In the Hall-Heroult process, aluminium is formed at the cathode. The cathode is made out of
(a) plat

Q: Why can't we just use an aqueous solution of aluminium salts for electrolysis?

Aluminium is highly reactive. If we use an aqueous solution, water will be reduced to hydrogen gas at the cathode instead of aluminium ions. Therefore, a molten state is strictly required.

Q: Why is the anode also made of carbon?

Carbon is a good conductor and relatively cheap. However, it reacts with the evolved oxygen to form $CO$ and $CO_2$, which actually helps lower the overall energy required for the process.

Q: What is the exact role of cryolite in this process?

Pure alumina has a very high melting point (around $2050^\circ C$) and is a poor conductor. Cryolite ($Na_3AlF_6$) lowers the melting point of the mixture to about $950^\circ C$ and significantly increases its electrical conductivity.

Q: Does the cathode also get consumed like the anode?

No, the cathode does not react with the products. Only the anode reacts with the evolved oxygen gas and gets consumed over time.

Q: Why does molten aluminium settle at the bottom of the tank?

Molten aluminium is denser than the molten electrolyte mixture. Due to gravity, it naturally sinks to the bottom where it can be easily tapped off without mixing back into the electrolyte.

Question

In the Hall-Heroult process, aluminium is formed at the cathode. The cathode is made out of

Options:

(A) platinum

(B) carbon

(C) pure aluminium

(D) copper

Sigma Percentile · Accepted Answer

### The Industrial Behemoth

Welcome to the fascinating world of metallurgy! When we talk about extracting aluminium, one of the most abundant metals on Earth, we inevitably discuss the **Hall-Heroult process**. Imagine a massive industrial setup. The heart of this process is a large steel tank. This isn't just any tank; it is a highly specialized electrolytic cell designed to withstand extreme temperatures and corrosive chemicals.

### The Heart of the Cell: Cathode and Anode

For electrolysis to occur, we need electrodes. If you look closely at the inner lining of this steel tank, you will notice a thick, black layer. This lining is made entirely of **carbon**, specifically in the form of graphite. In this electrolytic cell, this carbon lining acts as the **cathode**, which is the negative terminal.

To complete the setup, the tank is filled with an electrolyte. This is a molten mixture of alumina ($Al_2O_3$), cryolite ($Na_3AlF_6$), and fluorspar ($CaF_2$). The addition of cryolite is a stroke of genius; it significantly lowers the melting point of the mixture and increases its electrical conductivity. Suspended from the top into this molten bath are thick graphite rods. These rods act as the **anode**, the positive terminal.

### The Chemical Dance

When the power is turned on, a beautiful chemical dance begins. The aluminium ions ($Al^{3+}$), which are positively charged, are attracted to the negative cathode—our carbon lining. Here, they undergo reduction by gaining three electrons to form pure, molten aluminium:

$$Al^{3+} + 3e^- \rightarrow Al(l)$$

Because molten aluminium is denser than the electrolyte, it settles at the bottom of the tank, where it can be easily tapped off.

Meanwhile, the oxide ions ($O^{2-}$) migrate towards the positive graphite anodes. At these high temperatures, the oxygen gas released doesn't just bubble away. It reacts with the carbon of the anode itself, forming carbon monoxide and carbon dioxide gases:

$$C(s) + O^{2-} \rightarrow CO(g) + 2e^-$$
$$C(s) + 2O^{2-} \rightarrow CO_2(g) + 4e^-$$

This means the carbon anodes are slowly consumed and must be replaced periodically. In fact, for every kilogram of aluminium produced, about half a kilogram of carbon anode is burnt away!

### The Final Verdict

Returning to our original question, it is clear that the cathode, where the pure aluminium is formed and collected, is the inner lining of the cell. This lining is made out of **carbon**. Therefore, the correct choice is option (b).

Animated Solution for Chemistry - Metallurgy: In the Hall-Heroult process, aluminium is formed at the cathode. The cathode is made out of

Select Answer:

Visualized Solution

$H a l l - H er o u l t C e l l$

$T h e C a t h o d e$

$E l ec t r o l y t e an d A n o d e$

$R e a c t i o n a t C a t h o d e$

$R e a c t i o n a t A n o d e$

$C o n c l u s i o n$

$F oo d f or T h o ug h t$

The Sigma Insight: Principles of Metallurgy and Extraction

The Industrial Behemoth

The Heart of the Cell

The Chemical Dance

The Final Verdict

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