OCR GCSE Chemistry A C3.4 Electrolysis Exam Questions

About OCR GCSE Chemistry A C3.4 Electrolysis

C3.4 explains how electricity is used to decompose ionic compounds. On the OCR Gateway Chemistry A course it completes Topic C3 Chemical reactions, assessed in Paper 1 (Foundation) or Paper 3 (Higher). You predict the products of electrolysis of molten and aqueous compounds with inert electrodes, compare this with using copper electrodes, and write half equations for the reactions at each electrode. It builds on ionic bonding in C2.2 and redox in C3.3, and links to metal extraction in C6.1. Examiners often ask you to predict products at each electrode, explain why hydrogen forms instead of a reactive metal, write half equations and describe the PAG electrolysis practical.

What you need to know

• Principles of electrolysis An electrolyte is an ionic compound that is molten or dissolved in water, so its ions can move. Positive ions (cations) move to the cathode (negative electrode) and negative ions (anions) move to the anode (positive electrode).
• Molten compounds In a molten binary compound, the metal forms at the cathode and the non-metal at the anode. For example, molten lead bromide gives lead at the cathode and bromine at the anode.
• Aqueous solutions: cathode In aqueous solutions water also provides H+ and OH- ions. At the cathode, hydrogen is produced if the metal is more reactive than hydrogen; otherwise the metal, such as copper, is deposited.
• Aqueous solutions: anode At the anode, oxygen is produced from hydroxide ions unless the solution contains halide ions, in which case the halogen (chlorine, bromine or iodine) is produced. For example, brine gives hydrogen at the cathode and chlorine at the anode.
• Half equations At the cathode, ions gain electrons (reduction), for example Cu2+ + 2e- -> Cu. At the anode, ions lose electrons (oxidation), for example 2Cl- -> Cl2 + 2e-, and 4OH- -> O2 + 2H2O + 4e-.
• Copper electrodes When copper sulfate solution is electrolysed with copper electrodes, the copper anode dissolves (Cu -> Cu2+ + 2e-) and copper is deposited on the cathode. The anode loses mass, the cathode gains mass, and this is used to purify copper.

Common mistakes in C3.4 Electrolysis

• Saying solid ionic compounds can be electrolysed. The ions must be free to move, so the compound must be molten or dissolved.
• Stating that electrons flow through the electrolyte. Ions move through the electrolyte; electrons flow through the external circuit.
• Predicting sodium at the cathode for aqueous sodium chloride. Sodium is more reactive than hydrogen, so hydrogen is produced.
• Mixing up the electrodes. Cations go to the negative cathode; anions go to the positive anode.
• Writing unbalanced half equations, such as Cl- -> Cl2 + e-. Balance atoms and charge: 2Cl- -> Cl2 + 2e-.

Try three C3.4 Electrolysis exam-style questions