Electrolytic cells | Applications of thermodynamics | AP Chemistry | Khan Academy

Electrolytic cells use an electric current to drive a thermodynamically unfavorable reaction. Before we look at a diagram of an 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 an 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 a 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. Before we do that, we have to make sure the number of electrons is 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 would 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, going 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. Let's start with the power source. 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. 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." Oxidation occurs at the anode—that's an ox—and reduction occurs at the cathode—that's a red cat.

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. 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.

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 ions 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. For the half reaction, it'd be nickel two plus plus two electrons turning 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. 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. 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. 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. Finally, an electrolytic cell is used for electroplating, where a thin layer of one metal is plated onto another metal.