d and f Block Elements — CBSE Class 12 · 2010 · All India Board Question with Solution

a) Complete the following reactions in an aqueous medium:
i) MnO₄⁻ + C₂O₄²⁻ + H⁺ →
ii) Cr₂O₇²⁻ + H₂S + H⁺ →
b) How would you account for the following?
i) Metal–metal bonding is more extensive in the 4d and 5d series of transition elements than in the 3d series.
ii) Mn(+3) undergoes a disproportionation reaction easily.
iii) Co(+2) is easily oxidised in the presence of strong ligands.

Solution

(a)(i)

2Mn O_{4}^{-} + 5 C_{2} O_{4}^{2 -} + 16 H^{+} \to 2M n^{2 +} + 10C O_{2} + 8 H_{2} O

(ii)

C r_{2} O_{7}^{2 -} + 3 H_{2} S + 8 H^{+} \to 2C r^{3 +} + 3S + 7 H_{2} O

(b)(i) The 4d/5d orbitals are larger and more diffuse than 3d, overlapping strongly between adjacent metal atoms (also seen in their higher atomisation enthalpies) — hence frequent M–M bonds and clusters among the heavier metals.
(ii) Mn

^{3 +}

lies between two more stable states — Mn

^{2 +}

(half-filled d

^{5}

) and Mn

^{4 +}

(as insoluble MnO

_{2}

) — so it disproportionates:

2M n^{3 +} + 2 H_{2} O \to M n^{2 +} + Mn O_{2} + 4 H^{+}

(iii) With strong-field ligands (CN

^{-}

, NH

_{3}

), the product Co

^{3 +}

^{6}

) gains a very large crystal-field stabilisation energy as t

_{2 g}^{6}

; this added stability lowers the oxidation potential so much that Co(+2) is oxidised easily — even air converts [Co(NH

_{3}

)

_{6}

]

^{2 +}

to [Co(NH

_{3}

)

_{6}

]

^{3 +}

CBSE Class 12 Board · 2010 · All India · Subjective

CBSE Class 12 · 2010 · All India · d and f Block ElementsLong Answer

Solution

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