Required Practical: Electrolysis
Chemistry: AQA GCSE Higher, topic 4.4.5
Electrolysis of Aqueous Solutions Using Inert Electrodes
This experiment investigates which ions are discharged during the electrolysis of aqueous ionic solutions using inert electrodes, and identifies the products formed at each electrode.
Apparatus and materials
- DC power supply
- Two inert electrodes (carbon or platinum)
- Beaker
- Test tubes or gas collection tubes
- Retort stand and clamps
- Leads and crocodile clips
- Range of aqueous solutions (such as copper(II) sulfate, copper(II) chloride, sodium chloride, sodium nitrate, sodium bromide)
- Splints and litmus paper for gas testing
Method
- Assemble the electrolysis apparatus:
- Connect the electrodes to the DC power pack, ensuring one electrode is attached to each terminal.
- Pour an aqueous solution into the beaker so that both electrodes are submerged.
- Switch on the power supply and allow electrolysis to happen for several minutes.
- Observe any visible changes, such as gas bubbles or metal deposition on the electrodes.
Method (continuation)
- Collect any gases formed using inverted test tubes.
- Test the gases using the following standard gas tests:
- Hydrogen: a lighted splint produces a squeaky pop
- Oxygen: a glowing splint relights
- Chlorine: damp blue litmus paper turns red, then bleaches white
- Record all observations, including the identity of gases and any metal deposits.
- Repeat for at least five different electrolyte solutions.
Variables
- Independent variable:
- Type of aqueous solution used (e.g., copper sulfate, sodium chloride)
- Dependent variable:
- Products formed at each electrode
- Control variables:
- Current supplied, size and type of electrodes, volume and concentration of solution, and duration of electrolysis
Hazards and safety precautions
- Copper(II) compounds:
- Harmful if ingested; avoid contact with skin.
- Chlorine gas:
- Toxic; carry out experiments in a well-ventilated room or fume cupboard.
- Electric current:
- Avoid contact with solutions while the circuit is live.
- Always wear safety goggles and a lab coat, and wash hands after handling chemicals.
Analysing the results
- After electrolysis, identify which products appeared at the anode and cathode for each solution. Compare the outcomes with your original hypothesis.
- Typical observations might include:
- Copper(II) sulfate: copper metal at the cathode; oxygen at the anode.
- Sodium chloride: hydrogen at the cathode; chlorine at the anode.
Test yourself on Required Practical: Electrolysis
A student collects a gas at the cathode during electrolysis of sodium chloride solution and wants to confirm it is hydrogen. Which test would confirm this?
Why should chlorine gas produced during electrolysis be handled in a well-ventilated room or fume cupboard?
What is the dependent variable in this investigation?
What forms at the cathode during the electrolysis of copper(II) sulfate solution using inert electrodes?
A student collects a gas at the anode during electrolysis of copper(II) sulfate solution. A glowing splint relights in the gas. What is the gas?