Edexcel Jun 2014 (IAL) Paper 6 Q2

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2 A student wishes to measure the Ecell value of an electrochemical cell in which the following reaction occurs. Fe(s) + Cu2+(aq) (cid:314) Fe2+(aq) + Cu(s) Equation 1 The solutions and apparatus available to the student are listed below. Solution A: : Solution B copper(II) sulfate iron(II) sulfate Solution C: potassium nitrate Solution D: barium chloride 1.00 mol dm3 concentration unknown saturated saturated Copper foil electrodes Iron foil electrodes Platinum foil electrodes Voltmeter W : low resistance : high resistance Voltmeter X Ammeter Y: low resistance Ammeter Z: high resistance Beakers Connecting leads Crocodile clips Strips of filter paper (a) Draw a labelled diagram of the cell that the student should set up to measure Ecell for the reaction in Equation 1. Only use items selected from the list above. (4) *P42981A0312* Turn over<br /> (b) (i) The student measured Ecell as +0.79 V. The electrode dipping into the copper(II) sulfate solution was the positive electrode. For this half-reaction Cu2+(aq) + 2e (cid:85) Cu(s) E (cid:57) = +0.34 V where E (cid:57) is the standard electrode potential. Use the above information to calculate the electrode potential (E) in the students cell for the half-reaction Fe2+(aq) + 2e (cid:85) Fe(s) (ii) For the half-reaction Fe2+(aq) + 2e (cid:85) Fe(s) E (cid:57) = 0.44 V where E (cid:57) is the standard electrode potential. For this half-reaction, the electrode potential (E) at a particular concentration is related to the standard electrode potential (E (cid:57)) by the equation E = E (cid:57) + 0.013 ln [Fe2+] Equation 2 where ln is the natural logarithm and [Fe2+] is the concentration of Fe2+ ions in mol dm3. Use Equation 2, and your answer to (b)(i), to calculate the concentration of Fe2+ ions in solution B. (2) (2) *P42981A0412*<br /> (c) The concentration of another solution of iron(II) sulfate, Q, was found by titration. 25.0 cm3 samples of Q were titrated with a solution of acidified potassium manganate(VII), concentration 0.0300 mol dm3. The results are as follows: Titration Rough 25.00 24.40 24.40 25.70 1.00 2.10 1.60 3.30 Burette reading (final) / cm3 Burette reading (initial) / cm3 Titre /cm3 Titres used to calculate mean ((cid:57)) (i) Complete the table and calculate the mean titre. Indicate with a ((cid:57)) the titres that you have used in your calculation. (2) Mean titre(ii) State the colour change at the end-point. (1)(iii) Complete the equation for the reaction occurring during the titration. State symbols are not required. (2) MnO4+ 8H+ + 5Fe2+ (cid:314) *P42981A0512* Turn over<br /> (4) (1) (iv) Calculate the concentration, in mol dm3, of the iron(II) sulfate solution, Q. Give your answer to three significant figures. (v) The concentration of the iron(II) sulfate solution, Q, was also measured on a previous day using the method described in part (a). The concentration was found to be 0.157 mol dm3. Calculate the percentage difference between this value and the value you calculated in (c)(iv). You should assume that the correct concentration is 0.157 mol dm3. *P42981A0612*<br /> (vi) In the titration, the volume delivered by the pipette is accurate to 0.06 cm3. Each burette reading is accurate to 0.05 cm3. Calculate the percentage error of the pipette for a volume of 25.00 cm3 and of the burette for your mean titre. (2) PipetteBurette(vii) Comment on the magnitudes of the values you have calculated in (c)(v) and (c)(vi). (1)(viii) Suggest why the concentration of iron(II) sulfate in solution Q calculated in (c)(iv) is lower than the value given in (c)(v). (1)(Total for Question 2 = 22 marks) *P42981A0712* Turn over<br />

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