Question

Cryolite, $N{a_3}Al{F_6}$ is added to $A{l_2}{O_3}$ in Hall-Heroult process to ___________ m.p. and to ___________ electrical conductivity.
A.higher, increase
B.higher, decrease
C.lower, increase
D.lower, decrease

Answer 1

Hint: To answer this question, you must go through or recall the Hall-Heroult process. This process is used in smelting of Aluminium. Electric current cannot pass through the solid form of alumina, cryolite is added to alumina in a small amount to carry out electrolysis easily. This mixture acts as a good conductor of electricity. On passing electric current, molten aluminium is deposited at the cathode and oxygen gas is released at the anode.

Complete Step by step answer: Step 1: Firstly, go through the Hall-Heroult process. The process is used in the extraction of aluminium, in which, pure $A{l_2}{O_3}$ is mixed with $Ca{F_2}$ or $N{a_3}Al{F_6}$ . This results in lowering of the melting point of the mixture and in turn increases its ability to conduct electricity.
Recycled aluminum requires no electrolysis, thus it does not end up in this process. This process contributes to climate change through the emission of carbon dioxide in the electrolytic reaction.
A steel vessel with the lining of carbon and graphite rods is used. And now we will conclude the statement in the 2nd step.
Step 2: Clearly we can conclude that, melting point is lowered and the electrical conductivity is increased. And hence we got the answer and found the correct option.

So, the correct answer is option C (lower, increase).

Note: Hall–Heroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell. The process is applied at industrial scale at 940–980 °C and produces 99.5–99.8% pure aluminium.