Video Transcript
In this lesson, we will learn how
to describe conditions and applications for the electrolysis of molten salts and
salt solutions. The electrolysis of a blue solution
of copper sulfate is a very simple experiment to set up in a school or college
lab. All that is needed is a solution of
copper sulfate in a glass beaker. A source of dc electricity, such as
a battery or dc power supply, is also required. Some conductive electrodes, usually
graphite electrodes, are used. These are necessary to make an
electrical contact with the solution. Finally, some wires are needed to
connect the circuit together. A light bulb could be added to the
circuit to indicate when current is flowing.
This solution conducts electricity
because it contains dissolved ions that behave as mobile charge carriers. When copper sulfate dissolves in
water, two types of ion are released from this solid ionic compound. Positively charged copper ions or
cations are present in the solution. Negatively charged sulfate anions
are also present. From the water, there are a few
hydrogen ions which are positively charged and also a few hydroxide ions which are
negatively charged. So there are, in fact, four types
of ion present in the solution. The electrodes make an electrical
contact with the solution because the graphite that they’re made from conducts
electricity. Graphite is a form of carbon that
contains mobile electrons within its structure.
Remember that graphite can also be
used for high-temperature electrolysis of molten ionic compounds due to its very
high melting point. The electrolysis experiment
described here is carried out on a solution at room temperature. The battery or power supply
provides a direct current, also known as dc. In a direct current circuit, the
current only flows in one direction around the entire circuit. The wires complete the connection
to the battery to the circuit. We have built an electrolytic cell
here, which is a circuit where electricity is used to drive a reaction forwards, one
that would not otherwise occur spontaneously or all by itself.
The electrolytic cell is used to
carry out the electrolysis. Remember that electrolysis is the
decomposition of a compound using direct current electricity. When the current is flowing, some
changes will be observed after a short time in the glass beaker and particularly at
the electrodes. At the negative electrode, also
known as the cathode, the grayish-to-black graphite will become a pink-to-orange
color as copper metal is deposited here.
This electrode would in fact gain
mass as copper atoms are coating the surface of the electrode. These copper atoms originated as
copper ions in the copper sulfate solution. Copper is deposited at the negative
electrode or cathode instead of hydrogen gas being formed as it’s less reactive than
hydrogen. Remember that hydrogen ions were
the other positive ions in the solution that could’ve arrived at the negative
electrode to gain electrons and become hydrogen gas.
At the positive electrode or anode,
bubbles of a gas will be seen. The gas is in fact oxygen, which
originated from the hydroxide ions in the water. These products are formed as a
direct result of the electrolysis process that is taking place. This process would carry on for as
long as current is flowing and providing there are still ions remaining in the
solution. If we make a small change to the
experiment by changing the graphite electrodes for metallic copper electrodes, the
results will be a little different. We will now have a copper cathode
and a copper anode. The ions in the solution are the
same as before, but some slightly different processes will be observed at each
electrode.
The main difference is that there
will be no gas formed at the anode or positive electrode. The negative sulfate ions and
hydroxide ions will be attracted to the positive electrode, but they will not
change. They will remain in solution. Instead, metallic copper atoms from
the anode will lose electrons and become positive copper cations in the
solution. This process is known as
oxidation. The oxidation process can be shown
using the half equation, Cu turns into Cu2+ plus 2e−. The positively charged copper ions
and hydrogen ions in the solution will be attracted to the negative electrode or
cathode, which is also made from copper. Here, the copper ions only will
gain electrons and become copper atoms.
These copper ions are getting
reduced at the cathode. This process can be represented by
the half equation, Cu2+ plus 2e− makes Cu. Fresh copper atoms are therefore
deposited at the copper cathode. If we checked the mass of the
copper anode before and after the experiment, we would find that it has lost
mass. This makes sense as the copper ions
created here have migrated through the solution to the copper cathode. The electrons left behind at the
copper anode flow around the external circuit to the copper cathode. The copper cathode will have gained
mass here as fresh copper atoms are deposited. The concentration of copper ions in
the original copper sulfate solution will not change much at all. Since all we’re doing here is
effectively transferring copper atoms from the anode to the cathode, this experiment
can be used as a way of purifying impure copper metal.
Copper needs to be very pure for
applications such as microelectronic circuits. This is because high-purity copper
is a very good conductor of electricity. If we take a lump of impure copper
produced by smelting copper ore and make this the anode in this experiment, only the
copper atoms from this piece of copper will enter the solution as copper ions. At the anodes, the copper atoms in
the impure copper leave their electrons behind and enter the solution as copper two
plus ions. This process is oxidation. These copper ions from the impure
copper will be attracted to the cathode, where they will gain two electrons and
become copper atoms. They will become pure copper.
At the cathode in this cell,
reduction is taking place. Impurities in the impure copper
anode will fall to the bottom of the electrolysis cell tank. This anode sludge, as it is known,
can be further processed to recover other precious metals, such as nickel and
silver, which were present in trace amounts in the impure copper. So the impure copper anode will
shrink or disappear. It is losing mass as copper ions
are formed here, and they enter the solution. The pure copper cathode will grow
with fresh copper as the copper ions arrive here and become copper atoms. It will gain mass. The pure copper can be scraped off
the cathode and used in high-purity copper wiring and microelectronics.
