The correct answer is (4) (C), (D) only.
\(CaF_2\) is added in the electrolytic reduction of \(Al_2O_3\) dissolved in fused cryolite (Hall-Héroult process) and how it serves the purposes mentioned in the options:
In the Hall-Héroult process, aluminum is extracted from alumina (\(Al_2O_3\)) using electrolysis. This process takes place in a high-temperature electrochemical cell called an aluminum reduction cell. The main components of the cell are:
1. Cathode: A layer of molten aluminum forms at the bottom of the cell, and this acts as the cathode. The aluminum ions (\(Al^{3+}\)) in the alumina (\(Al_2O_3\)) are reduced at the cathode to form aluminum metal (\(Al\)).
2. Anode: Carbon blocks or rods are used as the anode. At the anode, oxygen ions (\(O^{2-}\)) from alumina combine with carbon to form carbon dioxide (\(CO_2\)). This process is essentially the oxidation of carbon:
\[2C + O_2 \rightarrow 2CO_2\]
Now, let's address each purpose mentioned in the options:
(A) To decrease the rate of oxidation of carbon at the anode: This statement is not accurate. The addition of \(CaF_2\) does not directly decrease the rate of oxidation of carbon at the anode. Instead, it helps with other aspects of the process.
(B) To act as a catalyst: \(CaF_2\) itself is not a catalyst. However, it plays a crucial role in reducing the melting point of the mixture. The addition of \(CaF_2\) to the alumina-cryolite mixture lowers the overall melting point of the mixture. This is important because it reduces the energy required to keep the mixture in a molten state. In other words, it makes the process more energy-efficient. So, while \(CaF_2\) is not a catalyst in the traditional sense, it does enhance the efficiency of the process.
(C) To make the fused mixture conducting: This statement is accurate. The primary purpose of adding \(CaF_2\) is to increase the electrical conductivity of the mixture. When \(CaF_2\) is added to the alumina-cryolite mixture, it forms compounds like calcium fluoride (\(CaF_2\)) and other calcium-aluminum fluorides (\(CaAlF_4\)). These compounds are more conductive than pure alumina (\(Al_2O_3\)). Enhanced conductivity is essential for the efficient flow of electric current through the molten mixture.
(D) To lower the fusion temperature to melt: This statement is accurate. Adding \(CaF_2\) lowers the fusion temperature of the alumina-cryolite mixture. This is important because it reduces the energy needed to initially melt the mixture and maintain it in the molten state during the electrolysis process. Lowering the fusion temperature improves the energy efficiency of the process.
In summary, the addition of \(CaF_2\) in the Hall-Héroult process serves primarily to increase the electrical conductivity of the molten mixture and lower the fusion temperature. These effects enhance the energy efficiency of the aluminum production process, making it more cost-effective. |
