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.
Select the Incorrect statement from the following:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cr2+ ion act as strong oxidising agent in aqueous solution
Whether an ion is an oxidising or a reducing agent depends on which change brings it to a more stable configuration:
An ion that becomes more stable by gaining an electron is an oxidising agent (positive E° for M3+/M2+).
An ion that becomes more stable by losing an electron is a reducing agent (negative E°).
Cr (Z = 24) is [Ar]3d54s1, so \( \ce{Cr^{2+}} \) is 3d4 and \( \ce{Cr^{3+}} \) is 3d3, a half-filled t2g level that is especially stable in water. So \( \ce{Cr^{2+}} \) readily loses an electron:
\( \displaystyle \ce{Cr^2+ -> Cr^3+ + e^-},\quad E^\circ(\ce{Cr^3+}/\ce{Cr^2+})=-0.41\ \mathrm{V} \)
Option (1) Mn3+ ion act as strong oxidising agent in aqueous solution — A correct statement. Mn (Z = 25) is [Ar]3d54s2; \( \ce{Mn^{3+}} \) (3d4) gains one electron to give the half-filled 3d5 \( \ce{Mn^{2+}} \) (E° = +1.57 V).
Option (2) Co3+ ion act as strong oxidising agent in aqueous solution — A correct statement. Co (Z = 27) is [Ar]3d74s2; the \( \ce{Co^{3+}} \)/\( \ce{Co^{2+}} \) couple has a large positive E° (+1.97 V), so aqueous \( \ce{Co^{3+}} \) is readily reduced.
Option (3) Cr2+ ion act as strong oxidising agent in aqueous solution — The incorrect statement. \( \ce{Cr^{2+}} \) readily loses an electron to give the stable 3d3 \( \ce{Cr^{3+}} \), so it is a strong reducing agent.
Option (4) Ti2+ ion act as strong reducing agent in aqueous solution — A correct statement. Ti (Z = 22) is [Ar]3d24s2; \( \ce{Ti^{2+}} \) (3d2) readily loses electrons.
NCERT (2026-27 edition):
Trends in the M2+/M Standard Electrode Potentials – …
