JEE Main 2022 · 24 June · evening · Single correct · +4 / −1

Which of the following chemical reactions represents Hall-Heroult Process?

  1. (a) Cr₂O₃ + 2Al → Al₂O₃ + 2Cr
  2. (b) 2Al₂O₃ + 3C → 4Al + 3CO₂
  3. (c) FeO + CO → Fe + CO₂
  4. (d) 2[Au(CN)₂]_((aq)) - + Zn(s) → 2Au(s) + [Zn(CN₄)]²⁻
Answer and solution

Answer (b) 2Al₂O₃ + 3C → 4Al + 3CO₂

Solution

Chemistry ma’am · 3 steps

Hall-Heroult means electrolysis of molten alumina, where the carbon anode burns away as CO₂.

  1. The process. Purified Al₂O₃ is dissolved in molten cryolite, Na₃AlF₆, with some CaF₂ added. This lowers the melting point and makes the melt conduct. It is then electrolysed between carbon (graphite) electrodes.
  2. At the electrodes. At the cathode, Al³⁺ gains electrons and molten aluminium collects. At the anode, oxide ions give oxygen, which reacts with the carbon anode to form CO and CO₂, so the anodes are slowly used up.

    2Al₂O₃ + 3C → 4Al + 3CO₂

  3. So. The overall reaction of the Hall-Heroult process is option (b).

The idea to keep. Link each named process to its reaction: Hall-Heroult is alumina plus carbon giving Al and CO₂, thermite is Al reducing a metal oxide, and the cyanide process uses Zn to displace gold or silver.

Answer from the NTA final answer key. Solution by PrepWiser.

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Asked about 80 times in JEE Main since 2020. This question is from 2022.