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प्रश्न
Describe with the help of chemical equations, the preparation of potassium dichromate from chromite ore.
Describe how ·potassium dichromate is made from chromite ore and give the equations for the chemical reactions involved.
How is potassium dichromate prepared from chromite ore? Write five reactions to show the oxidising character of this salt.
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उत्तर
Potassium dichromate is generally manufactured from chromite ore (FeO · Cr2O3 or FeCr2O4). The process involves the following steps.
Conversion of chromite ore to sodium chromate: The chromite ore is finely mixed with soda ash (Na2CO3) and quicklime. The mixture is then roasted in a reverberatory furnace using extra air. Roasting yields a yellow material containing sodium chromate.
\[\ce{4FeCr2O4 + O2 -> 2Fe2O3 + 4Cr2O3}\]
\[\ce{4Cr2O3 + 8Na2CO3 + 6O2 -> 8Na2CrO4 + 8CO2}\]
\[\ce{\overline{4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \leftarrow 2\text{Fe}_2\text{O}_3 + 8\text{Na}_2\text{CrO}_4 + 8\text{CO}_2}}\]
Quicklime produces a porous mass, allowing air to oxidize the ore.
The reaction can also be performed with NaOH instead of Na2CO3.
\[\ce{4FeCr2O4 + 16NaOH + 7O2 -> 2Fe2O3 + 8Na2CrO4 + 8H2O}\]
Conversion of chromate into dichromate: The sodium chromate solution from the previous step is treated with strong sulphuric acid to produce sodium dichromate.
\[\ce{2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O}\]
The solution becomes concentrated as less soluble sodium sulphate (Na2SO4 · 10H2O) crystallizes. This is filtered hot. Allow the solution to cool until sodium dichromate crystals (Na2Cr2O7 . 2H2O) separate upon standing.
Conversion of sodium dichromate into potassium dichromate: To create potassium dichromate, treat a solution of sodium dichromate with potassium chloride, as the latter is less stable and more soluble than the former. To produce orange crystals of potassium dichromate, a heated and concentrated sodium dichromate solution is mixed with a determined amount of potassium chloride and allowed to cool.
\[\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}\]
