Video Transcript
In this video, we will learn how to
describe the components of electrolytic cells and predict the products of the
electrolysis of molten salts.
This video is about the
electrolysis of molten salts. The electro- means we’re using
electricity, and -lysis means separation. What are we separating? Well, molten means that something
has been heated up to its liquid state. And a salt is another word for an
ionic compound made up of positive and negative ions. During electrolysis, we will use
electricity to separate these two types of ions.
Let’s take a look at the
electrolysis of lead(II) bromide as an example. In its solid state, the ions in
lead bromide are bonded to one another and cannot float freely. If we heat it up so that the solid
reaches the molten or liquid phase, the ions will no longer be bonded to one another
and instead float freely in the liquid. In fact, a sample of molten liquid
lead(II) bromide contains only lead ions and bromide ions floating around to make up
the liquid.
When we heat lead(II) bromine till
liquid phase, it becomes an electrolyte. An electrolyte is a substance made
of ions that can conduct electricity. Electricity is the flow of charged
particles. So liquid lead(II) bromide, with
its free-floating ions, can conduct electricity, while solid lead(II) bromide, with
its bonded ions, cannot. We could also turn solid lead(II)
bromide into an electrolyte by adding water to create an aqueous solution. We will learn about the
electrolysis of aqueous salt solutions in another video.
The electricity in electrolysis
comes from a battery or another similar power source. The battery is connected by wires
to two electrodes, usually made out of an inert substance like platinum or
carbon. With an inert electrode, electrons
can pass through the electrode without the atoms of the electrode taking part in the
reaction. We will sometimes see a battery
diagrammed like this, as two parallel lines connected to the wires. The longer parallel line represents
the battery’s positive terminal, and the shorter parallel line represents the
battery’s negative terminal.
During electrolysis, when the
circuit is turned on, the negative ions, also known as anions, move to the positive
electrode. The positive ions, known as the
cations, move to the negatively charged electrode. In fact, the names of the
electrodes and the ions that are attracted to them correspond to one another. The anions are attracted to the
anode, and the cations are attracted to the cathode. Soon after turning on the circuit,
we will see a brown gas bubble up at the anode and waft up into the air. In addition, we will see a liquid
silver-colored metal bead form beneath the cathode.
What is going on to produce these
two substances? Well, the bromide ions gather at
the anode. At the anode, each bromide ion
donates an electron. A pair of bromide ions can donate
their electrons and then come together to form a bromine gas molecule. That brownish gas that we see is
bromine gas. The half reaction for this process
is two bromide ions produce a Br2 gas molecule and two electrons. The electrons donated by the
bromide ions flow through the wire to the cathode.
The lead ions gather around the
cathode. There, each lead ion is able to
accept two electrons, turning into a solid lead atom, which then sinks to the bottom
of the vessel, forming this silver-colored liquid bead. The bead is made of molten metal
lead. The half reaction for this process
is a lead ion plus two electrons produces lead.
Overall, this is the process of
electrolysis. Electricity separates the positive
ions and the negative ions, which then form new products at the electrodes. We sometimes refer to electrolysis
as the decomposition of an electrolyte using electricity. To put this in simpler terms, it
means the breaking down of a substance that conducts electricity. In our example, we broke down lead
bromide to form lead and bromine gas. If we had instead electrolyzed
aluminum oxide, the products would be aluminum and oxygen gas.
We can extend this pattern and say
that for the electrolysis of molten salts, the products will be the elemental form
of the cation and the anion. The cation will tend to form a
metal, and the anion will tend to form a gas. Earlier, we wrote down the chemical
equations that show what is happening at each electrode. At the cathode, a lead two plus ion
combines with two electrons to form lead. At the anode, two bromide ions form
Br2 gas and two electrons. These chemical equations might look
a little different than the ones we’re used to seeing because they are half
reactions. Half reactions show the formation
of one product and the electrons involved.
There are two main categories of
half reactions. Either the atoms or ions gain
electrons or the atoms or ions lose electrons. For the reaction on the left, the
lead ion gains electrons to form lead. In the reaction on the right, each
bromide ion loses an electron during the formation of bromine gas. When the atom or ion gains
electrons, we call this a reduction. In this case, the lead ion is being
reduced. On the other hand, when an atom or
ion loses electrons, we call this an oxidation. The bromide ions are being oxidized
to form bromine gas.
Because the full reaction here is
the combination of a reduction half reaction and an oxidation half reaction, we call
this full reaction an oxidation–reduction reaction. While this oxidation–reduction
reaction describes the entire electrolysis, each half reaction gives us some extra
information about what’s happening to the electrons at each electrode.
