Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sulphuric acid : H2SO4
sodium hydroxide : NaOH
molybdenum trioxide : MoO3
Class:
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elem=Mo
coordination
redox
Summary:
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Molybdate forms strongly colored, but unstable, complexes with hydrogen
peroxide.
Description:
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Sequence 1:
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Dissolve some solid MoO3, together with some solid NaOH in water: A clear
colorless liquid is formed.
Add some H2O2 (3% by weight): The liquid becomes red/brown, but fairly
quickly this color fades, until the liquid has become pale yellow. The liquid
bubbles and especially when it is shaken the bubbling is fairly strong.
Add an excess amount of sulfuric acid: The liquid becomes lemon-yellow and
its color is much more intense, than the pale yellow of the liquid to which
the acid was added. (When acid is added to a liquid, without hydrogen
peroxide, then the liquid remains colorless). The yellow color does not
fade, as is the case with the alkaline liquid.
Sequence 2:
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Dissolve some solid MoO3, together with some solid NaOH in water and add
some H2O2 (3% by weight) and boil for a while: The liquid becomes colorless
after being red/brown initially.
Add some new H2O2: The liquid becomes red/brown again. Again this color
quicky fades away.
Sequence 3:
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Dissolve some solid MoO3, together with some solid NaOH in water and add
some H2O2 (3% by weight) and boil for a while: The liquid becomes colorless
after being red/brown initially.
Add an excess amount of sulfuric acid: The liquid becomes lemon-yellow and
this color does not fade quickly.
Sequence 4:
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Dissolve some solid MoO3 in a solution of NaOH: The solid dissolves, the
resulting liquid is clear and colorless.
Add an excess amount of H2SO4 (appr. 15% by weight): Liquid remains clear
and colorless.
Add some H2O2 (3%): Liquid turns yellow.
Add an excess amount of solid NaOH: The solid dissolves, a gas is evolved
(very slowly) and when all NaOH is dissolved, the liquid has become colorless.
Sequence 5:
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Dissolve some solid MoO3 in a solution of NaOH: The solid dissolves, the
resulting liquid is clear and colorless.
Add some H2O2 (3%): The liquid remains colorless. Now no brown/red color is
observed in the alkaline solution.
Add an excess amount of H2SO4 (appr. 15% by weight): Liquid turns intense
yellow and remains clear.
Heat and boil gently for a while: The liquid remains intense yellow. The
H2O2-complex apparantly is fairly stable in acidic environment.
Hypothesis: When adding hydrogen peroxide to alkaline molybdate, then a red/
brown complex is formed, which decolorizes again, but apparently the H2O2
still exists (probably bound to the molybdate), even if the liquid has
turned colorless again. This assumption is made, because of the lemon-yellow
color after adding acid to the colorless liquid.
The red/brown complex is not always formed in alkaline solution. Probably
this depends on pH.
On the other hand, the colorless liquid does not contain free H2O2, because
when new H2O2 is added, again a red/brown color is observed, which quickly
fades again.