Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfide : Na2S . 3H2O
hydrochloric acid : HCl
nitric acid : HNO3
antimony sesquioxide : Sb2O3
Class:
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elem=S,Cl,Sb
precipitation
redox
Summary:
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Antimony trioxide does not dissolve in concentrated nitric acid, not even when
the liquid is heated to boiling. When some hydrochloric acid is added as well,
then it quickly dissolves, producing a colorless gas as well. It is oxidized to
the +5 oxidation state.
When this solution is diluted, then a white precipitate is formed. This white
precipitate must be hydrous Sb2O5. When a dilute solution of sodium sulfide is
added to this still strongly acidic solution, then H2S bubbles out of solution,
but also a lot of precipitate is formed, which has a beautiful bright
orange/red color.
Description:
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Add some concentrated nitric acid (appr. 60% HNO3 by weight) to solid antimony
trioxide: No reaction occurs. Even when the liquid is heated, still no reaction
occurs. No brown gas is produced, the white solid does not change.
Add an equal volume of concentrated hydrochloric acid (appr. 30% HCl by weight)
to the still hot liquid: The white solid quickly dissolves and a colorless gas
is produced. The liquid turns orange/yellow and an orange/brown gas mix is
produced above the liquid. This orange/yellow and orange/brown color most
likely is due to formation of nitrosyl chloride. The colorless gas probably is
NO.
Dilute the liquid by adding approximately two times its volume of water: The
orange/yellow color of the solution disappears at once. The liquid becomes
totally clear and colorless. This quick disapearance of the yellow/orange color
confirms that this color is due to nitrosyl chloride. Nitrosyl chlorid very
easily hydrolyses in water. However, a few seconds later, it becomes milky
white. This change from clear to white/opaque is not an instantaneous reaction,
but it takes several seconds.
Add a dilute solution of sodium sulfide to this white and still strongly acidic
liquid: Some colorless gas escapes (bad smell, H2S). A lot of yellowish/brown
precipitate is formed, which quickly turns bright orange/red and becomes
flocculent. The precipitate fairly quickly settles as a bright orange/red layer
with a still somewhat turbid yellow liquid above it.
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Antimony trioxide is not easily oxidized by nitric acid, but things change when
also hydrochloric acid is present in the mix. The antimony(III) then quickly is
oxidized to the +5 oxidation state. Antimony in oxidation state +5 is quickly
hydrolysed just like antimony(III). With hydrogen sulfide, this forms the
beautifully colored compound antimony pentasulfide. The pentasulfide probably
is not a true pentasulfide, but some compound of indeterminate stoichiometry,
which contains antimony and sulphur in a 2