Video Transcript
In this video, weāll learn about
galvanic cells, simple devices used to generate electrical energy. Weāll learn how galvanic cells
work, how we can create galvanic cells with a variety of materials, and how we can
compare galvanic cells using standard electrode potentials.
Letās imagine we have a beaker that
contains a strip of zinc metal and a solution that contains zinc ions like zinc
nitrate. Metals have a tendency to lose
electrons and form ions. The extent that a metal will do
this depends on its reactivity. More reactive metals tend to lose
more electrons and form more ions. At the same time that the zinc
metal loses electrons and forms ions, zinc ions in the solution can accept electrons
to form zinc metal. This establishes an equilibrium
between the zinc metal, losing electrons to form ions, and zinc ions, accepting
electrons to form zinc metal.
But this effect isnāt unique to
zinc. The same thing would happen with
any metal such as copper. The copper metal can lose electrons
and form ions, and copper ions can accept the electrons to form copper metal, which
establishes an equilibrium. Zinc is more reactive than copper,
which means the zinc metal will tend to form more electrons and ions than the copper
metal will.
Both the container with the zinc
metal and the container with the copper metal are examples of half-cells. Half-cells are systems containing a
conductive electrode in an electrolyte solution. Electrodes are some kind of
conductive material thatās used to transfer charge. In our half-cells, the zinc metal
and copper metal are serving as the electrodes. Electrolyte solutions contain
ions. In these half-cells, the zinc
nitrate and copper nitrate solutions are the electrolyte solutions, as both zinc
nitrate and copper nitrate will dissociate into ions.
In both of these cells, we have
electrons that are being shed by the metal. When we have free electrons like
this, we can use them to power something like a phone or a remote. But we canāt do that with this
current setup because the electrons have nowhere to go. But we could if we added a few
things to our setup. The first thing we need to add is a
wire. The wire will allow electrons to
flow from one half-cell to the other. The other thing we need to add is a
salt bridge. A salt bridge completes the circuit
by allowing ions to travel from one side of the cell to the other, balancing the
charge.
Thereās different ways to construct
a salt bridge, depending on the setup of our cell, but the most common is the glass
U tube thatās filled with an ionic solution like potassium nitrate. Potassium nitrate is a good choice
here because potassium ions and nitrate ions are unlikely to form a precipitate with
the electrolyte solution in the half-cells. With these additions to our
half-cells, we now have a galvanic cell, a simple device thatās capable of
generating electricity. Galvanic cells are also referred to
as voltaic cells in honor of their inventor, Alessandro Volta. Letās take a closer look at how
galvanic cells work.
We mentioned earlier that zinc is
more reactive than copper. Because of this, the zinc metal
will tend to lose more electrons and form more ions than the copper metal will. Because oxidation is the loss of
electrons, we could also say that zinc has a higher potential to be oxidized than
copper does. This potential difference between
the zinc and copper half-cells is called a voltage. Itās also historically referred to
as the electromotive force. We can think of the voltage as the
force that pushes the electrons from one cell to the other. Itās the driving force for the
spontaneous chemical reaction occurring in the galvanic cell. We can measure the voltage of any
cell using a voltmeter. This would give us a reading in
volts.
Weāll look more in depth at voltage
readings in a moment. But for now, letās take a closer
look at the reactions and flow of charge in the cell. Because zinc has a higher oxidation
potential than copper, that means itās more likely to form ions and lose
electrons. The copper electrode will likely
oxidize to some extent also because of the equilibrium reaction we discussed
earlier. But since zinc has a higher
oxidation potential, there will be more electrons at the zinc electrode. These excess electrons from the
zinc electrode can travel through the wire where they can be used to power something
like a light bulb.
Now that the electrons have
traveled from the zinc electrode to the copper electrode, there are extra electrons
at the copper electrode. These extra electrons will attract
the positively charged copper ions in the electrolyte solution. The copper ions will be reduced to
form copper metal. Next, ions from the salt bridge can
travel to either of the half-cells to balance the charge and complete the
circuit. The negatively charged nitrate ions
in the salt bridge might travel to the zinc half-cell because there are extra
positively charged zinc ions being created there. And the positively charged
potassium ions might travel to the copper half-cell because the positively charged
copper ions are being used up here.
So galvanic cells are all powered
by spontaneous redox reactions. In galvanic cells, the electrode
with the higher oxidation potential is where oxidation occurs, and the electrode
with the lower oxidation potential is where reduction occurs. We call the electrode where
oxidation occurs the anode and the electrode where reduction occurs the cathode. The overall redox reaction for this
galvanic cell is zinc metal plus copper two plus ions reacting to form copper metal
and zinc two plus ions. The spontaneous redox reaction in
this cell will continue until our reactants, zinc metal and the copper two plus
ions, are used up.
Now itās inconvenient to draw an
entire galvanic cell every time we want to talk about one. To easily describe the setup of our
galvanic cell, we can use cell notation, which is sometimes also called cell
diagrams. To represent our galvanic cell in
cell notation, weāll place the anode information on the left and the cathode
information on the right. Weāll start off with a chemical
symbol for the metal electrode in the anode. Next, weāll place a vertical line
to indicate the change in phase between the metal electrode and the aqueous
electrolyte solution. Next, weāll place the chemical
symbol and charge of the ions at the anode. In parentheses will be the state
symbol. Weāll also sometimes include the
concentration or other conditions for the half-cell if they arenāt standard.
