CCEA GCSE Chemistry 2.7 Electrochemistry Exam Questions

About CCEA GCSE Chemistry 2.7 Electrochemistry

Electrochemistry is a Unit 2 topic in CCEA GCSE Chemistry about using electricity to decompose ionic compounds. You need to define the key terms, explain why ionic compounds must be molten or dissolved to conduct, and predict and explain the products at each electrode. Specific examples include molten lead(II) bromide and dilute sulfuric acid. The topic then applies electrolysis to the extraction of aluminium from bauxite and explains why recycling aluminium is important. It links to bonding and structures, redox, the reactivity series and quantitative chemistry. Examiners often ask for half equations, observations at the electrodes and explanations of why cryolite is used and why anodes are replaced.

What you need to know

• Key terms Electrolysis is the decomposition of a liquid ionic compound (electrolyte) using electricity. The cathode is the negative electrode and the anode is the positive electrode. Inert electrodes such as graphite or platinum do not react.
• Molten lead(II) bromide Ions are free to move only when molten. At the cathode: Pb2+ + 2e- → Pb (silver liquid). At the anode: 2Br- → Br2 + 2e- (red-brown vapour). Positive ions gain electrons (reduction) and negative ions lose electrons (oxidation).
• Dilute sulfuric acid Hydrogen forms at the cathode: 2H+ + 2e- → H2. Oxygen forms at the anode: 4OH- → 2H2O + O2 + 4e-. The volume of hydrogen is twice the volume of oxygen, and overall water is decomposed.
• Extracting aluminium Bauxite is purified to aluminium oxide, which is dissolved in molten cryolite to lower the operating temperature and save energy. At the cathode: Al3+ + 3e- → Al. At the anode: 2O2- → O2 + 4e-. The graphite anodes react with oxygen to form carbon dioxide, so they burn away and must be replaced.
• Recycling aluminium Recycling uses much less energy than extraction, conserves bauxite, reduces mining damage and reduces landfill.

Common mistakes in 2.7 Electrochemistry

• Saying ionic solids conduct electricity. The ions cannot move in a solid; the compound must be molten or in solution.
• Saying electrons flow through the electrolyte. In the electrolyte, current is carried by moving ions.
• Writing unbalanced half equations, for example Br- → Br2 + e-. Balance atoms and charge: 2Br- → Br2 + 2e-.
• Explaining cryolite as a catalyst. It lowers the melting temperature of the electrolyte.
• Mixing up the electrodes. Positive metal ions and hydrogen ions go to the negative cathode.

Try three 2.7 Electrochemistry exam-style questions