Which species will oxidize Cr2+ (E°red = -0.407) but not Mn2+ (E°red = +1.224)?

• A. O3 in acid (E°red = +2.076)
• B. Zn2+ (E°red = -0.762)
• C. V3+ (E°red = -0.255)
• D. Pb4+ (E°red = +1.68)

Answer: (C)

The key idea is comparing standard reduction potentials to see which species can pull electrons from Cr2+ but cannot pull electrons from Mn2+. A species will oxidize Cr2+ if its own reduction potential is more positive than the Cr3+/Cr2+ couple, which is -0.407 V. At the same time, to avoid oxidizing Mn2+, its reduction potential must not exceed the Mn3+/Mn2+ couple, which is +1.224 V.

Among the options, only the V3+ couple has a reduction potential between these two values: -0.255 V, which is more positive than -0.407 V, so it can oxidize Cr2+ to Cr3+. But it is less than +1.224 V, so it cannot oxidize Mn2+ to Mn3+. The other candidates either are strong enough to oxidize Mn2+ (like ozone and Pb4+) or are not strong enough to oxidize Cr2+ (like Zn2+).

Thus, V3+ will oxidize Cr2+ but not Mn2+.