CBSE Class 12 · 2018 · d and f Block ElementsShort Answer

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Give reasons:
(a) The E° value for the Mn³⁺/Mn²⁺ couple is much more positive than that for Fe³⁺/Fe²⁺.
(b) Iron has a higher enthalpy of atomisation than copper.
(c) Sc³⁺ is colourless in aqueous solution whereas Ti³⁺ is coloured.

Solution

(a) Gaining an electron converts Mn

^{3 +}

(3d

^{4}

) into Mn

^{2 +}

(3d

^{5}

, exactly half-filled, extra stable) — reduction is strongly favoured, E° = +1.57 V. For iron the same step takes stable Fe

^{3 +}

(3d

^{5}

) to Fe

^{2 +}

(3d

^{6}

), so the drive is weaker (+0.77 V).
(b) Iron ([Ar]3d

^{6}

^{2}

) has four unpaired d electrons participating in metallic bonding; copper ([Ar]3d

^{10}

^{1}

) has none. Stronger interatomic bonding in iron means a higher enthalpy of atomisation.
(c) Sc

^{3 +}

: 3d

^{0}

— no d electron, no d–d transition, hence colourless.
Ti

^{3 +}

: 3d

^{1}

— the single d electron undergoes d–d transitions absorbing visible light, so Ti

^{3 +}

(aq) is purple.

CBSE Class 12 Board · 2018 · Subjective

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CBSE Class 12 · 2018 · d and f Block ElementsShort Answer

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