Electrolysis 

Electrolysis is a common process tested in GCSE and A-level Chemistry exams. Below are some key facts, definitions, and concepts about electrolysis. Our full set of Revision Notes give you all the specifics that you need to know, broken down and tailored to your specification, so you can master what matters for your exams! 

What is electrolysis?

Electrolysis is a chemical process, which splits an ionic compound into its ion using electricity. 

What is the etymology of electrolysis?

The word comes from ‘electro-’, which shows that electricity is involved, and ‘-lysis’, which comes from the Greek word for ‘split’. 

When will electrolysis happen?

For electrolysis to work, you need an ionic compound that has either been melted into a liquid or which has been dissolved in water to make a solution. 

The electricity breaks the compound down into its positive ions (cations) and negative ions (anions).

What keyword definitions do I need to know for electrolysis? 

Some keyword definitions you need to know are:

• An electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte
• An electrolyte is an ionic compound in a molten or dissolved solution that conducts the electricity and is the substance that you are going to split into ions using electrolysis 
• An anode is the positive electrode of an electrolysis cell
• An anion is a negatively charged ion that is attracted to the anode
• A Cathode is the negative electrode of an electrolysis cell
• A Cation is a positively charged ion that is attracted to the cathode 

What happens during electrolysis? 

During electrolysis, the electrolyte is split up into its constituent ions when electricity is passed through it. The ions move to the electrodes, and then either lose or gain electrons when they get there. The anion (negative ions) move to the anode (positive electrode), because opposite charges attract each other. For the same reason, the cations (positive ions) move to the cathode (the negative electrode). 

Electrolysis exam tip:

One way you can remember which electrode is which, is to use the mnemonic, PANIC.

Positive (is)

Anode

Negative 

Is

Cathode 

Electrolysis works using a practical setup, like the one below. This is called an electrolytic cell. 

• During the process, metals and hydrogen will form cations (positive ions)
  • Because of this, they will move to the cathode (negative electrode) and either the metal or hydrogen gas will form at the cathode 
• Non-metals will form anions (negative ions)
  • Because of this, non-metals (except for hydrogen) will move towards the anode (positive electrode) and the non-metal is formed at this electrode 

What word is used to describe the electrodes used in electrolysis that do not react?

• The word used to describe electrodes used in electrolysis that do not react is inert. 
• Graphite (carbon) electrodes are used in electrolysis because graphite is a great conductor of electrolysis, it is not an ionic compound and, most importantly, is inert. 

Why is aluminium extracted by electrolysis? 

• Aluminium is extracted from its oxide using electrolysis because of its reactivity. 
• Sometimes, carbon can be used to extract metals from their ores. 
• However, for this to happen, the metal you want to extract must be less reactive than carbon, so that a displacement reaction can take place. 
• If the metal is more reactive than carbon, like aluminium is, then electrolysis is used. 
• This is a key reason why aluminium is so expensive! 
• Electrolysis uses a large amount of electricity, and so a lot of electricity is needed to use electrolysis to extract aluminium. 

What are some examples of electrolysis?

In your exam, you need to understand the process of electrolysis and what is going on. But, there are some common examples which come up a lot. 

A key example of electrolysis is the electrolysis of molten lead (II) bromide. 

The Electrolysis of Lead (II) bromide

• During the electrolysis of lead (II) bromide, a sample of lead bromide will be melted in a beaker so that the ions are mobile and free to move
• Two graphite rods will be added, which will be connected to a power pack, creating the simple electrolytic cell 
• When the power is turned on and electricity is passed through the molten substance, the positive metal lead ions will move to the cathode
• Here, the lead ions will gain electrons to become lead atoms, and lead will form 
• The lead will be deposited on the surface of the electrode 
• The negative bromide ions will move to the anode
• Here, each bromide ion will lose one electron, forming bromine atoms 
• Two of the bromine atoms formed will combine to form bromine molecules, and you will see bubbles of bromine gas given off 

There are lots of other examples of electrolysis - use the other Revision Notes in this topic to make sure you know everything that you need for your exam:

• Electrolysis of Molten Compounds
• Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid
• Electrolysis of Aqueous Solutions

What is electroplating?

• There are lots of applications of electrolysis in everyday life. Two common ones include electroplating and fuel cells
• Electroplating is a process where the surface of one metal is coated with a layer of a different metal
• A common example of this is coating a strip of iron with tin 
• Electroplating is done because it makes the metal more resistant to corrosion or damage
• It also improves the appearance of the metal 
• For example, when jewellery or cutlery is coated with silver