question_answer

In acidic medium, \[{{H}_{2}}{{O}_{2}}\] changes \[C{{r}_{2}}O_{7}^{2-}\] to \[Cr{{O}_{5}}\] which has two \[(OO)\] bonds. Oxidation state of Cr in \[Cr{{O}_{5}}\] is                                                             [AIPMT 2014]

A)  + 5     

B)  +3

C)                   + 6            

D)  - 10

Correct Answer: C

Solution :

When \[{{H}_{2}}{{O}_{2}}\] is added to an acidified solution of a dichromate, \[C{{r}_{2}}O_{7}^{2-}\], a deep blue coloured complex, chromic peroxide, \[Cr{{O}_{5}}\] \[[or\,CrO\,{{({{O}_{2}})}_{2}}]\] is formed.

\[C{{r}_{2}}O_{7}^{2-}+2{{H}^{+}}+4{{H}_{2}}{{O}_{2}}\xrightarrow{\,}\underset{Chromic\,peroxide}{\mathop{2CrO{{({{O}_{2}})}_{2}}}}\,+5{{H}_{2}}O\]

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

\[x+(-1)4+(-2)=0\]

\[x-6=0\]

\[x=+6\]