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प्रश्न
Indicate the steps in the preparation of K2Cr2O7 from chromite ore.
उत्तर
Dichromates are generally prepared from chromate, which in turn is obtained by the fusion of chromite ore \[\ce{(FeCr_2O_4)}\] with sodium or potassium carbonate in free air. The reaction with sodium carbonate occurs as follows:
\[\ce{4FeCr_2O_4 + 8Na2CO3 + 7O_2 ->8Na_2CrO_4 + 2Fe_2O_3 + 8CO2}\]
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, \[\ce{Na2Cr2O7. 2H2O}\] can be crystallised.
\[\ce{2Na_2CrO_4 + 2H^+ -> Na_2Cr_2O_7 + 2Na^+ + H_2O}\]
Sodium dichromate is more soluble than potassium dichromate. Therefore, the latter is prepared by treating the solution of sodium dichromate with potassium chloride.
\[\ce{Na_2Cr_2O_7 + 2KCl -> K_2Cr_2O_7 + 2NaCl}\]
Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in an aqueous solution, depending on the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.
\[\ce{2CrO^{2-}_4 + 2H^+ -> Cr2O^{2-}_7 + H2O}\]
\[\ce{Cr2O^{2-}_7 + 2OH^- -> 2CrO^2-_4 + H2O}\]
Hence, when the pH is increased, making the solution alkaline, dichromate ions (orange in colour) are converted into chromate ions, and the solution becomes yellow.
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संबंधित प्रश्न
Complete the following equations:
When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C).
(i) Write the formulae of the compounds (A), (B) and C.
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Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
Indicate the step in the preparation of K2Cr2O7 from chromite ore.
Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
