Comparing Electrolytic Cells & Galvanic Cells

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Electrolytic cells and galvanic cells both involve the transfer of electrons and energy. In this lesson, we will go through the similarities and differences between the two.

Rocks and Hills

Imagine you are on a hike in the mountains and you come to steep terrain. Before beginning the trek uphill, you notice a rock at the base of the incline. Guess what? It isn't moving, and you decide to pick it up and begin to walk up the hill. On the way up the hill you see a rock rolling down the incline toward you, and it comes to rest at the bottom of the incline. This is an analogy for galvanic cells and electrolytic cells. The galvanic cell could be compared to the rock rolling down the hill, and the electrolytic cell could be compared to the rock sitting at the base of the hill. Let's break this analogy down further and compare and contrast electrolytic and galvanic cells.

Galvanic Cell

A galvanic cell is a setup involving two metal electrodes connected with a wire. The metals must be different so that one of them spontaneously gives up electrons (oxidized), called the anode, and the other accepts the electrons (reduced), called the cathode.

An example of this is copper and zinc. Zinc is oxidized and copper is reduced. Both of these metals are in separate aqueous solutions connected by a salt bridge, and are called half-cells. One solution is losing negative charge and the other is gaining negative charge, which is the reason a salt bridge is required between the half-cells. A salt bridge is another aqueous solution that allows ions to travel into each solution to keep them electrically neutral.

Galvanic cell where electrons flow spontaneously from the zinc bar to the copper bar
galvanic

This setup turns chemical energy into electrical energy in the form of current flow. Electrical current flow occurs when there is a voltage difference. This occurs naturally between the half-cells. Going back to our hill analogy, we can see that the spontaneous flow of electrons between the anode and cathode compare to the spontaneous movement of a rock tumbling downhill.

Electrolytic Cells

An electrolytic cell involves two carbon electrodes in the same molten electrolytic solution and are connected to the positive and negative terminals of a battery. An example of a molten solution for an electrolytic cell is sodium chloride, NaCl.

When the wires are connected to the positive and negative terminals of the battery, electrical energy is transferred to the solution, causing the element, which is a stronger reducing agent, to become oxidized (lose electrons). The element that is the stronger oxidizing agent will be reduced (gain electrons). In the case of sodium and chlorine ions, the sodium ions will be reduced and the chlorine will be oxidized. The carbon rod connected to the negative terminal of the battery is the cathode and the carbon rod connected to the positive terminal of the battery is the anode.

In the case of the sodium chloride solution, the chlorine ions move toward the anode where they are oxidized and combine with another chlorine atom to form a molecule of chlorine gas, Cl2.

Electrolytic cell where electrons are forced to flow due to an external voltage source
EC

This setup turns electrical energy into chemical energy. Our hill analogy relates to an electrolytic cell because the molten solution has to have a external source of energy to move the ions in the solution toward the cathode and anode, just like the rock has to have someone carry it up the hill. Now let's make a table comparing the two types of cells.

Similarities and Differences

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