Video Transcript
Which of the following reactions is
the half-equation for the reaction that occurs at the cathode in a mercury cell.
To answer this question, we need to
remember that the cathode is the site of reduction in the galvanic cell. In a galvanic cell, oxidation
happens at the anode. In a reduction reaction, electrons
are gained. So we’re looking for a
half-equation where the species on the left, that is, the reactants, gain
electrons. The electrons have to appear on the
left side of our half-equations.
We can rule out answer (C) as the
electrons are being lost in this half-equation. This half-equation represents an
oxidation process. We can also rule out answer (D), as
this is not a half-equation; it’s a full redox equation. In (D), zinc loses electrons, its
oxidation state increases, and it’s being oxidized. Hg or mercury is being reduced. It gains electrons as its oxidation
state decreases. For the same reason that we ruled
out answer (C), we can also rule out answer (E). Although it appears to be a
half-equation, we can see that the electrons appear on the right-hand side. They’re being lost, and it’s an
oxidation reaction. Remember, we’re looking for a
reduction process.
On closer inspection of answer (B),
we can see that there appears to be no redox processes taking place at all. The oxidation state of mercury
remains at plus two throughout the equation. Since we’re looking for a reduction
reaction, it can’t be the right answer. Mercury cells contain mercury two
oxide or HgO at the cathode. In answer (A), we see mercury two
oxide gaining electrons. Mercury two oxide is the correct
species for the cathode in our cell, and it’s involved in a reduction reaction
here. This is the correct answer.
