Question

All the following species are strong oxidizing agents. Their strength as oxidizing agents in acidic solution is such that

• A. $S_{2}O_{8}^{2 -}>Cr_{2}O_{7}^{2 -}>MnO_{4}^{-}$
• B. $MnO_{4}^{-}>Cr_{2}O_{7}^{2 -}>S_{2}O_{8}^{2 -}$
• C. $S_{2}O_{8}^{2 -}>MnO_{4}^{-}>Cr_{2}O_{7}^{2 -}$
• D. $MnO_{4}^{-}>S_{2}O_{8}^{2 -}>Cr_{2}O_{7}^{2 -}$

Answer 1

Answer: (C)

The standard reduction potentials of the above species in acidic medium are as given below
$S_{2}O_{8}^{2 -}\left(\right.aq\left.\right)+2e^{-} \rightarrow 2SO_{4}^{2 -}\left(\right.aq\left.\right);E^\circ =+2.01\text{V}$
$MnO_{4}^{-}\left(\right.aq\left.\right)+8H^{+}+5e^{-} \rightarrow Mn^{2 +}\left(\right.aq\left.\right)+4H_{2}O;E^\circ =+1.77\text{V}$
$Cr_{2}O_{7}^{2 -}\left(\right.aq\left.\right)+14H^{+}+6e^{-} \rightarrow 2Cr^{3 +}+7H_{2}O;E^\circ =+1.33\text{V}$
The more the standard reduction potential, the more the tendency of the species to undergo reduction and more strongly it (species) would work as oxidizing agent.
As a result, their strength as oxidizing agent in acidic solution decreases in the following order
$S_{2}O_{8}^{2 -}>MnO_{4}^{-}>Cr_{2}O_{7}^{2 -}$