Video Transcript
Given the contents of the
electrolytic cell used for the extraction of aluminum, which of the following is
unlikely to be a waste gas? (A) CO2, (B) O2, (C) H2, (D) CO.
To answer this question, let’s
have a look at the electrolytic cell that is used during the extraction of
aluminum. First of all, we know that in
the cell, alumina, or Al2O3, is mixed with molten cryolite to form an
electrolyte solution. Electricity is passed through
the electrolyte, causing the alumina to decompose. At the cathode, reduction takes
place. Each Al3+ ion gains three
electrons to form aluminum metal atoms. The molten aluminum sinks to the
bottom of the cell and can then be collected. At the anodes, oxidation takes
place. The O2− ions lose electrons to
form molecules of oxygen gas. The oxygen gas bubbles out of
the electrolyte solution and leaves the cell. Since we know oxygen gas is a
waste gas, we can eliminate it from the answer choices.
Now, the electrolytic cell
operates at a very high temperature, around 1000 degrees Celsius. Both the anodes and the cathode
in the electrolytic cell are composed of carbon. When we heat pure carbon to such
a high temperature, some of it will react with the oxygen gas being produced in
the cell. This reaction tends to occur at
the carbon anodes. When the carbon in the anode
reacts with oxygen gas, it forms carbon dioxide gas. This carbon dioxide gas is
another waste gas, so let’s eliminate it from the answer choices.
The chemical reaction we just
wrote is for the complete oxidation of solid carbon. However, it’s also possible that
incomplete oxidation could take place. The incomplete oxidation of
solid carbon would produce carbon monoxide gas. So, let’s eliminate this waste
gas from the answer choices. This leaves only hydrogen gas,
or H2, which must be the correct answer.
In conclusion, in the
electrolytic cell used for the extraction of aluminum, the gas that is unlikely
to be a waste gas is H2, or answer choice (C).