Now, weāll separate the anode
information and the cathode information with two vertical lines which represents the
salt bridge. Now weāll do the same thing for the
cathode information, but this time weāll start with the copper two plus ions in the
electrolyte solution. There will be another vertical line
for the phase boundary and the chemical symbol of the copper electrode. Youāll notice that spectator ions
were not included in our cell notation.
There are many different types of
half-cells that can be used to create galvanic cells. Both the zinc and copper half-cells
we looked at previously are examples of metalāion half-cells. We can also have half-cells that
involve a gas. In this half-cell, hydrogen gas is
oxidized to form hydrogen ions. Of course, weāll still need
something that can conduct electricity, so we have a platinum electrode in this
cell. Platinum is a good choice here
because itās inert, so it wonāt react with the hydrogen gas or hydrogen ions that
are in the cell. Weāll use these square brackets in
our cell notation to indicate that the hydrogen gas is being bubbled over the
platinum electrode.
A final type of cell we might
encounter is an ionāion cell. For example, in this cell, iron two
plus can be oxidized to form iron three plus. Again, weāre going to need
something that can serve as the electrode to conduct electricity in the cell. So we have the platinum
electrode. This is included in the cell
notation. We can also see that both iron two
plus and iron three plus are included on the same side of the phase boundary in the
cell notation because theyāre both ions. We can use any combination of
half-cells to create a galvanic cell. But how would we know ahead of time
which half-cell will be the anode and which will be the cathode?
As we know, different half-cells
have different oxidation potentials. The one with the higher oxidation
potential is the anode and the one with the lower oxidation potential is the
cathode. The potential difference between
these two cells is the voltage. We can measure the voltage using a
voltmeter, giving us a reading in volts. But the voltage is only telling us
the difference in potential between these two cells. The voltmeter reading doesnāt tell
us the potentials of our half-cells on their own. So if we want to know the potential
of each half-cell, weāll need to choose a standard half-cell to measure all of the
rest against. The electrode that was chosen to be
the standard is the hydrogen electrode. The standard hydrogen electrode is
often called the SHE for short.
To measure the standard electorate
potential of a cell, we simply set up a galvanic cell with the standard hydrogen
electrode and the cell weāre interested in measuring, for example, the copper
half-cell involving solid copper and copper two plus ions. Weāll connect those two cells with
a voltmeter. We want to make sure that weāre
measuring the electrode potentials under standard conditions, as different
conditions can affect the potential difference in the cell. Standard conditions are a
concentration of one molar, temperatures of 298 kelvin, and pressures of one
bar.
We also want to make sure weāre
using a high-resistance voltmeter. With a high-resistance voltmeter,
there will be a low flow of electrons between the two cells. We want to limit the flow of
electrons between the two half-cells so that the redox reaction doesnāt occur too
quickly. This is important because we want
to measure the maximum voltage reading for our galvanic cell and the voltage will
decrease as the reactants are used up.
With all this in mind, we can
determine the standard electrode potential for the copper half-cell by looking at
the voltage reading. The voltage reading for this setup
would be positive 0.34 volts. This positive voltage reading
indicates that the potential for hydrogen to be oxidized is greater than the
potential for copper to be oxidized. In other words, hydrogen gas will
be oxidized, making it the anode, and the copper two plus ions will be reduced,
making this the cathode.
We could do the same thing to
measure the standard electrode potential for the zinc half-cell. This would give us a voltage
reading of negative 0.76 volts. This negative voltage reading
indicates that the potential for hydrogen to be oxidized is less than the potential
for zinc to be oxidized. In other words, in this galvanic
cell, the hydrogen ions will be reduced, making the standard hydrogen electrode the
cathode this time. Meanwhile, zinc will be oxidized,
making this half-cell the anode.
If we compile a list of all the
standard electrode potentials that we can measure, weāll end up with the
electrochemical series. Here is an abbreviated
electrochemical series involving the half-cells that we looked at in this video. A more negative standard electrode
potential indicates a greater oxidation potential. In other words, if we pair up any
two half-cells such as copper and zinc, we can use the standard electrode potentials
to determine which half-cell will be the anode and which will be the cathode.
The standard electric potential of
zinc is more negative than the standard electrode potential of copper, meaning that
zinc has a greater oxidation potential than copper does. So if we were to set up a galvanic
cell using zinc and copper half-cells, zinc would be oxidized, making it the anode,
and copper would be reduced, making it the cathode.
Now weāve covered everything we
need to know about galvanic cells, so letās summarize what we learned. A half-cell is a system composed of
a conductive electrode in an electrolyte solution. A galvanic cell is composed of two
half-cells connected by a wire and a salt bridge. The potential difference between
the two half-cells drives a spontaneous redox reaction in the galvanic cell. The potential difference is also
called a voltage. It can be measured using a
voltmeter. The standard hydrogen electrode is
used to measure standard electrode potentials for each half-cell.