Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
zinc : Zn
niobium pentachloride : NbCl5
Class:
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elem=Nb
acid/base
redox
Summary:
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Niobium pentachloride reacts violently with water, releasing hydrogen chloride.
Niobium pentachloride also reacts vigorously with hydrochloric acid. The liquid
first remains clear, but on standing it becomes turbid.
When zinc is added, then a deep blue compound is formed, which on dilution
becomes brown, and finally becomes colorless and turbid.
Description:
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Add some NbCl5 to water: A strong hissing noise is produced, and a lot of heat
is produced, the liquid becomes very hot. A strong and pungent smell of
hydrochloric acid is produced, the liquid becomes turbid. The insoluble hydrous
Nb2O5 is formed.
Add some NbCl5 to concentrated hydrochloric acid: Now the reaction is less
vigorous. The NbCl5 quickly dissolves and a lot of bubbles are produced,
somewhat like adding a piece of sodium carbonate to dilute hydrochloric acid.
The bubbles are HCl-gas. The amount of heat produced is much less, the liquid
only becomes luke-warm. The lower heat in this experiment can be explained. In
this experiment, the HCl escapes as a gas, in the experiment with water only,
the HCl is hydrated and dissolved, and that generates a lot of heat.
The liquid remains clear now. The NbCl5 now is partially hydrolysed to
colorless NbOCl3. On standing, however, the liquid becomes turbid, so also
here, the water in the concentrated acid causes further hydrolysis.
To the solution of NbOCl3 add some powdered zinc: A violent reaction occurs,
and a lot of hydrogen gas is evolved. The liquid becomes very dark blue and a
little turbid. The niobium is reduced to a lower oxidation state.
Dilute the dark blue liquid with a lot of water: The liquid at first still is
very dark blue, but it slowly becomes lighter and finally it becomes colorless.
The changes are not easily reproducible. Different (very similar) experiments
were done. Sometimes, the liquid first becomes brown and then it becomes
colorless, sometimes, the color goes through shades of grey before it becomes
colorless, and yet other times, the blue color simply fades. But the final
result always is that the liquid becomes colorles