A-LevelChemistryElectrochemistryOct/Nov 2017Paper 4 Q313 Marks
(c) Car batteries are made up of rechargeable lead-acid cells. Each cell consists of a negative electrode made of Pb metal and a positive electrode made of PbO₂. The electrolyte is H₂SO₄(aq). When a lead-acid cell is in use, Pb²⁺ ions are precipitated out as PbSO₄(s) at the negative electrode. Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ (d) The diagrams show how the voltage across two different cells changes with time when each cell is used to provide an electric current. [Figure: Two graphs showing Voltage/V vs time/hours. One for a lead-acid cell, showing decreasing voltage over time. One for an H₂/O₂ fuel cell, showing constant voltage over time.]
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
🔒
Unlock the Examiner's Analysis
Sign up for free to reveal the full examiner report, trap analysis, and mark scheme breakdown for this question.
Sign Up Free to Unlock →Join thousands of Cambridge students already using Oracle Prep