The dichromate ion is in equilibrium with chromate ion in aqueous solution as :
${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons 2{\mathrm{CrO}}_4^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$
The oxoanion has

In the given equations, there are two oxoanions, ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-} \mathrm{and} {\mathrm{CrO}}_4^{2-}.$

${\mathrm{E}}_{{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}/{\mathrm{Cr}}^{3+}}^{\circ }=1.33\mathrm{V};{\mathrm{E}}_{{\mathrm{CrO}}_4^{2-}/{\mathrm{Cr}}^{3+}}^{\circ }=-0.11\mathrm{V}$

In acidic medium, oxoanions shows better oxidizing property because oxidation reaction favors in low pH.