Chemistry General Principles and Processes of Isolation of Elements ( Not In Syllabus ) Mix Single Correct MCQ
Published on: August 11, 2026

In electrolysis of Al 2 O 3 by Hall-Heroult process :

A
cryolite Na 3 [AlF 6 ] lowers the melting point of Al 2 O 3 and increases its electrical conductivity.
B
Al is obtained at cathode and probably CO 2 at anode.
C
oxygen liberated at anode reacts with carbon anode liberating CO and CO 2 .
D
All of the above are correct.

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The correct answer is:
C

Na 3 [AlF 6 ] — → 3NaF + AlF 3

NaF and AlF 3 both are ionic compounds and so ionise to give ions. This increases the electrical conductivity and lowers the melting point of Al 2 O 3 .

At cathode : Al 3+ (melt) + 3e – — → Al.

At anode : C(s) + O 2– (melt) — → CO (g) + 2e – ; C(s) + 2O 2– (melt) — → CO 2 (g) + 4e – .

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