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Electrolysis: Definition, Types & Uses

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Electrolysis is an important chemical process that is used in many different industries. In this lesson, learn about how electrolysis works and a few applications of electrolysis.

What is Electrolysis?

What do you think would happen if you ran an electric current through a glass of water? If you set it up just right, you might see bubbles of gas rising to the surface. What is happening inside the water to make the bubbles? It's a process called electrolysis that uses electricity to separate chemical compounds. Inside the glass of water, water molecules are being split to produce hydrogen and oxygen gas, which bubbles to the surface. The word electrolysis comes from the Greek lysis which means 'to dissolve' or 'break apart.'

Although the bubbling water looks pretty cool, electrolysis can do much more than just make water bubble. It can be used to create oxygen for astronauts on the space station, extract pure metals from metallic compounds, and even make pennies!

Electrolytic Cells and Oxidation-Reduction Reactions

Specially-designed devices called electrolytic cells are used to perform electrolysis. An electrolytic cell contains two electrodes and a solution with dissolved ions, called an electrolyte. You also need a source of electrical energy, like a battery. Electrolysis won't happen spontaneously and always requires electricity. When the electrodes are connected to a battery (or other source of electrical energy), one becomes positively charged and one becomes negatively charged. These charged electrodes can then react with the ions in the electrolyte.

An electrolytic cell like this is used to perform electrolysis. It always contains two electrodes (a cathode and anode) and an electrolyte solution.
electrolytic cell

The type of chemical reaction that occurs in an electrolytic cell is called an oxidation-reduction reaction. The negatively charged electrode, known as a cathode, reduces positive ions by adding electrons to them, creating neutral atoms. At the same time, the positively charged anode oxidizes negative ions to produce more free electrons. These two half-reactions generate new chemicals and also complete the circuit with the battery to keep current flowing through the cell.

Oxidation half of the reaction: X- → X + e-

Reduction half of the reaction: Y+ + e- → Y

Electrolysis of Water

To understand the process of electrolysis, let's first look at one of the most common and simplest electrolytic processes, the electrolysis of water. In this process, water molecules (H2 O) are split to produce both hydrogen (H2) and oxygen (O2) gas.

To perform an electrolysis of water, you need an electrolytic cell with two electrodes inside a solution of water with electrolytes in it. Although water molecules will dissociate into hydrogen (H+) and hydroxide (OH-) ions, pure water does not conduct electricity very well, so you need to add some extra ions to make electrolysis feasible.

Once you connect the electrodes to a battery, the H+ ions will be reduced at the cathode to produce hydrogen gas, and water molecules will be oxidized at the anode. The result of this oxidation-reduction reaction is the production of both oxygen and hydrogen gas.

Reduction reaction: 2H+ + 2e- → H2

Oxidation reaction: 2 H2 O → O2 + 4 H+ + 4 e-

Overall result of the reaction: 2 H2 O → 2 H2 + O2

Why would you want to perform electrolysis of water? Hydrogen is used in many industrial applications, including the production of hydrogen fuel cells, and this is a relatively easy way to make it. It's also a good way to make oxygen, especially in situations where transporting large amounts of oxygen gas isn't all that feasible, like on the International Space Station. The oxygen that the astronauts breathe while they're on the space station is created by electrolysis of water!

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