Purifying copper by electrolysis

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

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

2. The pure copper rod is connected to the negative terminal of a battery, and the impure rod is connected to the positive terminal

3. The pure copper rod has increased in size, while the impure rod has deteriorated, leaving a pool of anode sludge at the bottom of the beaker

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

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

2. A battery is connected between the rods and the Cu ions are pulled towards the left rod

3. There are now four Cu ions attached to the left rod, with four Cu²+ ions floating in the middle

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

These are the half-equations:

anode: Cu → Cu²⁺ + 2e^- (oxidation)

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.