Description of experiment
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Needed compounds:
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ammonium thiocyanate : NH4SCN
sodium borohydride : NaBH4
hydrochloric acid : HCl
ferric chloride : FeCl3 . 6H2O
Class:
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elem=Fe
redox
Summary:
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Ferric ions are reduced by borohydride to a black compound. Is this
metallic iron or is this iron (II) oxide?
Description:
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Add a slightly acidified solution of ferric chloride to an excess of a
solution of NaBH4: Formation of a curdy black precipitate and formation of
quite some gas.
Decant the liquid and rinse the black precipitate a few times with tap
water. Next add dilute HCl (appr. 10% by weight) to the black precipitate:
The precipitate slowly dissolves and a small amount of gas is evolved at
the precipitate. The evolution of gas quickly ceases and further dissolving
of the precipitate is without evolution of a gas. Apparently the precipitate
is no iron, otherwise evolution of gas would continue, until it has
dissolved completely.
Finally all of the precipitate dissolves and the liquid becomes almost
colorless (slightly green/yellow).
Add some NH4SCN: The solid quickly dissolves and the liquid becomes reddish
pink. Only a small amount of the iron (III) - thiocyanate complex is formed.
So, one can conclude that borohydride reduces iron (III) to iron (II).
What is the black compound? Iron (II) oxide?