CBSE Class 12 · 2005 · Delhi; also 2006 · d and f Block ElementsShort Answer

Describe the steps involved in the preparation of either potassium dichromate from sodium chromate or potassium permanganate from manganese dioxide.

Answer: Na₂CrO₄ —H₂SO₄→ Na₂Cr₂O₇ —KCl→ K₂Cr₂O₇ (crystallises out). [Or MnO₂ —KOH/O₂→ K₂MnO₄ —electrolytic oxidation→ KMnO₄.]

Solution

_{2}

_{2}

_{7}

from Na

_{2}

CrO

_{4}

:
Step 1 — acidify the yellow chromate to the orange dichromate:

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

Step 2 — treat the concentrated solution with KCl; the less soluble potassium salt crystallises:

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

(Alternative, KMnO

_{4}

from MnO

_{2}

: fuse with KOH/O

_{2}

to the green manganate, then oxidise electrolytically —

2Mn O_{2} + 4KOH + O_{2} \to 2 K_{2} Mn O_{4} + 2 H_{2} O; Mn O_{4}^{2 -} anode Mn O_{4}^{-} + e^{-})

CBSE Class 12 Board · 2005 · Delhi; also 2006 · Subjective