Electrolysis (AQA GCSE Chemistry Combined Science): Revision Notes
Required practical - Electrolysis
What is this practical about?
This practical helps you find out what happens when you pass electricity through different solutions. You'll see what substances are made at each electrode when ionic compounds dissolve in water and undergo electrolysis.
Aim of the experiment
You need to investigate what products form when different water-based solutions have electricity passed through them. This process is called electrolysis - the breakdown of ionic compounds using electrical energy.
Equipment you need
- Safety equipment: Eye protection (very important!)
- Electrodes: Graphite rods (these don't react during the experiment)
- Power source: Direct current (dc) electrical supply
- Container: Electrolysis cell to hold your solution
- Collection: Two small test tubes to catch any gases
- Solutions: Different ionic compounds dissolved in water:
- Potassium bromide solution
- Calcium nitrate solution
- Copper nitrate solution
- Copper chloride solution
Key science concepts
Why do these solutions conduct electricity?
When ionic compounds dissolve in water, the ions can move freely. This movement of charged particles allows electricity to flow through the solution. The dissolved ions act as charge carriers, making the solution conductive.
What happens to the ions during electrolysis?
Ion Movement During Electrolysis:
- Positive ions (cations) are attracted to the negative electrode (called the cathode)
- Negative ions (anions) are attracted to the positive electrode (called the anode)
- This happens because opposite charges attract each other - a fundamental principle of electrostatics
Method steps
- Set up your apparatus with graphite electrodes connected to the dc power supply
- Pour one of the solutions into the electrolysis cell
- Switch on the dc power supply
- Watch what happens at each electrode and note any colour changes
- Collect any gases that bubble up in the test tubes
- Record your observations carefully, noting which electrode produces which product
What you should observe
When you electrolyse different solutions, you'll see different products forming at each electrode:
At the cathode (negative electrode):
- Potassium bromide: Colourless gas (hydrogen)
- Calcium nitrate: Colourless gas (hydrogen)
- Copper nitrate: Red-brown solid (copper metal)
- Copper chloride: Red-brown solid (copper metal)
At the anode (positive electrode):
- Potassium bromide: Orange solution (bromine)
- Calcium nitrate: Colourless gas (oxygen)
- Copper nitrate: Colourless gas (oxygen)
- Copper chloride: Pale-green gas (chlorine)
Why do you get these products?
The products you observe depend on which ions are present and their reactivity:
Product Formation at the Cathode:
At the cathode, either hydrogen gas forms from the reduction of water molecules, or the metal from the ionic compound is deposited as a solid. The outcome depends on the reactivity of the metal - reactive metals like potassium and calcium remain as ions, while less reactive metals like copper are deposited.
Product Formation at the Anode:
At the anode, either oxygen gas forms from the oxidation of water molecules, or a halogen gas appears if the original compound contained a halide ion (like chloride or bromide). Halide ions are more easily oxidised than water, so they are preferentially discharged.
Remember the Key Rule:
- Cathode = Reduction (gaining electrons)
- Anode = Oxidation (losing electrons)
This is often remembered as "RED CAT, AN OX" - Reduction at Cathode, Oxidation at Anode.
Important safety notes
Critical Safety Requirements:
- Always wear eye protection - some products can cause serious eye damage
- Work in a well-ventilated area as some gases produced can be harmful to breathe
- The chlorine gas produced from copper chloride is toxic and should only be produced in a fume cupboard
- Never touch the electrodes while the power supply is switched on
- Handle all solutions with care and wash hands thoroughly after the experiment
Practice Understanding: Copper Chloride Electrolysis
Consider the electrolysis of copper chloride solution and think about these questions:
Question 1: Why do you use graphite electrodes instead of metal ones? Answer: Graphite electrodes are inert (unreactive) so they don't interfere with the electrolysis process. Metal electrodes might react and give misleading results.
Question 2: How would you test if the pale-green gas is really chlorine? Answer: Use damp blue litmus paper - chlorine will bleach it white. You could also test with damp universal indicator paper.
Question 3: Why should this be done in a fume cupboard? Answer: Chlorine gas is toxic and can cause serious respiratory problems if inhaled.
Question 4: Why might you collect less chlorine than expected? Answer: Some chlorine dissolves in water, and some may escape before being collected in the test tube.
Key Points to Remember:
- Electrolysis breaks down ionic compounds using electricity
- Positive ions go to the negative electrode (cathode)
- Negative ions go to the positive electrode (anode)
- You need direct current for electrolysis to work properly
- Different solutions produce different products at each electrode
- Safety first - always wear eye protection and work in ventilated areas
- The products formed depend on the reactivity of the ions present