In the context of the Hall-Heroult process for the extraction of Al, which of the following statements is false?

Wait a moment while we are looking for your question.

.

In the context of the Hall-Heroult process for the extraction of Al, which of the following statements is false?

Engineering

Chemistry

Electrolysis

Extraction of Iron and Copper

Extraction of Gold Silver Aluminium

Question

In the context of the Hall-Heroult process for the extraction of Al, which of the following statements is false?

A

Na₃AlF₆ serves as the electrolyte

B

CO and CO₂ are produced in this process

C

Al₂O₃ is mixed with CaF₂ which lowers the melting point of the mixture and brings conductivity

D

Al³⁺ is reduced at the cathode to form Al

(B) In Hall Heroult process alumina (Al₂O₃) is used as electrolyte.

Cryolite (Na₃AlF₆) and Fluospar (CaF₂) are used to lowers the melting point of the mixture and brings conductivity

The Hall-Héroult process is the primary industrial method for extracting aluminum from alumina (Al₂O₃). Let's evaluate each statement step by step:

Step 1: "Na₃AlF₆ serves as the electrolyte" – This is true. Cryolite (Na₃AlF₆) is used as the electrolyte because pure alumina has a very high melting point (around 2072°C) and is a poor conductor of electricity. Cryolite lowers the melting point to about 900–1000°C and enhances conductivity.

Step 2: "CO and CO₂ are produced in this process" – This is true. The anode is made of carbon, which reacts with oxygen liberated during electrolysis. The reactions are: $2 O^{2-} + C \to C O_{2} + 4 e^{-}$ and $O^{2-} + C \to C O + 2 e^{-}$ . So, both CO and CO₂ are produced.

Step 3: "Al₂O₃ is mixed with CaF₂ which lowers the melting point of the mixture and brings conductivity" – This is false. Alumina is mixed with cryolite (Na₃AlF₆), not CaF₂ (calcium fluoride). While CaF₂ is sometimes added in small amounts to further lower the melting point and improve performance, it is not the primary component; cryolite is. The statement is misleading because it incorrectly identifies CaF₂ as the main additive.

Step 4: "Al³⁺ is reduced at the cathode to form Al" – This is true. The cathode reaction is: ${Al}^{3 +} + 3 e^{-} \to Al$ . Aluminum metal is deposited at the cathode.

Final Answer: The false statement is: "Al₂O₃ is mixed with CaF₂ which lowers the melting point of the mixture and brings conductivity." The primary additive is cryolite (Na₃AlF₆), not CaF₂.

Related Topics and Formulae

Electrolysis in Metallurgy: Electrolysis is used to extract highly reactive metals like aluminum, sodium, and magnesium. The key reactions involve reduction at the cathode and oxidation at the anode.

Role of Cryolite: Cryolite (Na₃AlF₆) lowers the melting point of alumina from ~2072°C to ~950°C and increases electrical conductivity, making the process feasible.

Anode Reactions: Carbon anodes are consumed, producing CO and CO₂: $2 O^{2 -} + C \to C O_{2} + 4 e^{-}$ and $O^{2 -} + C \to C O + 2 e^{-}$ .

Cathode Reaction: ${Al}^{3 +} + 3 e^{-} \to Al$ (reduction).