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Characteristic Reactions of Chromium Ions (Cr³⁺)

  • Page ID
    97267
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    • Most common oxidation state: +3; +2 and +6 also exist. The +3 oxidation state is the most stable.
    • M.P. 1857º
    • B.P. 2672º
    • Density 8.94 g/cm3
    • Characteristics: Chromium is a silvery, rather brittle metal. Similar to aluminum, but exhibits several oxidation states.

    Aqueous Ammonia

    Ammonia reacts with chromium(III) ion to precipitate gray-green chromium(III) hydroxide:

    \[\ce{Cr^{3+}(aq) + 3NH3(aq) + 3H2O(l) <=> Cr(OH)3(s) + 3NH4^{+}(aq)} \nonumber \]

    Cr3.gif

    \(\ce{Cr(OH)3}\) dissolves only to a slight extent in excess ammonia. Boiling the solution causes the chromium(III) hydroxide to reprecipitate.

    Sodium Hydroxide

    Strong bases such as \(\ce{NaOH}\) also precipitate \(\ce{Cr(OH)3}\), but the precipitate dissolves in excess hydroxide.

    \[\ce{Cr^{3+}(aq) + 3OH^{-}(aq) <=> Cr(OH)3(s)} \nonumber \]

    Cr4a.gif

    \[\ce{Cr(OH)3(s) + OH^{-}(aq) <=> Cr(OH)4^{-}(aq) (green) } \nonumber \]

    Cr4bppt.gif

    Hydrogen Peroxide

    In basic solution, hydrogen peroxide oxidizes \(\ce{Cr(III)}\) to \(\ce{Cr(VI)}\):

    \[\ce{2Cr(OH)4^{-}(aq) + 3H2O2(aq) + 2OH^{-}(aq) -> 2CrO4^{2-}(aq) + 8H2O(l)} \nonumber \]

    Cr5ppt.gif

    To confirm the oxidation, addition of \(\ce{Ba^{2+}}\) solutions precipitate the yellow chromate ion, \(\ce{CrO4^{2-}}\), as yellow barium chromate.

    No Reaction

    \(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\)


    This page titled Characteristic Reactions of Chromium Ions (Cr³⁺) is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James P. Birk.

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