Video Transcript
Which of the following products is
unlikely to be powered by a primary cell? (A) An electric car, (B) an alarm
clock, (C) an SLR camera, (D) a child’s toy, (E) a hearing aid.
Galvanic cells are types of
electrochemical cells that produce electrical energy from chemical energy. One common example is a
battery. Inside batteries, spontaneous
chemical reactions occur, which produce electricity. These redox reactions cause
electrons to flow through an external circuit and power our devices. There are two types of galvanic
cells. Primary galvanic cells are only
used one time before they must be discarded or recycled. Secondary galvanic cells can be
reused. The chemical reactions inside them
are reversible, and they can be recharged. We will look more closely at
primary galvanic cells.
Let us look at a mercury battery as
an example of a primary galvanic cell. The anode contains zinc, and this
is where oxidation takes place. The cathode contains mercuric
oxide, and this is where reduction takes place. Electrons will flow from the anode,
or the negative electrode, through the device and back to the positive cathode. The potassium hydroxide solution
between the anode and cathode functions as a salt bridge. It completes the circuit by
allowing charged ions through but keeps the chemicals separated.
Let us look at the chemical
reactions in the cell. We see zinc oxidized to zinc oxide
in the anode half equation. And we see mercuric oxide reduced
to liquid mercury in the cathode half equation. When writing the overall equation,
the water electrons and hydroxide ions will cancel out and we can omit them. As zinc oxide and liquid mercury
are produced, the battery becomes used up. And once there are no more
reactants, it cannot be used any longer.
In secondary galvanic cells, an
external current can be used to cause the redox reactions to run in reverse. This regenerates the reactants and
allows the cell to be reused. However, in primary galvanic cells,
the design of the cell and the chemicals involved do not allow an external current
to reset the redox processes. Since the chemical reactions in
primary galvanic cells are not reversible, primary galvanic cells are not
rechargeable.
Therefore, if we look at the list
of options from the question, we should be looking for the item that is most likely
to need to be recharged often. Option (A) should stand out the
most, as electric cars are famously rechargeable. The remaining answer choices all
rely on primary galvanic cells. Since they do not need to be
recharged, they are incorrect answers. The correct answer is an electric
car.
