Video Transcript
What is a possible similarity
between a hydrogen fuel cell and a mercury cell? (A) The mercuric oxide cathode. (B) The platinum anode. (C) The KOH electrolyte. (D) The potential difference under
standard conditions. (E) The reaction that takes place
at the cathode.
Galvanic cells are types of
electrochemical cells that produce electrical energy from chemical energy. They use redox reactions to
generate a flow of electrons, which powers electrical devices. There are two types of galvanic
cells. Primary galvanic cells are only
used one time before they must be discarded, while secondary galvanic cells can be
recharged and reused. This question is comparing two
well-known primary galvanic cells, the mercury cell and the hydrogen fuel cell. Let’s look at a mercury cell in
more detail.
Oxidation will occur at the anode,
which is highlighted in blue and contains zinc. Reduction will occur at the
cathode, which is highlighted in pink. The cathode contains mercuric
oxide. Electrons will flow from the anode
out the negative electrode through the external circuit and back into the cathode
through the positive electrode. The potassium hydroxide solution
between the anode and cathode functions as a salt bridge, allowing the circuit to be
completed by allowing the free flow of ions but keeping the chemicals at the
terminals separate.
Let us look at the other type of
primary galvanic cell, the hydrogen fuel cell. On the left side of the diagram, we
see hydrogen gas diffusing in. It then diffuses into the anode
with any excess hydrogen diffusing out. Electrons from the hydrogen gas
travel through the circuit, creating a current. The resulting hydrogen ions react
with the hydroxide ions from the potassium hydroxide electrolyte, forming water. On the other side of the diagram,
we see oxygen gas diffusing into the cathode. This combines with electrons and
water to form more hydroxide ions. Excess oxygen gas, water, and heat
generated leave the system.
Now that we have a basic
understanding of the way that these two cells work, we can look for any
similarities. Only mercury cells have a mercuric
oxide cathode, so answer (A) is incorrect.
Neither type of cell has a platinum
anode, so answer (B) is also incorrect. Both of our examples use a
potassium hydroxide electrolyte solution, so (C) could be the correct answer. Both cells use different redox
reactions, so their potential differences will be different. Answer (D) is therefore
incorrect. Since different redox reactions
occur in each cell, the reactions at the cathode will not be the same. Mercury cells reduce mercuric
oxide, and hydrogen fuel cells reduce oxygen gas. Answer (E) is not correct.
The KOH electrolyte is the correct
answer.
