Consider the following statements about the first row (3d) transition…

The correct answer is Option (2) → (C) and (D) only

The E°(M²⁺/M) value shows how readily a metal forms its M²⁺ ion, and the E°(M³⁺/M²⁺) value shows how readily M³⁺ is reduced to M²⁺. A large positive E°(M³⁺/M²⁺) means a strong oxidising M³⁺ ion.

(A) The general trend towards less negative E°(M²⁺/M) values across the series is related to the increase in the sum of the first and second ionisation enthalpies. — Correct statement. As Δ_iH₁ + Δ_iH₂ rises, forming M²⁺ becomes harder, so E° becomes less negative.

(B) Titanium and vanadium, in practice, are passive to dilute non-oxidising acids at room temperature. — Correct statement. Their E° values allow them to be oxidised by H⁺, but the reaction is slow in practice.

(C) Mn³⁺ and Co³⁺ ions are the strongest reducing agents in aqueous solutions. — Incorrect statement. Their E°(M³⁺/M²⁺) values are the most positive in the series, so these ions readily gain an electron: they are the strongest oxidising agents.

(D) The E° values for Mn, Ni and Zn are less negative than expected from the general trend. — Incorrect statement. They are more negative than expected: the half-filled d⁵ of Mn²⁺ and the filled d¹⁰ of Zn²⁺ are stable, and Ni²⁺ has the most negative hydration enthalpy.

(E) The ions Ti²⁺, V²⁺ and Cr²⁺ are strong reducing agents and liberate hydrogen from a dilute acid. — Correct statement. For example:

    \( \displaystyle \ce{2Cr^2+(aq) + 2H+(aq) -> 2Cr^3+(aq) + H2(g)} \)

NCERT (2026-27 edition):
4.3.8 Chemical Reactivity and E° Values – “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^o values for M²⁺/M (Table 4.2) indicate a decreasing tendency to form divalent cations across the series. This general trend towards less negative E^o values is related to the increase in the sum of the first and second ionisation enthalpies. It is interesting to note that the E^o 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^o values; for nickel, E^o value is related to the highest negative enthalpy of hydration.”

“An examination of the E^o values for the redox couple M³⁺/M²⁺ (Table 4.2) 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)”