Electrolysis (AQA GCSE Chemistry): Revision Notes
Core practical - Electrolysis
What is this experiment about?
This practical helps you understand what happens when we pass electricity through different ionic solutions. You'll see what products form at each electrode and learn why this happens.
During electrolysis, electrical energy is used to break down ionic compounds, producing different substances at the positive and negative electrodes.
Aim
To find out what products are made when different aqueous solutions are electrolysed.
Key fact: Aqueous solutions of ionic compounds can conduct electricity. This is because the ions are free to move around in the solution.
Equipment you need
Safety first: Eye protection must be worn at all times during this practical.
- Different ionic solutions:
- Potassium bromide solution
- Calcium nitrate solution
- Copper nitrate solution
- Copper chloride solution
- Electrodes: Graphite electrodes (these don't react)
- Electrical circuit with d.c. power supply
- Electrolysis cell
- Two small test tubes to collect gases
You must use direct current (d.c.) for electrolysis to work properly. Alternating current will not produce the expected results.
Method
- Set up the apparatus as shown in the diagram
- Pour one of the electrolyte solutions into the cell
- Turn on the d.c. power supply
- Collect any gases produced in test tubes and observe what happens at both electrodes
Understanding what happens
At the cathode (negative electrode)
- Positive ions are attracted here
- You might see:
- Hydrogen gas (from water splitting)
- Metal forming (like copper)
At the anode (positive electrode)
- Negative ions are attracted here
- You might see:
- Oxygen gas (from water splitting)
- Halogens forming (like chlorine or bromine)
Remember the mnemonic: PANIC - Positive Anode, Negative Is Cathode. This helps you remember which electrode is which!
Results you should expect
The table below shows what you should observe when different solutions are electrolysed. Notice how the products vary depending on which ions are present in the solution:
| Solution | What you see at cathode | What you see at anode |
|---|---|---|
| Potassium bromide | Colourless gas (hydrogen) | Orange solution (bromine) |
| Calcium nitrate | Colourless gas (hydrogen) | Colourless gas (oxygen) |
| Copper nitrate | Red-brown solid (copper) | Colourless gas (oxygen) |
| Copper chloride | Red-brown solid (copper) | Pale green gas (chlorine) |
Key points to remember
- Graphite electrodes are used because they don't react with the products
- Metal ions go to the negative electrode (cathode)
- Non-metal ions go to the positive electrode (anode)
- The products depend on what ions are in the solution
- Safety: Always wear eye protection and work in a fume cupboard if collecting gases
Common mistake to avoid: Don't confuse which ions go to which electrode. Remember that opposite charges attract - positive ions go to the negative electrode (cathode) and negative ions go to the positive electrode (anode).
Example exam question
Worked Example: Electrolysis Questions
In the electrolysis of copper chloride solution:
Question 1: Why are graphite electrodes used?
Answer: Because graphite is inert (doesn't react) with the products formed during electrolysis.
Question 2: What gas forms at the anode and how could you test for it?
Answer: Chlorine gas forms at the anode. Test with damp litmus paper - it turns red then bleaches white.
Remember!
Essential Takeaways:
- Electrolysis splits ionic compounds using electricity
- Positive ions move to the negative electrode (cathode)
- Negative ions move to the positive electrode (anode)
- Different solutions give different products
- Always use d.c. current and wear safety equipment