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

a) Complete the following chemical equations:
i) MnO₄⁻(aq) + S₂O₃²⁻(aq) + H₂O(l) →
ii) Cr₂O₇²⁻(aq) + Fe²⁺(aq) + H⁺(aq) →
b) Explain the following observations:
i) La³⁺ (Z = 57) and Lu³⁺ (Z = 71) do not show any colour in solutions.
ii) Among the divalent cations in the first series of transition elements, manganese exhibits the maximum paramagnetism.
iii) The Cu⁺ ion is not known in aqueous solution.

Solution

(a)(i)

8Mn O_{4}^{-} + 3 S_{2} O_{3}^{2 -} + H_{2} O \to 8Mn O_{2} + 6S O_{4}^{2 -} + 2O H^{-}

(ii)

C r_{2} O_{7}^{2 -} + 6F e^{2 +} + 14 H^{+} \to 2C r^{3 +} + 6F e^{3 +} + 7 H_{2} O

(b)(i) La

^{3 +}

has 4f

^{0}

and Lu

^{3 +}

has 4f

^{14}

— an empty or completely filled f-subshell permits no f–f transitions, so neither ion absorbs visible light: both are colourless.
(ii) Mn

^{2 +}

is 3d

^{5}

with five unpaired electrons — the largest possible number for a divalent 3d ion; paramagnetism grows with unpaired electrons, μ = √35 ≈ 5.92 BM, the series maximum.
(iii) In water Cu

^{+}

disproportionates,

2C u^{+} \to C u^{2 +} + Cu

the large hydration enthalpy of Cu

^{2 +}

outweighing copper's second ionisation enthalpy — hence no stable aqueous Cu(+1).

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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