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Difficulty: MediumAluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

During the industrial extraction of aluminium by the Hall-Héroult electrolytic process, why must the graphite anode rods be replaced at regular intervals?

  1. The oxygen gas discharged at the anode reacts with the graphite at high operating temperatures to form carbon dioxideAnswer
  2. B
    Molten aluminium is deposited directly onto the anode rods, causing severe corrosion and electrical shorting
  3. C
    Molten cryolite reacts chemically with graphite to produce gaseous carbon tetrafluoride
  4. D
    Graphite oxidizes directly by releasing carbon cations into the molten electrolyte solution

Answer

The oxygen gas discharged at the anode reacts with the graphite at high operating temperatures to form carbon dioxide
During the Hall-Héroult process, oxide ions (O2−O^{2-}) are discharged at the carbon (graphite) anode to produce oxygen gas. Because the cell operates at approximately 950°C, the freshly liberated oxygen gas reacts with the carbon anodes to form carbon dioxide (CO2CO_2) and carbon monoxide (COCO). As a result, the graphite anodes are burned away over time and must be routinely replaced.

Step-by-Step Solution

1
Identify the anode half-reaction during the electrolysis of molten alumina in cryolite
Oxide ions (O2−O^{2-}) migrate to the positive graphite anode and undergo oxidation to form oxygen gas: 2O2−→O2(g)+4e−2O^{2-} \rightarrow O_2(g) + 4e^-
Anodes are the site of oxidation where negative ions discharge electrons.
2
Analyze the chemical interaction between liberated oxygen gas and the carbon anode at high cell operating temperatures
At temperatures around 950°C, the liberated oxygen gas reacts with the hot graphite anode to form carbon dioxide: C(s)+O2(g)→CO2(g)C(s) + O_2(g) \rightarrow CO_2(g)
Graphite carbon combusts in the presence of oxygen gas at elevated temperatures, leading to continuous depletion of the anodes.

Key Concept

Anode consumption due to oxygen reaction in the Hall-Héroult process
Estimated Time:1m 0s
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Topics

Aluminium: Extraction, Amphoteric Properties, Alloys, and Compounds

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