During electrolysis, electrons flow
through the wire from the anode to the cathode, where they are donated to ions. So the reduction half reaction,
where an ion gains electrons, will occur at the cathode. Conversely, since ions give up
their electrons at the anode, the anode is where the oxidation reaction takes
place. If we have trouble remembering the
definitions of oxidation and reduction, there’s a handy acronym we can use, OIL
RIG. Oxidation involves a loss of
electrons. Reduction involves a gain of
electrons. Another acronym we can use is “Leo
the lion says GER.” Lose, electrons, oxidation. Gain, electrons, reduction.
As a summary, during electrolysis,
one ion will gain electrons and be reduced at the cathode. The other ion will lose electrons
and be oxidized at the anode. Now that we’ve learned about the
electrolysis of molten salts, let’s do some practice problems to review.
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.
Barium metal can be obtained
through electrolysis of its molten salt. Which of the following equations
shows the reaction occurring at the negative electrode? (A) Barium plus two electrons
produces barium two plus. (B) Barium produces a barium two
plus ion plus two electrons. (C) A barium two plus ion plus two
electrons produces barium. (D) Barium two plus ion plus two
electrons produces two barium atoms. Or (E) a barium two plus ion
produces barium and two electrons.
This question is asking about the
electrolysis of a molten salt. This process occurs when we dip two
electrodes connected to a battery into the liquid form of a salt. The liquid salt is made of
free-floating positive and negative ions. In this question, the positive ions
or cations will be barium ions. The identity of the negative ions
isn’t important to answering the question, so let’s just use chloride as an
example. When the circuit is turned on, the
ions will be attracted to the electrodes of the opposite charge. When the ions reach each electrode,
a reaction will occur.
This question is asking, what
happens to the barium ions on the surface of the electrode shown here on the
left? Each choice is essentially the same
three pieces of information rearranged. In order to answer this question,
we need to ask, when are barium ions present? When are barium atoms present? And are electrons absorbed or
released during this process?
Let’s look at the wording of the
question for some clues about what’s going on. As we’ve just explained, a molten
salt involves ions. And the electrolysis of a molten
salt starts with ions. Similarly, if we are trying to
obtain barium metal, that must mean that the atomic form of barium is a product of
the reaction. That means we can eliminate choice
(A) and (B) from consideration. We want the ion to be on the left
side of the equation as a reactant and the atom to be on the right side of the
equation as a product. We know that barium metal will form
as a product of the reaction. It will do so by plating on the
electrode.
Our next question is, what is going
on with the electrons to make this happen? In electrolysis, electrons flow
from the anode to the cathode. They are taken from the ions at the
anode and given to the ions at the cathode. In our example here, the electrons
at the cathode are donated to the barium two plus ions. The two plus charge of the barium
ion and the combined two minus charge of the two electrons balance out. As a result, barium metal atoms
form. We can simplify this process by
saying that the barium ion has gained electrons to form the atom. Choice (E), where electrons are a
product of the reaction, is the opposite situation, where electrons are released
from the ion. We want the electrons to combine
with the ion to form the atom like they do in choice (C) and (D).
The last thing we need to consider
is whether the combination of a barium ion and two electrons would produce one
barium atom or two barium atoms. Simply put, the correct answer is
choice (C). One ion combines with electrons to
form one atom. This reaction describes what occurs
at the negative electrode when we electrolyze a molten salt containing barium. The barium ion will gain two
electrons to form a barium metal atom. When an atom or ion gains
electrons, we call that a reduction.
Electrolysis is one way to isolate
pure metals. In fact, barium was first isolated
by British chemist Sir Humphry Davy in 1808 when he electrolyzed molten barium
oxide. So when we obtain barium metal
through electrolysis of its molten salt, the equation that shows the reaction
occurring at the negative electrode is choice (C), a barium ion plus two electrons
produces a barium atom.
Now that we’ve done some practice
problems, let’s review the key points of the lesson. Electrolysis is essentially the
electrical separation of ions. And a molten salt is the liquid
form of an ionic compound. A molten salt is comprised entirely
of free-floating, positive and negative ions. The products of the electrolysis of
molten salt will be the elemental forms of the ions present. Typically, the cation will form a
metal, and the anion will form a gas.
When the circuit is turned on, the
negative anions will flow to the electrode known as the anode, where they will give
up their electrons. When an atom or ion loses
electrons, we call this oxidation. Conversely, the positively charged
cations will flow to the cathode, where they gain electrons. The process of gaining electrons is
known as a reduction. Lastly, half reactions show the
formation of one product and the electrons involved. For example, the half reaction that
shows the formation of lead is a lead two plus ion plus two electrons forms
lead.
