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Question Video: Explaining Why Cryolite Is Used in the Extraction of Aluminum Chemistry

Which of the following is not a reason why cryolite is part of a multielectrolyte in the extraction of aluminum? [A] Cryolite increases the amount of electricity used. [B] Cryolite reduces the temperature at which the aluminum melts. [C] Cryolite increases the conductivity of the electrolyte. [D] Cryolite provides sodium ions that help to carry the electric current. [E] Cryolite reduces the working temperature of the electrolytic cell.

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Video Transcript

Which of the following is not a reason why cryolite is part of a multielectrolyte in the extraction of aluminum? (A) Cryolite increases the amount of electricity used. (B) Cryolite reduces the temperature at which the aluminum melts. (C) Cryolite increases the conductivity of the electrolyte. (D) Cryolite provides sodium ions which help to carry the electric current. Or (E) cryolite reduces the working temperature of the electrolytic cell.

The extraction of aluminum is a two-step process. First, alumina is extracted from bauxite ore in the Bayer process. Then, pure aluminum metal is extracted from alumina in the Hall–Héroult process. This is an electrolytic process. Electrolysis is a chemical process where electricity is passed through an electrolyte, causing it to decompose into its constituent elements. The question asks us about a molten electrolyte, so we know we are looking at the electrolysis process. In the Hall–Héroult electrolytic cell, we have a carbon cathode and a carbon anode.

Electricity is passed through an electrolyte containing aluminum ions, which gain electrons from the cathode, are reduced to aluminum metal. And this liquid aluminum metal sinks to the bottom of the cell and can be collected through an outlet where it is then cast into blocks. If the electrolyte was only composed of molten alumina, in other words, liquid aluminum oxide, then solid alumina would need to be heated to its melting point, which is over 2000 degrees Celsius. This requires a lot of energy. So instead, cryolite, which is a sodium aluminum fluoride compound, is mixed with a solid alumina. Cryolite has a lower melting point than aluminum oxide and acts as a solvent to create an electrolyte mixture of molten cryolite and alumina.

This mixture melts at about 900 degrees Celsius instead of over 2000 degrees Celsius, as in the case of pure alumina. Less energy or less electricity is therefore used. This decreases production costs. The question asked which of the following is not a reason why cryolite is used. Option (A), cryolite increases the amount of electricity used, is not a reason why cryolite is used. Cryolite is used to decrease the amount of electricity needed.

The remaining four answer options are reasons why cryolite is added. Cryolite does reduce the temperature at which the aluminum melts. It does increase the conductivity of the electrolyte. It does provide sodium ions that help to carry the electric current. And it does reduce the working temperature of the electrolytic cell. Finally, “which of the following is not a reason why cryolite is part of a molten electrolyte in the extraction of aluminum?” The answer is cryolite increases the amount of electricity used.

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