Chemical Changes: Electrolysis

Understanding Electrolysis

Electrolysis involves breaking down ionic compounds into individual elements using electricity.
This process requires a liquid (molten or aqueous) ionic compound allowing the ions to move freely, promoting the transfer of charged particles.
An electric current is passed through the ionic compound, causing the elements to separate.
Electrolytes, compounds that break down during electrolysis, can be covalent compounds like water or ionic compounds like sodium chloride.

Components of Electrolysis

An electrolytic cell is made of an energy source, two electrodes (anode and cathode) and an ionic compound.
The positive electrode, the anode, attracts negatively charged ions (anions).
The negative electrode, the cathode, attracts positively charged ions (cations).

Processes at the Electrodes

At the cathode, positive ions gain electrons in a process called reduction. Thus, they are reduced.
At the anode, negative ions lose electrons in a process called oxidation. They are said to have been oxidised.
Remember the pneumonic OIL RIG: Oxidation Is Losing (electrons), Reduction Is Gaining (electrons).

Examples of Electrolysis

Electrolysis of Molten Lead Bromide: When electrical current is passed through molten lead bromide, it splits into lead (at the cathode) and bromine (at the anode).
Electrolysis of Dilute Sulfuric Acid: On passing an electrical current through dilute sulfuric acid, it decomposes into hydrogen (at the cathode) and oxygen (at the anode).

Practical Applications of Electrolysis

Electrolysis is used in electroplating, which is the process of coating an object with a layer of metal.
It is also used in the extraction of reactive metals above carbon in the reactivity series. For example, aluminium is extracted from bauxite ore using electrolysis.
Electro-refining uses electrolysis to purify copper.

Investigating Electrolysis

Practical experiments can help in better understanding and visualisation of the electrolysis process.
For instance, copper chloride electrolysis experiment shows the formation of copper at the cathode and chlorine gas release at the anode.

Predicting Products of Electrolysis

The substance formed at each electrode during electrolysis depends on the reactivity of the ions in the electrolyte.
More reactive species are more likely to remain in solution. For instance, in a solution of copper chloride, copper ions are less reactive than hydrogen ions so copper is made at the cathode.
The electrolysis of brine, a solution of sodium chloride in water, produces chlorine gas at the anode, hydrogen gas at the cathode, and leaves sodium hydroxide in solution.