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.
