AQA GCSE Chemistry 4.4.3 Electrolysis Exam Questions

About AQA GCSE Chemistry 4.4.3 Electrolysis

AQA GCSE Chemistry 4.4.3 explains how electricity is used to break down ionic compounds. You learn why ions must be free to move, what forms at each electrode when molten compounds and aqueous solutions are electrolysed, and how aluminium is extracted from aluminium oxide. At Higher tier you write half equations at each electrode. The topic is in section 4.4, assessed on Paper 1, and builds on ionic bonding and the reactivity series. Examiners often ask you to predict products for unfamiliar solutions, explain why the carbon anodes are replaced, and describe the required practical investigating electrolysis of aqueous solutions with inert electrodes.

What you need to know

• Principles When an ionic compound is melted or dissolved, the ions are free to move. Positive ions move to the negative electrode (cathode) and negative ions to the positive electrode (anode). The liquid or solution is the electrolyte.
• Molten compounds A molten binary compound forms the metal at the cathode and the non-metal at the anode, for example molten lead bromide gives lead and bromine.
• Extracting aluminium Aluminium oxide is dissolved in molten cryolite to lower the melting point and save energy. Aluminium forms at the cathode and oxygen at the anode; the oxygen reacts with the carbon anodes to make carbon dioxide, so the anodes must be replaced regularly. Electrolysis is used when the metal is more reactive than carbon, but it is expensive because of the energy needed.
• Aqueous solutions At the cathode, hydrogen forms if the metal is more reactive than hydrogen; otherwise the metal forms. At the anode, oxygen forms unless the solution contains halide ions, in which case the halogen forms. Brine (sodium chloride solution) gives hydrogen, chlorine and sodium hydroxide.
• Half equations (Higher tier only) Reduction at the cathode, for example 2H+ + 2e- → H2 or Cu2+ + 2e- → Cu. Oxidation at the anode, for example 2Cl- → Cl2 + 2e- or 4OH- → O2 + 2H2O + 4e-.
• Required practical Electrolyse aqueous solutions such as copper(II) sulfate or sodium chloride with inert electrodes and identify the products with tests: hydrogen (squeaky pop), oxygen (relights a glowing splint), chlorine (bleaches damp litmus paper).

Common mistakes in 4.4.3 Electrolysis

• Saying electrons move through the electrolyte. Ions move through the electrolyte; electrons move through the wires.
• Predicting sodium at the cathode for sodium chloride solution. Sodium is more reactive than hydrogen, so hydrogen is produced.
• Saying cryolite is a catalyst. It lowers the operating temperature by dissolving the aluminium oxide.
• Saying the anodes are replaced because they 'wear out'. They react with the oxygen produced to form carbon dioxide.
• Writing half equations with electrons on the wrong side. Reduction gains electrons (on the left); oxidation loses electrons (on the right).

Try three 4.4.3 Electrolysis exam-style questions