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The D-And-f-Block Elements

Question

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

Solution

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⁻

Some More Questions From The d-And-f-Block Elements Chapter

Which is a stronger reducing agent Cr²⁺ or Fe²⁺ and why?

Explain why Cu⁺ ion is not stable in aqueous solutions?

Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not?

The outer electronic configuration of copper is 3d¹⁰ 4s¹, yet it is considered transition element. Why?

Why the melting points of transition elements are high?

Why Zn, Cd and Hg are not regarded as transition elements?

Name the first element of 3rd transition series.

Though copper, silver and gold have completely filled sets of d-orbitals yet they are considered as transition metals. Why?

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