Principles of electrolysis and molten compounds

Last time

Before we start: last time, in Predicting products and half-equations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Splitting a compound with electricity

Aluminium foil, chlorine for swimming pools and the coating on a gold watch all have something in common. They are made using electricity to split compounds apart. This is electrolysis, and the rules are simple once you know what the ions are doing.

What is electrolysis?

Electrolysis is the breaking down of an ionic compound, when it is molten or dissolved in water, using electricity. The compound must be molten or in solution, because the ions must be free to move. A solid does not conduct. So electrolysis converts electrical energy into chemical energy.

The terms to know

The electrolyte is the molten or dissolved compound that is broken down. The electrodes are the rods that carry the current into and out of the electrolyte. The positive electrode is the anode. The negative electrode is the cathode. Inert electrodes, such as graphite or platinum, do not react. They only carry the current.

What the ions do

Positive ions, the cations, are attracted to the negative electrode, the cathode. Negative ions, the anions, are attracted to the positive electrode, the anode. Opposite charges attract. Remember: the anode is positive, so the anions go to it.

What happens at the electrodes

At the cathode, the positive ions gain electrons, and become atoms. This is reduction. At the anode, the negative ions lose electrons, and become atoms. This is oxidation. A good way to remember: oxidation is loss, and reduction is gain, of electrons.

What carries the current?

In the wires of the external circuit, the charge is carried by electrons. In the electrolyte, the charge is carried by ions, moving in both directions. Electrons never travel through the electrolyte. They leave the anode through the wire, and arrive at the cathode.

Molten binary compounds

A molten compound contains only its two kinds of ion. There is no water to complicate things. The metal is always formed at the cathode, because metal ions are positive. The non-metal is formed at the anode, because the non-metal ions are negative.

Molten lead(II) bromide

Here are the ions in molten lead bromide. The lead ions, Pb²⁺, move to the cathode, and the bromide ions, Br⁻, to the anode. Lead metal forms at the cathode, and bromine gas at the anode. The bromine is a red-brown fume.