Manufacture Of K2cr2o7 — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Preparation of Potassium Dichromate Potassium dichromate is manufactured industrially from chromite ore by oxidative roasting with sodium carbonate in air to form sodium chromate, followed by acidification to sodium dichromate and salt metathesis with potassium chloride to precipitate K₂Cr₂O₇. The process exploits the chromate–dichromate acid–base equilibrium and the much lower solubility of potassium dichromate compared to the sodium salt. Yellow solution of chromate after leaching turns orange on acidification to dichromate. During roasting, CO₂ evolves and a porous, yellow solid forms. On adding KCl and cooling, orange crystals of K₂Cr₂O₇ separate out; mother liquor contains NaCl. The oxidative roasting step is exothermic (ΔH < 0) due to oxidation of Cr(III) to Cr(VI) and formation of CO₂; it is effectively irreversible under process conditions. The chromate–dichromate interconversion is pH-dependent and reversible, while precipitation of K₂Cr₂O₇ is driven by its much lower solubility relative to Na₂Cr₂O₇. 1. Oxidative roasting/fusion: Chromite (FeCr₂O₄) is fused with Na₂CO₃ under an O₂-rich stream, oxidizing Cr(III) → Cr(VI) as chromate while Fe is converted to Fe₂O₃. 4 ,FeCr₂O₄(s) + 8 ,Na₂CO₃(s) + 7 ,O₂(g) 1100 – 1200 , K 8 ,Na₂CrO₄(s) + 2 ,Fe₂O₃(s) + 8 ,CO₂(g). 2. Leaching: The cooled roast is extracted with hot water; Na₂CrO₄ dissolves (yellow solution) while Fe₂O₃ and gangue remain insoluble. 3. Solid–liquid separation: Insoluble residue (mainly Fe₂O₃) is filtered off to obtain a clarified sodium chromate liquor. 4. Acidification to dichromate: The chromate liquor is acidified with H₂SO₄ to shift the equilibrium toward dichromate. Molecular: 2 ,Na₂CrO₄(aq) + H₂SO₄(aq) Na₂Cr₂O₇(aq) + Na₂SO₄(aq) + H₂O(l); Ionic: 2 ,CrO₄ 2- + 2 ,H + Cr₂O 7 2- + H₂O. 5. Salt metathesis with KCl: The sodium dichromate solution is treated with KCl; due to much lower solubility, K₂Cr₂O₇ crystallizes out. Na₂Cr₂O₇(aq) + 2 ,KCl(aq) K₂Cr₂O₇(s) + 2 ,NaCl(aq). 6. Crystallization and recovery: The hot mixture is cooled to induce crystallization of orange K₂Cr₂O₇ crystals, which are filtered, washed (cold water), and dried. Thinking K₂Cr₂O₇ is obtained directly from chromite—actually it proceeds via Na₂CrO₄ then Na₂Cr₂O₇. Mixing up colors: chromate (CrO₄²⁻) is yellow; dichromate (Cr₂O₇²⁻) is orange. Forgetting that acidification (e.g., H₂SO₄) shifts 2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O. Using the wrong reagent for conversion to potassium salt—KCl is used because K₂Cr₂O₇ is much less soluble and crystallizes out. Missing oxidation states: Cr(III) → Cr(VI) and Fe(II) → Fe(III) during roasting.