by weight): Liquid becomes intense yellow, like a solution
of a ferricyanide. The most reasonable assumption is that ferrocyanide is
oxidized to ferricyanide.
Boil liquid for a while: Liquid first becomes red/brown, after a while it
suddenly becomes turbid. A dark blue precipitate is formed, which sticks
to the glass of the test tube and the liquid becomes yellow/green. A lot
of gas is produced as well.
The result of the oxidation with H2O2 differs a lot from the result of
oxidation with KBrO3.
Decant the liquid and add a solution of NaOH in water to the blue precipitate:
The precipitate becomes brown and does not stick to the glass anymore.
Sequence 3:
-----------
Add some solid KBrO3 to a solution of potassium ferricyanide in dilute H2SO4
(1 mol/l): The KBrO3 dissolves (albeit slowly) and the liquid remains intense
yellow. Apparently, the bromate does not react with ferricyanide.
Add some KBr: The solid quickly dissolves and a lot of bromine is formed. The
liquid becomes turbid, due to formation of many small droplets of bromine and
a thick red/brown vapor of bromine is above the liquid. After a while, all
bromine droplets have settled at the bottom and the final result is a clear
red/brown liquid above a drop of bromine and a thick red/brown vapor of
bromine above the liquid. Apparently, bromine does not react with ferricyanide
(at least not visibly).
Add a small amount of potassium ferrocyanide: At the crystals, a dark (almost
black) compound is formed. When the liquid with the crystals of potassium
ferrocyanide is shaken, then it becomes dark brown (much darker, than the
color of a concentrated solution of bromine in water). The thick vapor above
the liquid does not disappear. The liquid remains clear.
Apparently, the dark compound, observed in sequence 1, is formed from bromine
and ferrocyanide.
Cap the test tube and put it upside down, with the open end under water and
bubble the air with the thick bromine vapor quickly into a second test tube,
taking care not to get any of the liquid in the second test tube. Cap the
second test tube, with some water trapped in it and shake well. In this way
some bromine water is obtained, with still some bromine vapor above the
water. Add some solid potassium ferrocyanide to the bromine water: The solid
dissolves and besides that, no further visible changes can be observed.
Add much more potassium ferrocyanide: This also dissolves. Now the liquid
becomes yellow and the bromine vapor above the liquid disappears. Apparently
the bromine oxidizes ferrocyanide to ferricyanide and no further special
reaction is observed.
Sequence 4:
-----------
Prepare some acidic bromine water by mixing potassium bromate with an excess
amount of sodium bromide and adding some dilute H2SO4 (1 mol/l) to the
mixture and shaking until all solid has dissolved. The resulting bromine
water is orange/brown and has a thick vapor of bromine above it.
Add half of the acidic bromine water to a solution of potassium ferrocyanide
in dilute H2SO4 (1 mol/l): As soon as the bromine water touches the pale yellow
solution of potassium ferrocyanide, a dark, almost black, compound is formed,
which looks like a very finely divided precipitate. When viewed under TL-
light, the liquid becomes very dark green/grey.
Add the other half of the acidic bromine water to a solution of potassium
ferricyanide in dilute H2SO4 (1 mol/l): The liquid becomes orange/brown, the
color is due to dilution of the bromine water. However, on standing, the
liquid slowly turns darker. After approximately 20 minutes the liquid has
become much darker and it is red/brown. In the dark liquid it looks as if
there are clouds of even darker liquid.
After appr. 1 hour, the liquid has become so dark, that it is not possible
to see any light through a layer of 1 cm thickness. Above the liquid, the
thickness of the bromine vapor also increases. It looks as if the excess
bromide in the bromine water is converted to bromine and that this causes
the formation of the very thick vapor above the liquid. However, the liquid
remains clear and no drop of precipitated bromine is formed.
Sequence 5:
-----------
Mix some sodium bromide and potassium ferricyanide and dissolve this mixture
in dilute H2SO4 (1 mol/l): The liquid becomes yellow.
Wait for approximately 1.5 hour: Liquid remains yellow, no visible coloration
of the air above the liquid and the liquid is odourless.
Add a very small amount of KBrO3 to the liquid: The solid dissolves slowly,
the liquid becomes fairly dark brown and above the liquid some bromine vapor
can be observed, but the vapor only is 'thin'.