Question

In which of the following reactions oxidation number of Chromium has been affected?

• A. $Cr{O}_{2}C{l}_2+O{H}^{-}\to Cr{O}_4^{2-}+2HCl$
• B. $(N{H}_4{)}_{2}C{r}_2{O}_7\stackrel{\Delta }{\to }{N}_2+C{r}_2{O}_3+3H_{2}O$
• C. $2Cr{O}_4^{2-}+2H^{+}\to C{r}_2{O}_7^{2-}+H_{2}O$
• D. $C{r}_2{O}_7^{2-}+2O{H}^{-}\to 2C{r}_2{O}_4^{2-}+H_{2}O$

Answer 1

The correct option is B $(N{H}_4{)}_{2}C{r}_2{O}_7\stackrel{\Delta }{\to }{N}_2+C{r}_2{O}_3+3H_{2}O$

Analysing the options
Option (A)
$2Cr{O}_4^{2-}+2H^{+}\to C{r}_2{O}_7^{2-}+H_{2}O$
Let oxidation state of Cr in $Cr{O}_4^{2-}=x$
$x+4(-2)=-2$
$\Rightarrow x-8=-2$
$\therefore x=+6$

Let oxidation state of Cr in $C{r}_2{O}_7^{2-}=x$
$2x+7(-2)=-2$
$\Rightarrow 2x-14=-2$
$\therefore x=+6$
In this reaction, oxidation no. of Cr is +6 in both $Cr{O}_4^{2-}$ and $C{r}_2{O}_7^{2-}$.

Option (B)
$C{r}_2{O}_7^{2-}+2O{H}^{-}\to 2C{r}_2{O}_4^{2-}+H_{2}O$
In this reaction also, oxidation no. of Cr is +6 in both $Cr{O}_4^{2-}andC{r}_2{O}_7^{2-}$.

Option (C)
$Cr{O}_{2}C{l}_2+2O{H}^{-}\to Cr{O}_4^{2-}+2HCl$
Oxidation state Cr in $Cr{O}_{2}C{l}_2$ be x
$x+2(-2)+2(-1)=0$
$\Rightarrow x-6=0$
$\therefore x=+6$
Oxidation state of Cr in $(Cr{O}_4^{2-}$) is 6 (as calculated)
$x+4(-2)=-2$
$\Rightarrow x-8=-2$
$\therefore x=+6$
In this reaction , oxidation no. of Cr is +6 in both $Cr{O}_{2}C{l}_{2}andCr{O}_4^{2-}$.

Option (D)
$(N{H}_4{)}_{2}C{r}_2{O}_7\stackrel{\Delta }{\to }{N}_2+C{r}_2{O}_3+3H_{2}O$
Oxidation state : Cr in $C{r}_2{O}_7^{2-}=+6$
Oxidation number of Cr in $C{r}_2{O}_3=+3$
The oxidation no. of Cr thus decreases from +6 to +3 in this reaction.
So, the correct answer is option (D).