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Course: AP®︎/College Chemistry > Unit 9
Lesson 7: Galvanic (voltaic) and electrolytic cellsElectrolytic cells
Electrolytic cells use an electric current to drive a thermodynamically unfavored redox reaction. As in galvanic cells, oxidation occurs at the anode and reduction occurs at the cathode. An application of electrolysis is the electroplating of metals, which uses an electric current to deposit a thin layer of metal onto the surface of another material. Created by Jay.
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How does the power source start the reaction? Since the wire doesn't go in a loop, how can there be a current? Is it because the reactions in the water act to finish the loop?(1 vote)
Well the power source is essentially just an energy storage device which is always ready to provide a voltage if it is connected in a circuit. In this electrolytic cell the two electrodes are submerged in the same solution. The electrons are able to move from one electrode through the solution to the other electrode. Having an electrolytic cell in the same solution accomplishes the same goal as the salt bridge does in a voltaic cell; they both complete the circuit.
Hope that helps.(3 votes)
If I join three 1v galvanic cells in series I get 3v total, does it work in reverse? i.e. if I have three identical electrolytic cells in series and pass 3v through them do I have 1v passing through each of the cells?(1 vote)- Electrolytic cells need an input of voltage to be able to function, so they would essentially behave as three resistors in a circuit. If they are identical, then they would cause the same drop in voltage as the current moved through the circuit. Assuming we have a voltage of 3V with three resistors in series, then yes, each electrolytic cell would cause a 1V drop each.
Hope that helps.(1 vote)
Does the solution surrounding the reaction have any impact on the electroplating?(1 vote)- Yes, we have to take the solution itself into account too in electrolytic cells. The solutions are usually either molten salt or aqueous.
If the solution is a molten salt with only a single salt present, like in the sodium chloride example, then the reaction is straightforward. The cations get reduced into neutral chemicals like Na^(+)(l) + e- → Na (s). And the anions get oxidized into their neutral chemicals like 2Cl^(-)(l) → Cl2(g) + 2e-.
If the solution is a molten salt with multiple salts present, then there are different possibilities as to which cation gets reduced and which anion gets oxidized. For example, if we have a molten salt mixture of sodium chloride and potassium chloride, the chloride will still be oxidized since it’s the only anion, but either the sodium or potassium cations have the possibility of reducing. The cation which is more easily reduced, the one with the more positive reduction potential, is reduced. Which in this case is the sodium cation which turns into solid sodium, and the liquid potassium remains unreacted. The same idea applies to anions, the anion which is more easily oxidized, the one with the more negative reduction potential, is oxidized first.
Electrolytic cells in aqueous solutions become problematic because of the possibility of oxidizing or reducing water instead of the target ions. The same logic applies here as it does in molten salt mixture cells though. For the oxidation whichever is more easily oxidized, the anion or water, will be oxidized first. For reduction whichever is more easily reduced, the cations or water, will be reduced first. If water is oxidized, then oxygen gas is produced instead of the neutral form the anion, and if water is reduced then hydrogen gas forms instead of the neutral form of the cation.
Hope that helps.(1 vote)
Video transcript
- [Instructor] Electrolytic cells use an electric current to drive a thermodynamically unfavorable reaction. Before we look at a diagram
of electrolytic cell, let's look at the half reactions that will occur in the cell. In one half reaction, liquid sodium ions react with an electron to
form liquid sodium metal. Because the liquid sodium
ion is gaining an electron, this represents the
reduction half reaction. The other way to tell
that this is a reduction half reaction is to
assign oxidation numbers. Liquid sodium ions have
an oxidation number of plus one and liquid sodium metal has an oxidation number of zero. So going from plus one to zero is a decrease or a reduction
in the oxidation number. Therefore, liquid sodium ions are reduced to form liquid sodium metal
in this half reaction. In our next unbalanced half reaction, liquid chloride anions
turn into chlorine gas. If we assign oxidation numbers, liquid chloride anions have a
oxidation number of minus one, and chlorine gas has an
oxidation number of zero. So going from minus one to zero is an increase in the oxidation number. Therefore, the liquid chloride anions are oxidized to chlorine gas. So this is the oxidation half reaction, and we need to balance it. Since we have two chlorines on the right, we need to put two as a coefficient in front of the chloride
anions on the left. Loss of electrons is oxidation. And since we're oxidizing
