In this tutorial on galvanic cells, aka voltaic cells, you will learn the basics of redox reactions and how to apply this information to electrochemical cells. You will also learn how to determine what half-reactions occur at which electrode, as well as using these standard half-reaction potentials to calculate cell potential.
There are two types of electrochemical cells – a voltaic cell, also called a galvanic cell, and an electrolytic cell. Voltaic cells produce electricity, while electrolytic cells use a power source to drive a reaction forward.
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Basics of Oxidation-Reduction Reactions
In an electrochemical process, electrons flow from one substance to another substance in what is known as an oxidation-reduction (redox) reaction. Redox reactions happen when a substance called an oxidizing agent oxidizes another substance by taking electrons and being reduced. In a redox reaction, a simple mnemonic to remember the direction of the transfer of electrons is “Oil Rig,” which stands for “Oxidation is losing, reduction is gaining.” This refers to the fact that in oxidation, a species loses electrons while in reduction, the species gains electrons.
In reactions that are not redox reactions, like acid-base reactions, the oxidation states of the elements do not change.
A simple example of an oxidation reaction is the reaction of iron (II) oxide with carbon monoxide as follows:
In this reaction, iron is reduced from an oxidation state of +3 to 0, while carbon is oxidized from an oxidation state of +2 to +4. All redox reactions will have one species being oxidized and one species reduced.
Some elements, like vanadium, can even progress through multiple oxidation states as a redox reaction progresses.
There are two types of electrochemical cells: galvanic and electrolytic.
The galvanic cell has a long history. It is named after Italian physicist Luigi Galvani (1737 – 1798), who observed that dissected frog leg muscles twitched when an electric current was applied to them. A galvanic cell uses the energy released from a spontaneous redox reaction to produce an electric current. Another common name for galvanic cells is voltaic cells, which is named after another Italian physicist, Alessandro Volta (1745 – 1827), who invented the galvanic (voltaic) cell.
On the other hand, an electrolytic cell uses an electric current to drive a normally non-spontaneous redox reaction. A typical example of an electrolytic cell is applying an electrical current to water to drive the decomposition of water into hydrogen and oxygen gases through the following reaction:
The main similarities between galvanic and electrolytic cells are that they both use two electrodes, an anode and a cathode, and contain electrolyte solution. This electrolyte solution allows the transfer of ions in and out of solution while keeping the overall charge of the cell neutral.
The major difference between a galvanic and electrolytic cell is the direction of work. A galvanic cell turns a spontaneous chemical reaction into usable work, whereas an electrolytic cell uses work to drive a non-spontaneous reaction.
Oxidation occurs at the anode while reduction occurs at the cathode. An easy mnemonic to remember this is “The red cat and an ox,” where “red cat” means reduction – cathode, and “an ox” means anode – oxidation.
Determining Potential of Galvanic Cells
Consider the classic Cu-Zn galvanic cell (voltaic cell) pictured below
As you can see from the image, there are two electrodes. One of them is zinc, and the other one is copper. In order to determine which electrode is the anode and which is the cathode, the two half-reactions must be considered. A half-reaction is the net reaction that occurs with the species being oxidized or reduced and shows the flow of electrons. Take the two half-reactions in the Cu-Zn cell:
When given two half-reactions like this, the species with a higher reduction potential will typically be reduced. Because of this, we can see that in the Cu-Zn cell, the copper will be reduced while the zinc will be oxidized. Next, we must flip the zinc half-reaction to show the oxidation reaction:
Note that although we flipped the chemical equation, the standard reduction potential remains negative.
From the copper reduction half-reaction and the zinc oxidation half-reaction, we can create the overall net redox reaction by adding them together and making sure the electrons add up properly:
We can then use the equation to determine the overall potential of the cell. For our Cu-Zn cell, the potential would be calculated as:
Recall that reduction occurs at the cathode and oxidation occurs at the anode. You can read more about calculating cell potentials here.
Understanding a cell diagram is very simple. The concept of a cell diagram is to give a more convenient line notation that easily shows the phase boundaries, notated by a single vertical line, and the salt bridge, notated by a double vertical line.
Following this convention for our Cu-Zn cell would lead us to the following cell diagram:
The Voltaic Cell & Galvanic Cell – Read Further
For calculations at non-standard conditions, use the Nernst Equation!