Q40 / 180 · COMEDK 2023 (Evening Shift) · Chemistry

Identify the incorrect statement.

  • A
    $\mathrm{Cr}^{2+}$ acts as reducing agent while $\mathrm{Mn}^{3+}$ acts as an oxidising agent though both the ions have $\mathrm{d}^4$ configuration.
  • B
    The second and third ionisation enthalpies of Mn and Fe, respectively, have lower values than expected.
  • C
    Transition metals readily form alloys because their metallic radii are within about $15 \%$ of each other.
  • Correct answer
    D
    Ability of Fluorine to stabilise higher oxidation states in transition metals is due to the low lattice enthalpy of the fluorides.

Solution

(1) $Cr^{2+}$ $(3d^4)$ gets oxidised to $Cr^{3+}$ $(3d^3)$, which has a stable $t_{2g}^3$ configuration, so it acts as a reducing agent. $Mn^{3+}$ $(3d^4)$ gets reduced to $Mn^{2+}$ $(3d^5)$, which has a stable half-filled configuration, so it acts as an oxidising agent. This statement is correct.

(2) Second ionisation of Mn: $3d^54s^1 \to 3d^5 + e^-$ and third ionisation of Fe: $3d^6 \to 3d^5 + e^-$ both lead to a stable half-filled $3d^5$ core, making these ionisation enthalpies lower than expected. This statement is correct.

(3) Transition metals have metallic radii within about 15% of each other, allowing easy substitution in crystal lattices to form alloys. This statement is correct.

(4) Fluorine stabilises higher oxidation states due to high lattice enthalpy (ionic compounds) or high bond enthalpy (covalent compounds), not low lattice enthalpy. This statement is incorrect.

The incorrect statement is (4).

Hence, the correct answer is option: 'D'.

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