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The d-And-f-Block Elements

Short Answer Type

301.

Assigns reasons for the following:
(i) Transition metals exhibit enthalpy of atomisation.
(ii) Transition metals form interstitial compounds.

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302.

Explain why:
(i) Transition elements are metals?
(ii) Lanthanides are uniformly trivalent?

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303.
Write the preparation of :
(i) Potassium dichromate from chromite.
(ii) KMnO₄ from pyrolusite.

1) Potassium dichromate (K2Cr2O7) is prepared from chromite ore FeCr2O4. The chromite ore is fused with sodium or potassium carbonate in free access of air.
4FeCr₂O₄+ 8Na₂CO₃ + 7O₂ -------> 8Na₂CrO₄ + 2FeO₃ + 8CO₂
The yellow solution of sodium chromate is filtered and acidified with sulfuric acid to give a solution from which orange sodium dichromate , Na2Cr2O7 2H2O can be crystallized.

2Na₂CrO₄ + 2H⁺ --------> Na₂Cr₂O₇ + 2Na + H₂O

Sodium dichromate is more soluble than potassium dichromate. Hence sodium dichromate when fused with KCl forms orange crystals of potassium dichromate.

Na₂Cr₂O₇+ 2KCl --------> K₂Cr₂O₇ + 2NaCl

(2) Potassium permanganate is manufactured on a large scale due to its manifold uses in th
laboratory. In the first stage, pyrolusite, which is the mineral form of manganese dioxide, is fused with potassium hydroxide and heated in air or with potassium nitrate (a source of oxygen). This leads to the formation of potassium manganate, which on electrolytic oxidation in alkaline solution gives potassium permangante.

MnO₂+ 2OH⁻ + O₂ → MnO₄²−+ H₂O

MnO₄²−+ Cl₂ → MnO₄−+ 2Cl⁻

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