Question

What is the colour of ${\mathrm{CrO}}_5$?

Answer 1

1. Chromium peroxide (${\mathrm{CrO}}_5$) is an unstable compound that consists of one oxo ligand and two peroxo ligands which form a total of five oxygen atoms with one chromium atom.
2. It is a strong oxidizing agent because of the two peroxo ligands but it is not used for organic synthesis due to its low solubility.

The reaction mechanism for the chromium peroxide color is given below:

1. The reaction between Hydrogen peroxide and Chromate: The conversion of either yellow or orange Chromate converts into dark blue colour Chromium peroxide. ${\mathrm{CrO}}_4^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\to {\mathrm{CrO}}_5\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
2. Chromium peroxide reduction: Chromium (${\mathrm{Cr}}^{6+}$) decomposes into Chromium (${\mathrm{Cr}}^{3+}$) during the reduction process; therefore dissolve the Chromium (VI) in an organic solvent to prevent this reduction and enhance the lifetime. $2{\mathrm{CrO}}_5\left(\mathrm{aq}\right)+7{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)+6{\mathrm{H}}^{+}\to 2{\mathrm{Cr}}^{3+}\left(\mathrm{aq}\right)+10{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+7{\mathrm{O}}_2\left(\mathrm{g}\right)$

Figure: Chemical structure of Chromium (VI) peroxide (Butterfly structure).