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Describe the Preparation of Potassium Dichromate from Iron Chromite Ore. What is the Effect of Increasing Ph on a Solution of Potassium Dichromate? - CBSE (Science) Class 12 - Chemistry

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Question

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Solution

Potassium dichromate is prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr2O3) with sodium or potassium carbonate in free excess of air. The reaction with sodium carbonate occurs as follows:

`4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 -> 8Na_2CrO_4+2Fe_2O_3+8CO_2`

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr,07.2H20 can be crystallised.

`2Na_2CrO_4+2H^+ -> Na_2Cr_2O_7+2Na^(+) + H_2O`

Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.

`Na_2Cr_2O_7 + 2KCL -> K_2Cr_2O_7 + 2NaCl`

Orange crystals of potassium dichromate crystallise out. The chromates and dichromates depending upon pH of the solution. If pH of potassium dichromate is increased it is converted to yellow potassium chromate

`2CrO_4^(2-) + 2H^+ -> Cr_2O_7^(2-) + H_2O`

`Cr_2O_7^(2-) + 2OH^(-) -> 2CrO_4^(2-) + H_2O`

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 NCERT Solution for Chemistry Textbook for Class 12 (2018 to Current)
Chapter 8: The d-block and f-block Elements
Q: 14 | Page no. 234
Solution Describe the Preparation of Potassium Dichromate from Iron Chromite Ore. What is the Effect of Increasing Ph on a Solution of Potassium Dichromate? Concept: Some Important Compounds of Transition Elements - Oxides and Oxoanions of Metals.
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