CBSE Class 12 · 2002 · Delhi · d and f Block ElementsShort Answer

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Explain why:
i) E° for the Mn³⁺/Mn²⁺ couple is more positive than that for Fe³⁺/Fe²⁺ (Z: Mn = 25, Fe = 26)
ii) Ce³⁺ can be easily oxidised to Ce⁴⁺ (Z of Ce = 58)

Solution

(i) Reduction Mn

^{3 +}

(3d

^{4}

) → Mn

^{2 +}

(3d

^{5}

) creates the exactly half-filled, extra-stable d

^{5}

shell, so it is strongly favoured — E° is large and positive (+1.57 V).
For iron, Fe

^{3 +}

(3d

^{5}

) → Fe

^{2 +}

(3d

^{6}

) destroys the half-filled shell, so the drive is much weaker (+0.77 V).
(ii) Losing one more electron takes Ce

^{3 +}

(4f

^{1}

) to Ce

^{4 +}

with the noble-gas 4f

^{0}

configuration — the extra stability of f

^{0}

makes the oxidation easy.

CBSE Class 12 Board · 2002 · Delhi · Subjective

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