Lesson Video: Galvanic Cells | Nagwa Lesson Video: Galvanic Cells | Nagwa

Lesson Video: Galvanic Cells Chemistry

In this video, we will learn how to describe a galvanic cell and explain how 𝐸^⦵ values are measured.

12:35

Video Transcript

In this video, we’ll learn about galvanic cells, simple devices used to generate electrical energy. We’ll 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.

Let’s 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 isn’t 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 that’s 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 can’t 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.

There’s different ways to construct a salt bridge, depending on the setup of our cell, but the most common is the glass U tube that’s 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 that’s capable of generating electricity. Galvanic cells are also referred to as voltaic cells in honor of their inventor, Alessandro Volta. Let’s 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. It’s 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. It’s 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.

We’ll look more in depth at voltage readings in a moment. But for now, let’s 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 it’s 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 it’s 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, we’ll place the anode information on the left and the cathode information on the right. We’ll start off with a chemical symbol for the metal electrode in the anode. Next, we’ll place a vertical line to indicate the change in phase between the metal electrode and the aqueous electrolyte solution. Next, we’ll place the chemical symbol and charge of the ions at the anode. In parentheses will be the state symbol. We’ll also sometimes include the concentration or other conditions for the half-cell if they aren’t standard.

Now, we’ll separate the anode information and the cathode information with two vertical lines which represents the salt bridge. Now we’ll do the same thing for the cathode information, but this time we’ll 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. You’ll 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 metal–ion 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, we’ll still need something that can conduct electricity, so we have a platinum electrode in this cell. Platinum is a good choice here because it’s inert, so it won’t react with the hydrogen gas or hydrogen ions that are in the cell. We’ll 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 ion–ion cell. For example, in this cell, iron two plus can be oxidized to form iron three plus. Again, we’re 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 they’re 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 doesn’t tell us the potentials of our half-cells on their own. So if we want to know the potential of each half-cell, we’ll 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 we’re interested in measuring, for example, the copper half-cell involving solid copper and copper two plus ions. We’ll connect those two cells with a voltmeter. We want to make sure that we’re 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 we’re 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 doesn’t 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, we’ll 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 we’ve covered everything we need to know about galvanic cells, so let’s 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.

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