Electrolysis » The Process of Electrolysis

Electrolytes: liquids that conduct

Ionic compounds are made of charged particles called ions. In a solid, the ions are locked in place, so the solid cannot carry a current. But when an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. Moving charged particles can carry an electric current.

These liquids and solutions conduct electricity, and they are called electrolytes.

Key terms:

• Electrolyte: a molten ionic compound, or a solution of an ionic compound in water, which conducts electricity because its ions are free to move.
• Electrode: a solid rod, often made of carbon or metal, that carries the current into and out of the electrolyte.
• Cathode: the negative electrode.
• Anode: the positive electrode.
• Electrolysis: passing an electric current through an electrolyte so that the ions move to the electrodes and are discharged, producing elements.
• Discharged: an ion that reaches an electrode and is changed into an uncharged atom or molecule of an element.

What happens during electrolysis

To carry out electrolysis, two electrodes are dipped into the electrolyte and joined to a power supply. One electrode becomes negative and the other becomes positive. The ions then move.

Opposite charges attract. That is why each type of ion heads for the electrode with the opposite charge.

When the ions arrive, they are discharged. This means they are turned into the uncharged element. So electrolysis breaks an ionic compound down into its elements.

A good way to remember which is which: PANIC - Positive Anode, Negative Is Cathode.

Electrolysis of molten ionic compounds

The simplest case is a molten ionic compound. There is no water, so the only ions present are the two that make up the compound.

Lead bromide is a good example. It contains lead ions (positive) and bromide ions (negative). When lead bromide is melted and electrolysed using inert electrodes:

• Positive lead ions move to the cathode, where lead metal is produced.
• Negative bromide ions move to the anode, where bromine is produced.

The overall reaction is: lead bromide → lead + bromine.

Inert means the electrodes do not react. They only carry the current. If they did react, they would get used up and the products would be confusing.

This makes sense. Metals form positive ions, so they go to the negative cathode. Non-metals form negative ions, so they go to the positive anode.

Why the compound must be melted

A solid ionic compound will not electrolyse, because its ions cannot move. The compound has to be melted first. This takes a lot of heat, because ionic compounds have high melting points.

Practical safety

Predicting the products

You should be able to predict the products of the electrolysis of a binary ionic compound in the molten state. Binary means the compound contains just two elements, such as lead bromide or zinc chloride.

Follow these steps:

1. Name the two elements in the compound.
2. The metal forms positive ions, so it is produced at the cathode.
3. The non-metal forms negative ions, so it is produced at the anode.

Notice the name change. The compound ends in -ide (bromide, chloride, iodide). The element made is named without the ending (bromine, chlorine, iodine).

Exam-style question

Molten lead bromide is electrolysed using inert electrodes. (a) Why must the lead bromide be molten? (b) Name the product formed at the cathode. (c) Name the product formed at the anode. (d) Explain why the product at the cathode is formed there. [5 marks]