HCI Prelim_H2_CHEM_P2_ans

7 Paper 2 1 (a) (i) Cu(s) + 2Fe3+(aq) → Cu2+(aq) + 2Fe2+(aq) [1] (ii) Positive. More aqueous ions produced and thus more ways that energy can be distributed in the system through the motion of ions. [1] 1 (b) M1 [1/2] voltmeter and salt bridge [1/2] a single water bath or hot plate (not separate) M2 Correct Fe3+/Fe2+ half cell: [1/2] electrode + [1/2] solutions (with correct concentration) M3 Correct Cu2+/Cu half cell: [1/2] electrode + [1/2] solution (with correct concentration) M4 [1/2] 5.56 g of solid [1/2] details for weighing – electronic balance, weighing bottle/ beaker, reweigh or tare and rinse out all solid M5 Preparation of acidified FeSO4 solution: [1/2] adding acid before topping up to 100 cm3 mark [1/2] 100 cm3 volumetric flask M6 Preparation of FeSO4 solution:  Transfer solid to beaker  Dissolve solid in beaker  Transfer solution to volumetric flask

2016 HCI C2 H2 Chemistry Preliminary Exam / Paper 2  Transfer washings to volumetric flask  Top up volumetric flask  Shake [1/2] any 3 – 5 points [1] all 6 points M7 [1/2] measure volumes of solutions used for both half -cells (minimum volume of electrolyte in each half-cell: 20 cm3) [1/2] use appropriate apparatus e.g. measuring cylinder for measurement of solutions for both half-cells M8 [1/2] measure temperature using thermometer or set temperature of thermostatically controlled water bath [1/2] range of temperature used at least 30 ºC – should have at least 5 readi ngs with at least 5 ºC intervals (lowest temperature should not be below 20 ºC) M9 Sketch of graph: [1/2] Correct choice of axes – E vs T (ignore units) [1/2] Linear graph with positive gradient Allow a plot of ∆G vs T. In this case, a linear graph with negative gradient would be obtained. Candidate needs to explain clearly how ∆G can be obtained. M10 [1] explain how entropy change could be obtained from the graph – e.g. Gradient = ∆S/ nF (for E vs T graph) Gradient = ∆S (for ∆G vs T graph) Sample procedure 1. Using an electronic weighing balance, weigh accurately 5.56g of FeSO4.7H2O in a weighing bottle. 2. Transfer this solid into a 100 cm3 beaker. Reweigh the weighing bottle to account for any residual solid. Record the mass of the solid used. 3. Dissolve the solid using about 20 cm3 of 0.50 mol dm3 H2SO4(aq). 4. Transfer this solution into a 100 cm3 volumetric flask. Rinse the beaker thoroughly and transfer all washings into the flask. Top up to the mark with deionised water/H2SO4(aq) and shake well to obtain a homogeneous solution. Label this solution as FA1.

8 5. Transfer 15 cm3 of FA1 and 15 cm3 of 0.20 mol dm-3 FeCl3 into a 100 cm3 beaker using separate burettes. 6. Transfer 30 cm3 of 0.10 mol dm-3 CuSO4 into a 100 cm3 beaker using a burette. 7. Set up the apparatus as shown in the diagram above. 8. Ensure that the water bath is maintained at 25oC by measuring the temperature with