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# Electrodes and voltage of Galvanic cell

Identifying the anode and cathode in a galvanic cell, and calculating the voltage using standard electrode potentials. Created by Sal Khan.

Video transcript

In the last video,
we talked about how we could create a voltaic cell
or galvanic cell or essentially a battery by separating the
oxidation and the reduction reactions and connecting
them with a wire, which forces the electrons
that zinc is losing to go through the wire
to go to the copper ions so that they can get them, so
the copper ions can be reduced. Now, that might have
raised some questions. If this is a battery, well,
what is the positive terminal? What is the negative terminal? If this is a battery, what is
the voltage of this battery? Well, I'll encourage
you, first of all, when you think about the
terminal, what's the positive and what's the
negative terminal, I encourage you to
pause this video and think about
that on your own. Where is the current coming
from and where is it going? Well, the negative
terminal of a battery is where the electrons
are coming from. So the electrons are coming from
the zinc bar right over here. So this is the
negative electrode of the battery that
right over there. Or the negative electrode
is often called the anode. That is the anode
of this battery. And on the other
side, the copper bar, this is where the
electrons are going to. This is the positive
electrode, or what tends to be called the cathode. Now, the next question is
what is the voltage across? This voltage is going
to depend on what's the concentration of
zinc ions you have, the concentration of
copper ions you have. It'll depend on the pressure. It'll depend on the temperature. But all of that has
been standardized, and you could actually
look up "standard electrode potentials." And I encourage you, you
could do a web search for "standard
electrode potentials," and you'll see a
bunch of voltages for the different ions,
which is essentially a measure of relative
to hydrogen-- and this is all
relative to hydrogen-- how much does this ion
want to grab its electrons? And so, if you were
to look that up, you would get it for this
reaction right over here. For the copper ions, with an
oxidation number of positive 2 to grab these two electrons
and turn it into solid copper, relative to what they call the
standard hydrogen electrode, that has a 0.34
voltage, which means that it's more likely to
happen than in the case with a standard
hydrogen electrode. And don't worry too
much about that. We're really just going
to compare the voltages and seeing, well, what is
the total electromotive force or the total voltage with
which this redox reaction is going to happen or the
total electromotive force with which
we're going to push these electrons across the wire. If you were to look
up the zinc reaction in a table of standard
electrode potentials, you might see negative 0.76. Now, you have to
be careful, though, because if they're
giving you this number, they're giving you the
opposite reaction here. They're giving the reaction
going from zinc ions, grabbing some electrons,
and becoming solid zinc. We want the other reaction. This is the reaction
that we need to occur for our galvanic cell. So this reaction
right over here is going to be the
negative of that. So it's going to be
positive 0.76 volts. So one way to think
about it is this wants to happen with
an electromotive force or the energy per Coulomb
relatively of 0.76 volts. This wants to happen with--
the electromotive force here is 0.34 volts. So combined, this
entire reaction is going to happen with
an electromotive force, or you could say that
the potential per Coulomb difference between
this side and that side is going to be the sum
of these two things. And so, if we have the
standard concentration, which would be one molar--
which is one mole per liter-- of the ions in their
aqueous solution, if we're at standard
temperature, standard pressure, then we would have essentially
constructed a 1.1-volt battery. And I've just added these
two things together. And of course, there's
other ways you could do it. You could literally just
take a voltmeter and measure what is the volt
without the current there, without the
current flowing? You could literally
just measure what is the voltage difference
between these two terminals, just as you would do in
a traditional battery.