d-Block Elements
The metals of the first series with the exception of copper are relatively more reactive and are oxidised by 1M H+, though the actual rate at which these metals react with oxidising agents like hydrogen ion (H+) is sometimes slow. For example, titanium and vanadium, in practice, are passive to dilute non oxidising acids at room temperature. The E° values for M2+/M indicate a decreasing tendency to form divalent cations across the series. It is interesting to note that the E° values for Mn, Ni and Zn are more negative than expected from the general trend. Whereas the stabilities of half-filled d subshell (d5) in Mn2+ and completely filled d subshell (d10) in zinc are related to their E° values; for nickel, E° value is related to the highest negative enthalpy of hydration.
E0 value for the Mn3+/Mn2+ couple is much more positive than that for Cr3+/Cr2+ due to
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Much larger third ionisation energy of Mn.
The E° value of an M3+/M2+ couple tells how easily M3+ takes an electron to become M2+. It is closely linked to the third ionisation energy (M2+ → M3+): if removing the third electron is very difficult, then M3+ will readily take that electron back, and E° is strongly positive.
Deriving the configurations:
Mn (Z = 25): [Ar]3d54s2; \( \ce{Mn^{2+}} \) is 3d5 (half-filled, very stable) and \( \ce{Mn^{3+}} \) is 3d4. Removing an electron from the stable 3d5 set needs a very large third ionisation energy (3260 kJ mol−1).
Cr (Z = 24): [Ar]3d54s1; \( \ce{Cr^{2+}} \) is 3d4 and \( \ce{Cr^{3+}} \) is 3d3 (half-filled t2g level, stable). \( \ce{Cr^{2+}} \) loses its electron easily, and the third ionisation energy is lower (2990 kJ mol−1).
\( \displaystyle E^\circ(\ce{Mn^3+}/\ce{Mn^2+})=+1.57\ \mathrm{V},\quad E^\circ(\ce{Cr^3+}/\ce{Cr^2+})=-0.41\ \mathrm{V} \)
Why the other options are incorrect
Option (1) Much lower third ionisation energy of Mn — Incorrect. A low third ionisation energy would make \( \ce{Mn^{3+}} \) stable and give a less positive E°; Mn’s value is in fact high.
Option (2) Much larger second ionisation energy of Mn — Incorrect. The second ionisation energy concerns M+ → M2+, not the M3+/M2+ couple.
Option (4) Much lower second ionisation energy of Mn — Incorrect. The same reason; the second ionisation energy does not decide this couple.
NCERT (2026-27 edition):
Chemical Reactivity and E° Values – …
