- [Voiceover] Before we
get into electrolysis, let's review the structure
of a voltaic cell so we can compare it to
an electrolytic cell. So a voltaic cell uses a
spontaneous redox reaction to produce an electric current. So if we started our zinc electrode, solid zinc turns into zinc two plus ions. So solid zinc turns
into zinc two plus ions. An atom of zinc would
have to lose two electrons to turn into zinc two plus. So loss of electrons is oxidation. And oxidation is occurring
at our zinc electrode. So the zinc electrode must be our anode. Remember oxidation occurs at the anode. And you can remember that by an ox. The zinc electrode is
our source of electrons. So this is our negative electrode, and those electrons move in our wire to produce an electric current. When those two electrons
reach our copper electrode, we know we have copper
two plus ions in solution. So copper two plus ions in solution, and when those copper two plus ions gain those two electrons, copper two plus turns into solid copper. So copper forms at our
copper electrode here. So this is reduction. Gain of electrons is reduction. So reduction is occurring
at our copper electrode, making this the cathode. Remember red cat. Red cat is how to remember that one. So the copper electrode must
be our positive electrode. This reaction is spontaneous. The standard cell potential, E zero, is positive 1.10 volts. Remember a positive value
for your cell potential means a spontaneous reaction. So we produce an electric current. Let's compare that voltaic cell to an electrolytic cell, so on the right. An electrolytic cell
uses an electric current to drive a nonspontaneous redox reaction. So if we look at the
overall reaction here, we're starting with solid copper and zinc two plus ions in solution. And we're going to copper two plus ions in solution and solid zinc. So this is the reverse of the
reaction we just talked about. The reactants for this reaction were the products for this reaction. To find the standard cell potential, all we have to do is
take the negative of this cell potential, because we
just reversed the reaction. So if we reverse the reaction,
we just change the sign. So the cell potential, the
standard cell potential, would be negative 1.10 volts. A negative value for
your cell potential means a non-spontaneous redox reaction. So this doesn't occur on its own. Solid copper doesn't just
turn into copper two plus and zinc two plus doesn't
turn into solid zinc. It needs some help in order to do that. It needs an external voltage source, like a battery, to drive this reaction and force it to happen. So we need a battery here. Let's put in a battery
in our little circuit. So we have a battery. We need the negative terminal
to be on the left side and the positive terminal
to be on the right side. So this is our voltage source. And we need at least 1.10 volts to force this reaction to occur. And in practice, it turns out
to be more than 1.10 volts. So our negative terminal of the battery is where we get electrons. So electrons come out
of the negative terminal of the battery and deliver
electrons to the zinc electrode. So electrons are forced
onto the zinc electrodes. Let me draw on here two electrons. And now we have two zinc
plus ions in solution. Those zinc two plus ions have
an opportunity to be reduced. If those zinc two plus ions
gain those two electrons, zinc two plus gains two electrons and turns into solid zinc. So solid zinc forms. Solid zinc forms on our zinc electrode. So zinc two plus gaining two electrons to form solid zinc is reduction. Gain of electrons is reduction. So this time reduction is
occurring at our zinc electrode and reduction occurs at the cathode. So the zinc electrode is our cathode. So once again, red cat. The zinc electrode is more negative. So we've had electrons forced
onto the zinc electrode. That makes this our negative electrode. The battery is pulling electrons away from the copper electrode. So the copper electrode
is losing electrons. Electrons are going
toward the battery here. So electrons are going toward the positive terminal of the battery. That makes the copper
electrode relatively positive. So this is our positive electrode. And this is the site, this
must be the site of oxidation. So solid copper turns
into copper two plus ions. Solid copper turns into
copper two plus ions. So we have solid copper
turning into Cu two plus. And so we're losing two
electrons in order to do that. So solid copper turns
into copper two plus, we lose two electrons. Loss of electrons is oxidation. So oxidation occurs at our
copper electrode this time and that makes this the anode. Because oxidation occurs at the anode. So an ox. So the copper electrode
loses mass over time. So you can see the difference between a voltaic cell and an electrolytic cell. So a voltaic cell created a current, because we had a
spontaneous redox reaction. But for an electrolytic cell, we had a non-spontaneous redox reaction. So to get it to occur we needed a current. We needed an external voltage source. Let's point out that the
fact that the signs are, the signs of the electrodes are opposite for voltaic and electrolytic cells. For example, let me use a
different color over here, we can see that the anode was the zinc electrode and this was our negative electrode here. And that's because the zinc electrode was our source of electrons. So this makes that our negative electrode. But over here, the negative
electrode is our cathode. Because electrons are
forced on it by the battery. So the signs of the
electrodes are opposite for voltaic and electrolytic cells. Notice for an electrolytic cell, for an electrolytic cell
the negative electrode matches up with the negative
terminal of the battery. And the positive electrode matches up with the positive terminal of the battery. So that's a good way to remember which electrodes are which
in an electrolytic cell. Just look at the battery
and you can figure it out.