Chromium, Cr3+

Most common oxidation state: +3; +2 and +6 also exist.

M.P. 1857o

B.P. 2672o

Density 8.94 g/cm3

Characteristics: Silvery, rather brittle metal. Similar to aluminum, but exhibits several oxidation states.

Characteristic reactions of Cr3+:

(The +3 oxidation state is the most stable.)

Aqueous Ammonia:

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

Cr3+(aq) + 3NH3(aq) + 3H2O(l) <==> Cr(OH)3(s) + 3NH4+(aq)

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 NaOH also precipitate Cr(OH)3, but the precipitate dissolves in excess hydroxide.

Cr3+(aq) + 3OH-(aq) <==> Cr(OH)3(s)

Cr(OH)3(s) + OH-(aq) <==> Cr(OH)4-(aq) (green) 

Hydrogen Peroxide:

In basic solution, hydrogen peroxide oxidizes Cr(III) to Cr(VI):

2Cr(OH)4-(aq) + 3H2O2(aq) + 2OH-(aq) --> 2CrO42-(aq) + 8H2O(l)

To confirm the oxidation, addition of Ba2+ solutions precipitate the yellow chromate ion, CrO42-, as yellow barium chromate.

 No Reaction:

Cl-, SO42-

Go to cation menu