We have just seen that a copper
cathode can become covered in fresh copper atoms in an electrolytic cell, provided
that there’s a source of copper ions and copper ions exist in the solution. It is possible to coat any metallic
object that’s placed at the cathode with fresh copper in this situation. In fact, if the solution contained
ions from a different metal, they could instead be deposited onto the metallic
cathode. This would only work if the anode
were made from the same metal as the ions in the solution. This is the basis of
electroplating, where a layer of metal is deposited onto a different metal surface,
that’s the cathode, in an electrolytic cell.
It may be desirable to plate a
layer of a more valuable metal onto a less valuable metal to make the object appear
that it’s made from the attractive precious metal. Silver or gold may be plated onto
less precious metals for this purpose. This has even been carried out by
fraudsters in the past to make a cheap piece of lead look like a piece of solid
gold. Sometimes the metal plating is less
reactive than the solid metal underneath. And the plated layer protects the
bulk metal from oxidation or corrosion. This is certainly the case with
silver or gold plating. Often, chromium or nickel are
plated onto steel to prevent rusting and make it look more attractive. This is seen as chrome plating on
vintage cars. The steel is protected as long as
the chrome plating is not severely scratched through.
As another example, if a brass fork
were to be electroplated with silver, the fork would need to be placed at the
cathode in the electrolytic cell. The electrolyte solution would need
to contain dissolved silver ions from a soluble silver salt, such as silver
nitrate. The anode would be made from pure
silver. As dc electricity is passed through
the cell, silver atoms at the anode would lose electrons and become silver ions in
the solution. Oxidation is occurring at the
silver anode. The silver ions would be attracted
to the brass cathode, where they will gain electrons and become silver atoms. Reduction is taking place at this
cathode.
The silver atoms build upon the
brass fork, and it becomes electroplated with silver metal. The more current that passes
through the circuit, the more silver will be deposited. It can take quite some time to get
a reasonably thick layer with an attractive finish. Although the fork will not have the
same density as pure solid silver, it will look like pure silver. And the silver will protect the
underlying metal from oxidation or corrosion. We will now look at a question to
test your understanding of electroplating.
A student electroplates a key with
copper. What aqueous solution and electrode
would be the best choices for this experiment? (A) H2SO4 aqueous and a graphite
electrode. (B) H2SO4 aqueous and a platinum
electrode. (C) NaOH aqueous and a copper
electrode. (D) CuSO4 aqueous and a graphite
electrode. (E) CuSO4 aqueous and a copper
electrode.
Electroplating involves the use of
an electrolytic cell to deposit a thin layer of metal onto another metal
surface. An electrolytic cell needs a power
supply. In this experiment, a simple cell
is used to provide the direct current electricity. Our simple cell has a positive
terminal and a negative terminal. The positive terminal is called the
anode. And at the anode, we would expect
to find a source of the metal that is going to provide the plating.
In this scenario, the metal that is
providing the plating is copper. So in this experiment, the anode
needs to be an electrode made from a piece of pure copper. At this electrode, copper two plus
ions will be produced. Copper atoms from the anode will
leave two electrons behind on the anode, and they will enter the solution as copper
two plus ions. The anode is therefore the site of
oxidation here, and the electrons will flow from the anode to the cathode, which is
the negative electrode.
In order to plate the key, which is
placed at the cathode in this circuit, with copper, we need copper ions to move
through the solution and become copper atoms again at the cathode. Since we need a copper electrode to
maintain the concentration of copper ions in the solution, we can reject any answers
that suggest any other electrode should be used. Answers (A), (B), and (D) are
therefore not correct. Graphite is a form of carbon, and
it won’t provide any copper ions at all. Platinum is a completely inert
metal, and it won’t provide ions either.
The aqueous solution in the
electrolytic cell must contain dissolved copper ions. These copper two plus ions will
move to the cathode where they will gain two electrons and become copper atoms
again, plating the key. This is the site of reduction. Since the solution must contain
copper two plus ions, we can reject answer (C). In answer (C), we see a solution of
sodium hydroxide. Aqueous sodium hydroxide will
contain aqueous sodium ions and aqueous hydroxide ions. These can be written as Na+ (aq)
and OH− (aq). Aqueous sodium hydroxide will not
yield any copper two plus ions, so it’s not the right answer here. Aqueous copper sulfate or CuSO4
(aq) does yield aqueous copper two plus ions. Answer (E) is the correct answer as
we have the correct solution and the correct electrode.
Let us now review the key points
from this lesson. An electrolytic cell uses an
external direct current power supply to drive a chemical reaction. Impure copper can be purified in an
electrolytic cell by placing it at the anode in a solution of copper sulfate. Electroplating involves depositing
a layer of a metal onto another metal surface using an electrolytic cell. The metal to be plated is placed at
the cathode in the electrolytic cell. Electroplating is done to protect
metals from corrosion or to improve their appearance.
