Electrolysis of aqueous solutions required practical

Aim: To investigate what is produced at each electrode when different aqueous solutions of ionic compounds are electrolysed using inert electrodes.

Variables

• Independent: The aqueous solution being electrolysed (for example copper(II) chloride, sodium chloride or copper(II) sulfate)
• Dependent: The product formed at each electrode (identified by observations and gas tests)
• Control: Potential difference (voltage) of the power supply; Type of electrode (inert graphite rods); Concentration and volume of the solution; Distance between the electrodes

Equipment

• Low-voltage DC power supply (or battery pack)
• Two graphite (carbon) rod electrodes in a holder or Petri dish lid
• Beaker (100 cm³)
• Connecting leads and crocodile clips
• Two small test tubes to collect gases
• Blue litmus paper, wooden splints
• Solutions such as copper(II) chloride, sodium chloride and copper(II) sulfate

Method

1. Pour about 50 cm³ of copper(II) chloride solution into a beaker.
2. Place two graphite electrodes into the solution so that they do not touch each other.
3. Connect the electrodes to the DC supply using leads and crocodile clips; note which is the cathode (negative) and which is the anode (positive).
4. If gas is to be collected, fill small test tubes with the solution and turn them upside down over each electrode.
5. Switch on at about 4 V and watch each electrode for a few minutes, noting any bubbles, colour or coating.
6. Test any gas at the anode by holding damp blue litmus paper near it (chlorine turns it red then bleaches it white).
7. Test any gas collected at the cathode with a lighted splint (hydrogen burns with a squeaky pop).
8. Switch off, rinse and dry the electrodes, then repeat with the other solutions and record the products at each electrode.

Safety

• Wear eye protection; copper compounds are harmful and some solutions are irritants.
• Chlorine is toxic: use a well-ventilated room, run the experiment for a short time only and do not sniff the gas. People with asthma should take extra care.
• Use a low voltage and switch off before touching or moving the electrodes.
• Keep leads and the power supply away from spilt solution.

Results and calculations

Record the observations and the product at the cathode and anode for each solution in a table. At the cathode, the less reactive of the metal and hydrogen is discharged (copper forms a pink-brown coating; with sodium, hydrogen is produced); at the anode, a halide gives the halogen (e.g. chlorine), otherwise oxygen is given off. Half equations can be written, e.g. Cu²⁺ + 2e^- → Cu.

Common mistakes and improvements

• Electrodes touching, which causes a short circuit; keep them apart in a holder.
• Missing a small amount of gas; collect it in an inverted test tube and test it straight away.
• Confusing which electrode is which; label the leads and check the terminals on the power supply before starting.
• Not cleaning the electrodes between solutions, which contaminates the next test; rinse with distilled water and dry each time.

Exam tips

• Remember the cathode is negative and attracts positive ions (cations); the anode is positive and attracts negative ions (anions).
• Learn the rules: metals less reactive than hydrogen (like copper) are deposited at the cathode, otherwise hydrogen forms; halide ions give halogens at the anode, otherwise oxygen forms.
• Know the gas tests: chlorine bleaches damp litmus paper, hydrogen gives a squeaky pop, oxygen relights a glowing splint.
• Explain why inert electrodes are used: they conduct but do not react with the products or the solution.
• Higher tier: balance half equations, e.g. 2Cl^- → Cl₂ + 2e^- and 2H⁺ + 2e^- → H₂, and identify reduction at the cathode and oxidation at the anode.