Electrolysis (AQA GCSE Chemistry Combined Science): Revision Notes
📚Revision Notes
Electrolysis
What is electrolysis?
Electrolysis happens when we break down ionic compounds using electricity. This only works when the ionic compound is molten (melted) or dissolved in water.
Note
When ionic compounds are solid, the ions cannot move. But when they are melted or dissolved, the ions become free to move around. This means they can carry an electric current.
How ionic compounds conduct electricity
In a solid ionic compound, ions are stuck in place in a crystal structure called a lattice. The ions cannot move, so no electricity can flow.
When we melt or dissolve the ionic compound:
- The ions break free from the lattice
- Positive and negative ions can now move around
- The moving ions carry electric current through the liquid
What happens during electrolysis?
During electrolysis, we put two electrodes into the molten or dissolved ionic compound:
- Anode - the positive electrode
- Cathode - the negative electrode
The ions move towards the electrodes:
- Positive ions move to the cathode (negative electrode)
- Negative ions move to the anode (positive electrode)
Note
Remember: opposites attract!
Oxidation and reduction in electrolysis
Two important processes happen at the electrodes:
At the anode (positive electrode):
- Negative ions lose electrons
- This is called oxidation
- The ions turn into atoms
At the cathode (negative electrode):
- Positive ions gain electrons
- This is called reduction
- The ions turn into atoms
Important
Memory tip: OIL RIG - Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)
Example: Lead bromide electrolysis
Example
Worked Example: Electrolysis of Lead Bromide
When we electrolyse molten lead bromide ():
At the anode:
- Bromide ions () lose electrons
- They become bromine atoms
- This is oxidation
At the cathode:
- Lead ions () gain electrons
- They become lead atoms
- This is reduction
The lead bromide breaks down into lead metal and bromine gas.
Why use graphite electrodes?
Graphite is often used for electrodes because:
- It conducts electricity well
- It doesn't react with the chemicals
- It's inert (unreactive)
Summary
Key Points to Remember:
- Electrolysis only works with molten or dissolved ionic compounds
- Positive ions go to the cathode (negative electrode)
- Negative ions go to the anode (positive electrode)
- Oxidation happens at the anode - ions lose electrons
- Reduction happens at the cathode - ions gain electrons