Why does the anode need replacing during the electrolysis of aluminium oxide?
The correct answer and explanation is:
The anode in the electrolysis of aluminium oxide (Al₂O₃) needs to be replaced because it undergoes continuous consumption during the process. The electrolysis of aluminium oxide takes place in a molten state at high temperatures (around 950°C to 1000°C) inside an electrolytic cell. The cell consists of a molten electrolyte of aluminium oxide dissolved in molten cryolite (Na₃AlF₆), and the two electrodes used are the cathode (which remains stable) and the anode (which gradually wears away).
At the anode, oxidation reactions occur, leading to the release of oxygen gas. Specifically, oxygen ions (O²⁻) from the aluminium oxide migrate to the anode, where they lose electrons (oxidation) and form oxygen gas (O₂). The overall reaction at the anode is:
2O2−→O2+4e−2O^{2-} \rightarrow O_2 + 4e^-
This release of oxygen causes the anode material, typically made of carbon, to react with the oxygen gas produced. The carbon in the anode combines with the oxygen to form carbon dioxide (CO₂):
C+O2→CO2C + O_2 \rightarrow CO_2
This reaction gradually depletes the carbon anode over time, leading to its reduction in size and eventual degradation. The loss of material means the anode must be replaced periodically to maintain the process.
Furthermore, the continuous erosion of the anode can be costly, and its replacement adds to the overall expense of the aluminium extraction process. This is one of the challenges of aluminium production, as the electrolysis process requires a significant amount of energy, and the degradation of the anode adds to both the operational and environmental costs of the process.
In summary, the anode needs to be replaced in the electrolysis of aluminium oxide because it is consumed by the reaction with oxygen, which produces carbon dioxide, resulting in the gradual loss of material.