two chloride anions, we're going to lose two electrons. As a quick reminder, one way to remember that loss of electrons is oxidation and gain of electrons is reduction is to think about LEO the lion goes GER. So loss of electrons is oxidation and gain of electrons is reduction. Next let's add our two
half reactions together to get the overall redox reaction. And before we do that, we have to make sure
the number of electrons are equal in both half reactions. Since we have two electrons in the oxidation half reaction and only one electron in
the reduction half reaction, we need to multiply everything in our reduction half
reaction through by two. So that'd be two liquid sodium ions, two electrons, and two liquid sodiums. When we add the two
half reactions together, the two electrons cancel out and we get two liquid sodium cations plus two liquid chloride anions goes to two liquid
sodiums and chlorine gas. So if we had some molten sodium chloride, we could form sodium and chlorine gas. However, delta G naught for this reaction is greater than zero, which means this reaction is thermodynamically unfavorable. So in order for this reaction to occur, we need some sort of a power source to provide an electric current, to drive this thermodynamically
unfavorable reaction. Here's a diagram showing
our electrolytic cell for the electrolysis of
molten sodium chloride, and let's start with the power source. So the negative terminal
of the power source is where the electrons come from. So imagine we have electrons
moving in this wire toward the inert electrode on the right. Inert means the electrodes aren't going to participate
in this reaction. For example, the electrode could be a piece of platinum metal, which is very unreactive. At the surface of the electrode, the electrons reduce
the liquid sodium ions into liquid sodium. Therefore, liquid sodium
will form at this electrode. The melting point of sodium chloride is higher than the
melting point of sodium. Therefore, at this high temperature sodium will remain a liquid. Eventually when the
liquid sodium cools down, you would have some solid sodium metal because reduction is occurring at the electrode on the right, the electrode on the
right must be the cathode. Next, let's think about the other inert electrode on the left. At this electrode,
oxidation is taking place because liquid chloride anions are turning into chlorine gas. So we would see bubbles of chlorine gas at this electrode. And as the liquid chloride
anions are oxidized, that's loss of electrons. So the electrons would
flow through this wire back toward the positive
terminal of the power source. Since oxidation is occurring at the inert electrode on the left, this electrode must be the anode. As a quick review,
remember that a good way to remember this is to
think about an ox and a cat. So oxidation occurs at the anode. That's an ox. And reduction occurs at the cathode. That's a red cat. And as a quick summary, this diagram shows the electrolytic cell for the electrolysis of
molten sodium chloride to form liquid sodium and chlorine gas. Let's look at another example
of an electrolytic cell. This electrolytic cell shows the process of electroplating. So let's say you have
a pendant made of steel and you do some engraving on it. I do a lot of engraving. And so I drew a little picture of some flowers on this steel pendant, however, steel rusts pretty easily. So if we want to protect the engraving, we could electroplate it and put a thin layer of
nickel on top of the steel. So electroplating uses electrolysis to plate one metal onto another metal, either to protect against
rust or corrosion, or if you want to plate silver or gold onto another metal to
make it more beautiful. So for this electrolytic cell, we have a steel electrode on the right and a nickel electrode on the left. And we have an aqueous
solution of nickel sulfate. We know that electrons come from the negative terminal
of the power source. So imagine that electrons move in the wire toward the steel electrode on the right. When the nickel two
plus ion is in solution come in contact with the electrons at the surface of the steel electrode, reduction takes place. So reduction takes place at the cathode and for the half reaction, it'd be nickel two
plus, plus two electrons turns into solid nickel. So a thin layer of nickel is now plated on the steel object. The electrode on the left
is a piece of solid nickel. And when solid nickel is oxidized, it turns into nickel two plus ions. When solid nickel turns into nickel two plus ions in solution, two electrons are lost. And so we can think about electrons moving in this wire back toward the positive terminal of the power source. So for the oxidation half reaction, solid nickel turns into
nickel two plus ions, and two electrons are lost. And because oxidation occurs at the anode, the nickel electrode is the anode for this electrolytic cell. Let's look at a quick summary
for an electrolytic cell. In an electrolytic cell, the redox reaction is
thermodynamically unfavorable. Because the redox reaction is thermodynamically unfavorable, an electrolytic cell requires a power source to supply a current to drive the unfavorable redox reaction. Oxidation occurs at the anode and reduction occurs at the cathode. And finally, an electrolytic cell is used for electroplating where a thin layer of one metal is plated onto another metal.