Question

Answer the following question based on the extraction of aluminium from alumina by Hall-Heroult's process:

1. What is the function of cryolite used along with alumina as the electrolyte?
2. Why is powdered coke sprinkled on the top of the electrolyte?
3. Name the electrode, from which aluminium is collected.

Answer 1

In the extraction of aluminium from alumina by Hall-Heroult's Process:

1. Cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$ is used along with alumina $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ to lower the temperature of the mixture from $2050°\mathrm{C}$ to $950°\mathrm{C}$ and enhance conductivity.
2. Powdered coke is sprinkled over the surface of the electrolytic mixture because it reduces heat loss by radiation. It prevents the burning of the anode.
3. Aluminium ions $\left({\mathrm{Al}}^{3+}\right)$ are positively charged which migrates towards the cathode, a negative electrode, and get reduced to form aluminium.

${\mathrm{Al}}^{3+}\left(\mathrm{aq}\right)+3{\mathrm{e}}^{-}\to \mathrm{Al}\left(\mathrm{s}\right)$

Hence, aluminium is collected at the cathode.