Video Transcript
In the electrolysis of molten lead
bromide, shown in the picture, which electrode is on the right? (A) The cathode, as it is oxidizing
the bromine. (B) The cathode, as it is reducing
the bromide ions. (C) The anode, as it is reducing
the bromide ions. (D) The anode, as it is oxidizing
the bromide ions. Or (E) the anode, as it is
oxidizing the bromine.
This picture depicts the
electrolysis of molten lead bromide with a brown gas wafting up from one of the
electrodes. This brown gas is bromine gas. This question is asking us to
describe how bromine gas forms and at which electrode. Each choice has three parts,
meaning there’s three pieces of information we need to gather in order to answer the
question. Is bromine gas produced at the
anode or the cathode? When bromine gas is produced, does
it involve an oxidation or a reduction? And does that oxidation or
reduction happen to bromide ions or bromine atoms?
Let’s start with the third piece of
information. Does this process involve bromide
ions or bromine atoms? Molten lead bromide is made up of
free-floating lead ions and bromide ions. Electrolysis requires an
electrolyte. An electrolyte is a substance made
up of ions or able to release ions that can conduct electricity. When the circuit is turned on, the
bromide ions are drawn to one of the electrodes to begin the formation of bromine
gas. Since the reactants in this process
are bromide ions and not bromine atoms, we can eliminate choice (A) and choice (E)
from consideration.
Next, let’s consider the name of
the electrode. The negatively charged bromide ions
are known as anions. This is in contrast to the
positively charged lead cations. Which electrode, the anode or the
cathode, are anions drawn to? Well, we can remember that their
names correspond with one another. Anions are drawn to the anode, and
cations are drawn to the cathode. In this example, bromide anions are
drawn to the anode, so we can eliminate choice (B) from consideration as well.
The last question to consider is
whether these bromide ions are being oxidized or reduced at the anode. Let’s take a look at what happens
to the bromide ions at the anode in order to answer this question. First, each bromide ion donates an
electron to the anode. Then, the ions that have donated
their extra electrons can pair up to form a bromine gas molecule. The half reaction for this process
is 2Br- ions produce Br2 and two electrons. With this process in mind, are the
bromide ions being oxidized or reduced?
In this situation, since the
bromide ions are giving up or losing their electrons, it’s an oxidation. On the other hand, a reduction is
when an atom or ion gains electrons. For example, at the cathode in this
experiment, the lead ions will gain electrons or be reduced to form elemental
lead. Since the bromide ions are losing
electrons or being oxidized, we can identify (D) as the correct answer. The electrolysis of molten lead
bromide produces the brown bromine gas as a product. The bromide ions are attracted to
the anode, where they donate electrons to form bromine gas. Since the bromide ions lose
electrons, we also call this process an oxidation.
So in the electrolysis of molten
lead bromide, shown in the picture, which electrode is on the right? That’s choice (D), the anode, as it
is oxidizing the bromide ions.
