Question

In acidic medium, $H_2O_2$ changes $Cr_2O_7^{2-}$ to $CrO_5$ which has two (-O-O-) bonds. Oxidation state of Cr in $CrO_5$ is

• A. 5
• B. 3
• C. 6
• D. -10

Answer 1

Answer: (C)

6

Answer 2

When $H_2O_2$ is added to an acidified solution of a dichromate $Cr_2O_7^{2-} ,$ a deep blue coloured complex, chromic peroxide $CrO_5 [or \, CrO(O_2)_2]$ is formed.
$Cr_2O^{2-}_7+2H^++4H_2O \rightarrow \underbrace { 2CrO(O_2)_2 }_{Chromic \, peroxide }$
\hspace20mm +5H_2O
This deep blue coloured complex has the following structure
Oxidation state of Cr is +6 due to the presence of two peroxide linkages which can be calculated as
Cr peroxide normal $x + (-1)4+ (-2) = 0 ,x - 6 = 0$ and $x = +6$