Description of experiment
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Needed compounds:
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sodium metabisulfite : Na2S2O5
sodium hydroxide : NaOH
nitric acid : HNO3
silver nitrate : AgNO3
sodium persulfate : Na2S2O8
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr,Ag,S,O
redox
Summary:
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Chromium (III) is oxidized to dichromate (chrome (VI)) by persulfate. This
reaction is catalyzed by silver (I).
Description:
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Sequence 1:
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Add an excess amount of a solution of sodium persulfate to a solution of
chrome alum in nitric acid (2 mol/l) and heat to appr. 60 C: No visible
reaction, not even after several minutes.
Add a few drops of a solution of AgNO3: The violet color of the chrome (III)
solution slightly changes. On further heating the color changes to orange,
but this orange is not a bright orange as one expects from a solution of
a dichromate. A brownish shade remains on the orange color.
After standing for one hour the situation has changed. The liquid has become
clear and it is bright orange. At the bottom a thin layer of ruby-red crystals
can be observed. In the liquid a few glittering crystals are floating around
as well.
On the crystals, a very slow evolution of a gas can be observed (persulfate,
decomposing??): A few tiny bubbles per second develop at the crystals and
go to the surface of the liquid.
Sequence 2:
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Add a solution of sodium persulfate and a little amount of silver nitrate
to a solution of chrome alum in nitric acid (2 mol/l) and heat to appr.
60 C: The liquid slowly turns yellow/orange/brown. The Cr3+ ions are converted
to dichromate ions.
After a long time of standing, a dark red (bordeaux-red) solid separates from
the liquid in the form of large crystals with a size of appr. 1 mm. The liquid
above the crystals is clear and has a bright yellow/orange color (color of
dilute dichromate).
Cleaning of the dark red crystals (several times) with tap water: Every time
when the crystals are cleaned, a little amount dissolves in the water, making
the liquid pale yellow and a little turbid (it is expected that the turbidity
is due to the presence of chloride and carbonate in the tap water, which
precipitates with silver ions from the crystals).
Rinsing of the crystals in dilute HNO3 (2 mol/l): A minute quantity dissolves.
The dilute acid turns pale yellow and remains clear.
Add a solution of sodium hydroxide in tap water to some of the dark red
crystals (after carefully removing the dilute HNO3): A fairly large part
of the crystals dissolves. The liquid becomes intense yellow/brown and
slightly turbid.
Add some H2O2 (3% by weight): A lot of oxygen is produced. The liquid turns
moss-green. After the gas evolution from the liquid has ceased, at the
crystals still some oxygen is evolved at a very low rate.
Add a large excess amount of Na2S2O5 to some of the dark red crystals: The
crystals dissolve partly. The liquid becomes pale green and slightly turbid
(turbidity may again be due to the presence of chloride in the tap water,
used in this experiment).
Remarks:
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It is assumed that the dark red crystals are silver chromate (Ag2CrO4), which
is slightly soluble in water.
Persulfate is capable of oxidizing Cr3+ in acidic environments (pH = 0), but
silver (I) is needed as a catalyst in order to have a reasonable speed
of reaction.
The fact that the produced dichromate solution by persulfate from chrome (III)
is not pure orange, but a little brownish orange may be due to the fact, that
persulfate reacts with silver (I) to form a brown compound (higher oxidation
state of silver). This may also explain the catalytic action of silver (I)
on the oxidation of chrome (III) to chrome (VI) by persulfate in acidic
environments.