Mention the different steps for the preparation of potassium dichromate from iron chromite ore and draw the structure of dichromate ion.

Question

Match List-I with List-II
\[ \begin{array}{|c|c|} \hline \text{List-I (Species)} & \text{List-II (Electronic distribution)} \\ \hline \text{(A) Cr}^{+2} & \text{(I) } 3d^8 \\ \text{(B) Mn}^{+} & \text{(II) } 3d^3 4s^1 \\ \text{(C) Ni}^{+2} & \text{(III) } 3d^4 \\ \text{(D) V}^{+} & \text{(IV) } 3d^5 4s^1 \\ \hline \end{array} \]

Answer 1

The manufacture of K₂Cr₂O₇ from iron chromite ore can be summarised as a roast, an acidify and a salt-exchange stage.
Step 1 (Roasting / alkali fusion): Finely ground chromite, FeCr₂O₄, is heated strongly with sodium carbonate in a current of air. Air oxidises Cr(III) of the ore to Cr(VI), producing soluble sodium chromate while iron is left behind as insoluble ferric oxide:
4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
Step 2 (Acidification): The filtered yellow chromate liquor is made acidic with sulphuric acid. In acid medium chromate ions condense to dichromate ions, so sodium dichromate is obtained:
2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
Step 3 (Metathesis with KCl): Adding KCl and cooling makes the far less soluble potassium dichromate separate as orange crystals, sodium chloride staying in solution:
Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl
Structure of Cr₂O₇^2-: Two tetrahedral CrO₄ units share a single bridging oxygen. Thus each Cr(+6) has three terminal oxygens plus one shared oxygen, and the ion has the shape O₃Cr-O-CrO₃ with a bent Cr-O-Cr bridge.

Mention the different steps for the preparation of potassium dichromate from iron chromite ore and draw the structure of dichromate ion.

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Solution and Explanation

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