Question
Indicate the steps in the preparation of:
K2Cr2O7 from chromite ore.
K2Cr2O7 from chromite ore.
✓
Answer
The preparation of potassium dichromate from chromite involves the following main steps:
- The chromate ore is finely ground and heated strongly with molten alkali in the presence of air.
$2\text{FeCr}_2\text{O}_4+8\text{NaOH}+7/2\text{O}_2\rightarrow4\text{NaCr}_2\text{O}_4+\text{Fe}_2\text{O}_3+4\text{H}_2\text{O}\\\text{Chromite}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Sodium chromate}$
- The solution of sodium chromate is filtered and acidified with dilute sulphuric acid so that sodium dichromate is obtained.
$2\text{Fe}_2\text{Cr}_2\text{O}_4+\text{H}_3\text{SO}_4\rightarrow\text{Na}_2\text{Cr}_2\text{O}_7+\text{Na}_2\text{SO}_4+\text{H}_2\text{O}\\\text{Sodium}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Sodium}\\ \text{Chromate} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{dichromate}$
- A calculated quantity of potassium chloride is added to a hot concentrated solution of sodium dichromate. Potassium dichromate is less soluble therefore it crystallizes out first.
$2\text{Na}_2\text{Cr}_2\text{O}_7+2\text{KCl}\rightarrow\text{K}_2\text{Cr}_2\text{O}_7+\text{NaCl}\\\text{Sodium}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Potessium}\\\text{dichromate} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{dichromate}$
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