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.]
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