The E°(M3+/M2+) values for Cr, Mn, Fe and Co are −0.41, +1.57, +0.77 and +1.97 V respectively. For which one of these metals is the change in oxidation state from +2 to +3 easiest?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Cr
The change from +2 to +3 is the oxidation \( \ce{M^2+ -> M^3+ + e^-} \). The lower (more negative) E°(M3+/M2+), the smaller the tendency of M3+ to take back the electron, so the easier it is to oxidise M2+.
Cr: −0.41 V, the only negative value: Cr2+ is a strong reducing agent, so +2 → +3 is easiest.
Fe: +0.77 V: harder than Cr.
Mn: +1.57 V: Mn2+ (d5) is particularly stable, so it is hard to oxidise.
Co: +1.97 V, the highest: Co3+ is a strong oxidising agent, so +2 → +3 is hardest.
NCERT (2026-27 edition):
4.3.8 Chemical Reactivity and E° Values – “An examination of the Eo values for the redox couple M3+/M2+ … shows that Mn3+ and Co3+ ions are the strongest oxidising agents in aqueous solutions. The ions Ti2+, V2+ and Cr2+ are strong reducing agents and will liberate hydrogen from a dilute acid, e.g., 2 Cr2+(aq) + 2 H+(aq) → 2 Cr3+(aq) + H2(g)”