Electrolysis is used to electroplate objects (coat them with a thin layer of metal). This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.
Here are two examples.
The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.
The object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is copper(II) sulfate solution.
Copper is purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper(II) sulfate. The anode (positive electrode) is made from impure copper and the cathode (negative electrode) is made from pure copper.
Pure copper forms on the cathode. The slideshow shows how this works.
Purifying copper by electrolysis
During electrolysis, the anode loses mass as copper dissolves, and the cathode gains mass as copper is deposited.
The slideshow shows what happens during the purification of copper by electrolysis:
The purification of copper by electrolysis
A half-equation shows what happens at one of the electrodes during electrolysis. Electrons are shown as e–.
These are the half-equations:
Oxidation happens at the anode because electrons are lost. Reduction happens at the cathode because electrons are gained.
One way to remember this is by using the mnemonic OIL RIG:
Oxidation Is Loss of electrons, Reduction Is Gain of electrons.