d and f Block Elements — CBSE Class 12 · 2007 · Delhi Board Question with Solution

a) Write the steps involved in the preparation of:
i) K₂Cr₂O₇ from Na₂CrO₄
ii) KMnO₄ from K₂MnO₄
b) What is meant by lanthanoid contraction? What effect does it have on the chemistry of the elements which follow the lanthanoids?

Solution

(a)(i) From sodium chromate:

2N a_{2} Cr O_{4} + H_{2} S O_{4} \to N a_{2} C r_{2} O_{7} + N a_{2} S O_{4} + H_{2} O

N a_{2} C r_{2} O_{7} + 2KCl \to K_{2} C r_{2} O_{7} + 2NaCl

The less soluble orange K

_{2}

_{2}

_{7}

crystallises on cooling.
(ii) From potassium manganate — oxidation at the anode of an electrolytic cell:

Mn O_{4}^{2 -} \to Mn O_{4}^{-} + e^{-}

(green manganate → purple permanganate).
(b) Lanthanoid contraction is the steady fall in atomic and Ln

^{3 +}

radii from La to Lu, due to poor shielding of the nuclear charge by 4f electrons.
Effect: the size increase normally expected on descending from the 4d to the 5d series is wiped out, so Zr ≈ Hf, Nb ≈ Ta, Mo ≈ W in radius — each pair has nearly identical chemistry, occurs together and is separated only with difficulty.

CBSE Class 12 Board · 2007 · Delhi · Subjective

CBSE Class 12 · 2007 · Delhi · d and f Block ElementsLong Answer

Solution

More board questions on d and f Block Elements