Cambridge IGCSE Chemistry 4 Electrochemistry Exam Questions

About Cambridge IGCSE Chemistry 4 Electrochemistry

Electrochemistry in Cambridge IGCSE Chemistry 0620 covers electrolysis and hydrogen-oxygen fuel cells. You need to know the terms electrolyte, anode and cathode, predict the products of electrolysing molten binary compounds and aqueous solutions, and describe electroplating. Extended candidates also write ionic half-equations, explain how charge is carried in the external circuit and in the electrolyte, and describe the refining of copper. The topic builds on ionic bonding and the reactivity series, and it links to the extraction of aluminium in the metals topic. Examiners often ask you to predict and test the products at each electrode, describe what is seen, and compare fuel cells with petrol engines.

What you need to know

• Key terms Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. The anode is the positive electrode and the cathode is the negative electrode. Inert electrodes are usually graphite or platinum.
• Molten binary compounds The metal forms at the cathode and the non-metal at the anode. Molten lead(II) bromide gives lead at the cathode and bromine at the anode.
• Aqueous solutions At the cathode, hydrogen forms unless the metal is less reactive than hydrogen (for example copper). At the anode, a halogen forms from a concentrated halide solution; otherwise oxygen forms. Concentrated aqueous sodium chloride gives hydrogen and chlorine; dilute sulfuric acid gives hydrogen and oxygen.
• Copper(II) sulfate With carbon electrodes, copper is deposited at the cathode and oxygen is given off at the anode, and the blue colour fades. With copper electrodes, the anode dissolves and copper is deposited on the cathode, so the colour stays the same; this is used to refine copper (Supplement).
• Half-equations (Supplement) Reduction at the cathode, e.g. Cu2+ + 2e- → Cu and 2H+ + 2e- → H2. Oxidation at the anode, e.g. 2Cl- → Cl2 + 2e- and 4OH- → O2 + 2H2O + 4e-. Electrons move in the external circuit; ions move in the electrolyte.
• Electroplating The object to be plated is the cathode, the plating metal is the anode and the electrolyte contains ions of the plating metal. It is used to improve appearance and resist corrosion.
• 4.2 Hydrogen-oxygen fuel cells A fuel cell uses hydrogen and oxygen to produce electricity, and the only product is water. Compared with petrol engines, there are no carbon dioxide emissions in use, but hydrogen is difficult to store and is often made using fossil fuels.

Common mistakes in 4 Electrochemistry

• Saying electrons move through the electrolyte. In the electrolyte the current is carried by moving ions; electrons only move in the wires and electrodes.
• Predicting sodium at the cathode when aqueous sodium chloride is electrolysed. Sodium is more reactive than hydrogen, so hydrogen is produced.
• Writing the anode half-equation with electrons on the wrong side. Oxidation is loss of electrons, so electrons appear on the right, e.g. 2Br- → Br2 + 2e-.
• Saying solid ionic compounds can be electrolysed. The ions must be free to move, so the compound must be molten or dissolved.
• Forgetting the test for chlorine. It bleaches damp litmus paper; it does not give a squeaky pop.

Try three 4 Electrochemistry exam-style questions