Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
sodium hydroxide : NaOH
calcium hypochlorite : Ca(ClO)2.2H2O
trichloroisocyanuric acid : C3N3O3Cl3
manganese(II) chloride tetrahydrate : MnCl2.4H2O
Class:
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elem=Cl,Mn
redox
Summary:
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When compounds of manganese(IV) or manganese(VII) are added to hydrochloric
acid, then chlorine gas is produced and the solution becomes very dark
brown/green. Only after a long time of heating, all dark brown/green material
is gone and what remains is a pale solution, containing manganese(II) ions.
In this series of experiments it was attempted to go the other way around. Is
it possible to go from almost colorless manganese(II) to the dark colored
compounds of manganese(III) or manganese(IV). This was done by adding chlorine
to solutions of manganese(II) salts in concentrated hydrochloric acid.
While doing so, an interesting observation was made. The dark colored compound
indeed can be made from manganese(II) in concentrated hydrochloric acid, but
only from hypochlorite and not from chlorine.
When calciumhypochlorite is added to hydrochloric acid, then chlorine is formed
vigorously, and the solution becomes green and clear.
It seems, however, that even in concentrated hydrochloric acid some of the
hypochlorite is not decomposed at once, but remains in solution as yellow/green
hypochlorous acid.
A freshly prepared solution, made from a pinch of calcium hypochlorite and
concentrated hydrochloric acid, reacts differently than a saturated solution of
chlorine in concentrated hydrochloric acid. This is shown with manganese(II)
ions in hydrochloric acid.
Description:
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Remarks: For this experiment it is important to use good quality hydrochloric
acid. The acid must be colorless. Acid, obtained from hardware stores usually
has a green color, due to the presence of metal salts (e.g. iron) or organic
impurities. Such acid is NOT suitable for this set of experiments. The acid,
used in the experiments below has a concentration of approximately 30%.
In the sequence below, manganese(II) chloride is used. All experiments work
equally well with manganese(II) sulfate. Again, the manganese salts must be
pure and free of other transition metals.
Chlorine gas is made by adding coarsely divided trichloroisocyanuric acid
(TCCA) to moderately concentrated hydrochloric acid. This makes a convenient
source of chlorine, which is released at a nice low rate.
Sequence 1:
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Add a small amount of manganese(II) chloride to a few ml of concentrated
hydrochloric acid and dissolve this: A colorless solution is obtained.
Bubble chlorine gas through this solution for several minutes, using a glass
tube with a fine tip, resulting in many fine bubbles of gas going through the
liquid: The solution slowly turns light green and does not seem to dissolve
much chlorine. Only a pale green color is obtained with a clearly visible green
gas above it.
Stopper the test tube with the pale green solution and the chlorine gas and
carefully heat the liquid while swirling: The liquid hardly changes color, it
looks as if the color shifts a little bit more towards yellow, but the effect
is only weak. The gas-mix above the liquid remains light green/yellow due to
gaseous chlorine.
Sequence 2:
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Bubble chlorine gas through a few ml of concentrated hydrochloric acid for
several minutes, using a glass tube with a fine tip: The liquid becomes green,
much more green than in sequence 1. Above the liquid, there is an atmosphere of
chlorine gas.
Add a small amount of solid manganese(II) chloride to the green liquid, loosely
stopper the test tube and swirl: The solid dissolves, at a certain moment the
stopper suddenly pops out of the test tube, some overpressure had developed.
Apparently, the dissolving of the manganese(II) chloride affects the solubility
of chlorine and chlorine escapes as gas. It might also be that the mere
swirling drives out some of the chlorine from the liquid. The color of the
solution hardly changes, it becomes a little weaker.
Sequence 3:
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Dissolve some manganese(II) chloride in concentrated hydrochloric acid and add
a few small pieces of solid TCCA: Chlorine gas is produced, the liquid becomes
turbid and light green. It does not look different from TCCA added to
concentrated hydrochloric acid without manganese(II) chloride added to it.
Sequence 4:
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Dissolve some manganese(II) chloride in concentrated hydrochloric acid and add
a tiny amount of solid calcium hypochlorite: The calciumhypochlorite dissolves
and chlorine gas is evolved vigorously. Around the particles of calcium
hypochlorite a very dark green/brown material is formed, which goes in
solution. After a few seconds, all calcium hypochlorite has dissolved and the
liquid is very dark brown, almost black. The liquid is clear though.
Based on the result of sequence 4 no real conclusions can be drawn. It could be
that the basic calciumhypochlorite (which always contains some calcium
hydroxide and calcium carbonate) oxidizes some of the manganese(II) to MnO2 and
that this in turn dissolves in the surrounding acid.
Sequence 5:
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Use the liquid from sequence 2 and add some solid sodium hydroxide to this: The
granules of sodium hydroxide, which stick to the glass are covered by a dark
brown solid. The granules, which are under the acid do not react. They are
covered by a layer of NaCl, which is almost insoluble in the concentrated acid.
Swirl the test tube, such that all granules are covered by acid: The liquid
becomes a little bit darker, it gets a brown/green color, but this color only
is somewhat stronger than the color of a saturated solution of chlorine in
concentrated hydrochloric acid.
Add a little water and shake the test tube: All of the sodium hydroxide now
dissolves and the liquid becomes light green/brown.
Sequence 6:
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Add a tiny pinch of calcium hypochlorite to a few ml of concentrated
hydrochloric acid: The calcium hypochlorite quickly dissolves, while fizzling
strongly. The solution becomes green and clear, more intense than a saturated
solution of chlorine in concentrated hydrochloric acid and above the liquid
there is green chlorine gas.
Add a small amount of solid manganese(II) chloride to this liquid: The solid
dissolves and the liquid becomes dark brown/green.
Sequence 6 is quite strongly conclusive: Only a small pinch of calcium
hypochlorite was added to a few ml of concentrated acid, so one can safely
assume that the solution still is highly acidic and at high concentration.
Sequence 7:
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Take some liquid from sequence 2 (manganese(II) chloride dissolved in saturated
solution of chlorine in concentrated hydrochloric acid).
In a separate test tube, prepare a solution of calciumhypochlorite in
concentrated hydrochloric acid and add a small amount of this clear green
solution to the liquid from sequence 2: Quickly (but not instaneously, it takes
a few seconds) the liquid becomes much darker brown/green. Manganese(II) is
oxidized to a higher oxidation state.
From all these experiments one can conclude that a solution of
calciumhypochlorite in concentrated hydrochloric acid differs from a solution
of chlorine in concentrated hydrochloric acid in an essential way. The former
can oxidize manganese(II) in concentrated hydrochloric acid, while chlorine
cannot do this. Most likely, this extra oxidizing power is due to the presence
of HOCl in the solution.