question_answer

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

A)  +5             

B)  +3

C)  4-6              

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 \[x+(-1)4+(-2)=0\] \[x-6=0\] \[x=+6\]