Required Practical: Electrolysis of Aqueous Solutions
The Cathode Rule: At the negative electrode, the least reactive positive ion is discharged. If the metal in the solution is more reactive than hydrogen, then hydrogen gas is produced; otherwise, the metal itself plates the electrode.
The Anode Rule: At the positive electrode, if halide ions (, , ) are present, the corresponding halogen gas is produced. If no halides are present, the hydroxide ions from water are discharged to produce oxygen gas.
Water Dissociation: The equilibrium reaction ensures that even in neutral salts, there are always hydrogen and hydroxide ions available to participate in the reaction.
Gas Identification Table: | Product | Test Method | Positive Result | | --- | --- | --- | | Hydrogen | Lighted splint held at the mouth of the tube | A characteristic 'squeaky pop' sound | | Oxygen | Glowing splint inserted into the tube | The splint relights (rekindles) | | Chlorine | Damp blue litmus paper held near the gas | Paper turns red then is bleached white |
Metal vs. Gas: A solid coating on the cathode indicates a metal less reactive than hydrogen (like copper), whereas bubbles indicate the discharge of hydrogen gas from the water.
Predicting Products: Always list all four ions present (, , and the two from the salt) before applying the discharge rules to avoid forgetting the contribution of water.
Reactivity Awareness: Memorize the position of hydrogen in the reactivity series; it is the 'tipping point' for determining if a metal or gas forms at the cathode.
Observation Accuracy: When describing results, distinguish between 'effervescence' (the process) and the 'gas' (the product). Examiners look for clear links between observations and conclusions.
Safety Checks: Ensure the electrodes do not touch, as this creates a short circuit, preventing electrolysis and potentially damaging the power supply.
Ignoring Water Ions: A common mistake is assuming only the salt's ions react; in aqueous solutions, the and ions are often the ones actually discharged.
Confusing Electrode Charges: Remember the mnemonic PANIC (Positive Anode, Negative Is Cathode) to ensure ions are being tracked to the correct terminal.
Incorrect Gas Tests: Students often swap the oxygen and hydrogen tests; remember that hydrogen is flammable (pop) while oxygen supports combustion (relight).