Characteristic Reactions of Chromium Ions (Cr³⁺)

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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:

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

\(\ce{Cr(OH)3}\) 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.

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

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

Hydrogen Peroxide

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

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

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

No Reaction

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

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