Electroplating

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.

How it works

  • The negative electrode should be the object to be electroplated.
  • The positive electrode should be the metal that you want to coat the object with.
  • The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate.

Here are two examples.

Electroplating with silver

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.

Electroplating with copper

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.

Purifying copper by electrolysis [Higher tier only]

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.

Beaker with pure and impure copper rods dipping into copper(II) sulfate solution.

Purifying copper by electrolysis

1. A beaker with pure and impure copper rods dipped into copper(II) sulfate solution

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:

Two vertical copper rods. Four Cu ions are attached to the rod on the right, and four Cu2+ ions are floating in the space between the rods.

The purification of copper by electrolysis

1. Four Cu ions are attached to the rod on the right, and four Cu2+ ions are floating in the space between the rods

A half-equation shows what happens at one of the electrodes during electrolysis. Electrons are shown as e.

These are the half-equations:

  • at the anode, Cu → Cu2+ + 2e (oxidation)
  • at the cathode, Cu2+ + 2e → Cu (reduction)

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.