JEE Main 2022 · 28 July · morning · Single correct · +4 / −1
Which of the reaction is suitable for concentrating ore by leaching process ?
- (a) 2 Cu₂ S+3 O₂ → 2 Cu₂ O+2 SO₂
- (b) Fe₃ O₄+CO → 3 FeO+CO₂
- (c) Al₂ O₃+2 NaOH+3 H₂ O → 2 Na[Al(OH)₄]
- (d) Al₂ O₃+6 Mg → 6 MgO+4 Al
Answer and solution
Answer (c) Al₂ O₃+2 NaOH+3 H₂ O → 2 Na[Al(OH)₄]
Solution
Chemistry ma’am · 3 steps
Leaching dissolves the ore and leaves impurities behind: find the reaction that gives a soluble product.
- What leaching is. Leaching is a concentration method. The powdered ore is treated with a reagent that dissolves the metal compound, while the impurities (the gangue) stay behind as a solid and are filtered off.
- What to look for. The reaction must turn the solid ore into a soluble product. It must not be a roasting or reduction step, because those come after concentration.
Option What the reaction does Stage of metallurgy (a) 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ Heats a sulphide in air, giving an oxide and SO₂ Roasting, not concentration (b) Fe₃O₄ + CO → 3FeO + CO₂ CO removes oxygen from the iron oxide Reduction in the blast furnace (c) Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄] Bauxite dissolves as soluble sodium aluminate. Impurities such as Fe₂O₃ and silicates remain undissolved. Leaching (Bayer process) (d) Al₂O₃ with Mg A more reactive metal pulls oxygen away from Al₂O₃ Reduction to the metal - So. Only (c) dissolves the ore and separates it from insoluble impurities. That is leaching of bauxite with NaOH, so the answer is (c).
The idea to keep. Leaching always makes a solution: the metal compound dissolves and the gangue is filtered off. If a reaction gives a gas, an oxide or the free metal, it is roasting or reduction, not concentration.
Answer from the NTA final answer key. Solution by PrepWiser.