Question

On passing $C{O}_2$ into aqueous yellow coloured $Cr{O}_4^{2-}$ solution, an orange coloured $C{r}_2{O}_7^{2-}$ solution is obtained. Is this oxidation or reduction? The reaction is given below:
$Cr{O}_4^{2-}\left(aq\right)\stackrel{C{O}_2}{\to }C{r}_2{O}_7^{2-}$

• A. Oxidation
• B. Reduction
• C. Neither of them
• D. Can't say

Answer 1

The correct option is C Neither of them

$Cr{O}_4^{2-}\to C{r}_2{O}_7^{2-}$. In both the anions, oxidation number of chromium is $+6$.
Hence, no change has occurred.
Acidic medium, due to $C{O}_2$, converts $Cr{O}_4^{2-}$ into $C{r}_2{O}_7^{2-}$.
$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3\rightleftharpoons 2H^{+}+C{O}_3^{2-}$ & $2Cr{O}_4^{2-}+2H^{+}\to C{r}_2{O}_7^{2-}+H_{2}O$