# Question 6bef5

May 24, 2015

Yes, chromium reacts with hot concentrated alkali to form the hydroxide and hydrogen gas.

Chromium is above hydrogen in the activity series, so we should expect it to react and form hydrogen gas and chromium hydroxide.

The half-reactions and standard reduction potentials are

$\text{Cr(OH)"_3"(s)" + "3e"^(-) → "Cr(s)" + "3OH"^(-)"(aq)}$; E^° = "-1.3 V"
$\text{2H"_2"O(l)" + "2e"^(-) → "H"_2"(g)" + "2OH"^(-)"(aq)}$; E^° = "-0.877 V"

The overall reaction is

2×["Cr(s)"+ cancel("3OH⁻(aq)") → "Cr(OH)"_3"(s)" + cancel("3e⁻")]; E^° = "+1.3 V"
3×["2H"_2"O(l)" + cancel("2e⁻") → "H"_2"(g)" + cancel("2OH⁻(aq)")]; E^° = "-0.877 V"

$\text{2Cr(s)" + "6H"_2"O(l)" → "2Cr(OH)"_3"(s)" + "3H"_2"(g)}$; E^° = "+0.4 V"#

However, the hydroxide is amphoteric.

It dissolves in excess base to give a dark green solution of hexahydroxochromate(III):

$\text{Cr(OH)"_3"(s)" + "3OH"^(-)"(aq)" → "Cr(OH)"_6^(3-)"(aq)}$

The overall reaction will be

$\text{2Cr(s)" + "6OH"^(-)"(aq)" + "6H"_2"O(l)" → "2Cr(OH)"_6^(3-)"(aq)" + "3H"_2"(g)}$