Electrolytic Cells

Electrochemical reactions are vital to many processes in technology and the human body and involve the transfer of electrons from one chemical species to another. These reactions are called reduction-oxidation reactions or redox reactions.

When a species loses electrons, it undergoes oxidation, and we say that it is oxidized. When a species gains electrons, it has undergone reduction, and we say that it is reduced. In an electrochemical galvanic cell, redox reactions occur spontaneously, which generate electrical energy. However, in an electrolytic cell, electrical energy is applied, which causes the redox reactions to occur.

An electrolytic cell consists of the reaction solution called the electrolyte, which contains any chemical species that are oxidized or reduced as well as ions needed to enable electron flow. The electrolytic cell has two metal electrodes that are immersed in the electrolytic solution. An external circuit connects the two electrodes, which completes the circuit and enables us to apply voltage or current.

The applied current or voltage is what causes the electrochemical reaction to take place. The oxidation half-reaction occurs at the anode, and the reduction half-reaction occurs at the cathode. To help you differentiate between the two, remember the phrase 'red cat,' which stands for 'reduction at cathode'.

But how do we determine which electrode is which? We can use the standard reduction potential of the metal electrodes, which is a measure of the metal's tendency to lose electrons. The higher the reduction potential, the more likely it is for the metal to be reduced.

Let's say we place a silver electrode and a copper electrode in an electrolytic cell and apply current. The silver electrode has a higher reduction potential, so it is reduced and acts as the cathode. The copper electrode has a lower reduction potential, so it is oxidized and acts as the anode.

One use of electrolytic cells is to perform electroplating, which is a reaction where one metal is oxidized and then reduced onto the surface of another metal. Since the anode is the metal doing the plating, in our example of silver and copper, the silver electrode is plated with a thin layer of copper.

In this lab, you'll assemble an electrolytic cell and perform the electroplating process by plating a brass key with copper.