Description of experiment
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Needed compounds:
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anhydrous chromic acid : CrO3
hydrochloric acid : HCl
Class:
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elem=Cl,Cr
redox
Summary:
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Chromium trioxide (CrO3) dissolves in concentrated hydrochloric acid, giving a
dark brown liquid. When this liquid is heated, then some orange/brown chromyl
chloride is formed, which can easily be observed as a vapor above the liquid.
On further heating the hydrochloric acid is oxidized and the hexavalent
chromium is converted to green trivalent chromium.
Description:
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Add some solid CrO3 to concentrated hydrochloric acid (30% HCl by weight): The
solid dissolves, the resulting liquid is dark red/brown.
Heat the liquid carefully: Bubbles of gas are produced. The air above the
liquid becomes pale orange/brown. This is a clear indication that some of the
CrO3 reacts to form CrO2Cl2, which partially escapes as vapor. The bubbles of
gas most likely are bubbles of Cl2, contaminated with some CrO2Cl2.
Heat the liquid more strongly, such that it starts boiling: The brown/orange
vapor above the liquid disappears, more gas bubbles are produced, and the
liquid turns green. All hexavalent chromium is reduced to trivalent chromium.
REMARK:
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In conc. HCl apparently the following reaction occurs:
CrO3 + 2HCl --> CrO2Cl2 + H2O