With respect to the Hall Heroult process, justify the following:

(a) Powdered coke is sprinkled over the electrolytic mixture undergoing electrolytic reduction.

(b) Graphite anodes are continuously replaced during the electrolysis.

(c) Cryolite and fluorspar must be added to the electrolytic mixture.

(d) Along with cryolite and alumina, another substance is added to the electrolyte mixture. Name the substance and give one reason for the addition.

(a) The layer of powdered coke is sprinkled over the electrolytic mixture as:

1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
2. It minimizes or prevents heat loss by radiation.

(b) The graphite anodes are continuously replaced during the electrolysis because:

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet.
   2C + O₂ ⟶ 2CO
   2CO + O₂ ⟶ 2CO₂

(c) Cryolite and fluorspar are added to the electrolytic mixture because their addition:

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(d) Fluorspar (CaF₂) is the additional substance added because it acts as a solvent and increases the fluidity of the electrolytic bath so the molten aluminium can settle at the bottom more easily.