26   8 This question is about electrode potentials, cells and equilibrium constants. (a) Chlorine gas can be prepared by the oxidation of chloride ions with manganate(VII) ions in acid solution. MnO4 –(aq) + 5Cl –(aq) + 8H +(aq)  Mn 2+(aq) + 2½Cl2(g) + 4H2O(l) E d cell = +0.15 V During this reaction, each manganate(VII) ion accepts five electrons. Calculate the equilibrium constant, K, for this reaction at 298 K using the expression lnK =  n F RT E d cell where n is the number of electrons transferred in the overall equation, F is the Faraday constant (96 500 C mol –1) and R is the gas constant (8.31 J mol –1 K –1). Units of K are not required. (2) Turn over 27   (b) A fuel cell produces a voltage from the reaction between a fuel and oxygen. The reaction occurring at one electrode in a methanol fuel cell is CH3OH(g) + H2O(l) → HCOOH(aq) + 4H +(aq) + 4e– Which reaction occurs at the other electrode? (1) A 4H +(aq) + O2(g) + 4e– → 2H2O(l) B 2H2(g) + 2O2(g) + 4e– → 4OH –(aq) C 4OH –(aq) → 2H2(g) + 2O2(g) + 4e– D 2H2O(l) → 4H +(aq) + O2(g) + 4e– (c) Lead‑acid batteries are used as storage cells in some cars. The electrolyte is sulfuric acid, one electrode is lead and the other is lead(IV) oxide, PbO2. As the cell discharges, the lead and the lead(IV) oxide are both converted to solid lead(II) sulfate, PbSO4 , and the concentration of the sulfuric acid decreases. Deduce, using the information given, the two half‑equations occurring in the lead‑acid battery. State symbols are required. (3) 28   (d) When solid lead(II) sulfate is added to aqueous sodium iodide, an equilibrium is established. PbSO4(s) + 2I –(aq)  PbI2(s) + SO4 2–(aq) The expression for the equilibrium constant, Kc , for this reaction is Kc =  [SO (aq)] [ (aq)]2 4 2− −I In an experiment, Kc may be determined by adding excess lead(II) sulfate to 25.0 cm3 of 0.100 mol dm –3 sodium iodide. The volume remains constant at 25.0 cm3. The mixture is left to reach equilibrium at room temperature. Ice‑cold water is added to freeze the position of equilibrium and the mixture is then titrated with standard silver nitrate solution. The whole mixture requires 12.20 cm3 of 0.0500 mol dm –3 silver nitrate solution to react with the aqueous iodide ions at equilibrium. Ag +(aq) + I –(aq) → AgI(s) Calculate the equilibrium concentrations of the sulfate ions and the iodide ions, and hence the value of Kc at room temperature. Give your answer to an appropriate number of significant figures and include units for Kc , if any. (7) 29   (Total for Question 8 = 13 marks) TOTAL FOR PAPER = 90 MARKS 30   BLANK PAGE 31   BLANK PAGE 32  