JEE Main · 2021 · 25 Jul 2021 · Shift 2 · Redox Reactions and Electrochemistry

Assign the oxidation number of the element that changes, on both sides of each process. Chromium goes from $+6$ in $\mathrm{Cr}{\mathrm{O}}_4^{2-}$ to $+3$, a change of 3, and the same drop of 3 per chromium atom occurs in $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}\to 2\mathrm{C}{\mathrm{r}}^{3+}$. Carbon rises from $+3$ in ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ to $+4$ in $\mathrm{C}{\mathrm{O}}_2$, a change of 1. Manganese falls from $+7$ in $\mathrm{Mn}{\mathrm{O}}_4^{-}$ to $+2$, a change of 5. The process in which the oxidation state changes by five is $\mathrm{Mn}{\mathrm{O}}_4^{-}\to \mathrm{M}{\mathrm{n}}^{2+}$.