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Edexcel Jun 2017 (IAL) Paper 6 Q3
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![3 A student measured the standard emf of a cell which uses the following half-reactions: Cu2+(aq) + 2eCu(s) E d = +0.34 V Fe3+(aq) + eFe2+(aq) E d = +0.77 V The following apparatus was provided: 1.0 mol dm3 copper(II) sulfate (a) Identify the parts of the apparatus. (i) Instrument C (1)(ii) The metal used for B (1)(iii) The metal used for D (1)(iv) The components needed to make item A (2)*P48386A0816*<br />
(v) Solution E (2)(b) Using the apparatus above, the student obtained a value of E d cell = +0.35 V for the cell reaction. (i) Write the equation for the cell reaction. State symbols are not required. (ii) Calculate the percentage error in the students measurement compared with the value calculated using the data at the start of the question. (1) (2) *P48386A0916* Turn over<br />
(c) In a further experiment, the student investigated the effect of changing the concentration of the copper(II) sulfate solution on its electrode potential. The following results were obtained: [CuSO4(aq)] / mol dm3 Electrode potential (E) / V log10[CuSO4(aq)] 1.0 0.10 0.010 0.0050 0.0010 0.35 0.31 0.28 0.27 0.24 0.0 1.0 2.0 2.3 3.0 (i) Outline how you would use the apparatus available in a school laboratory to prepare, as accurately as possible, at least 100 cm3 of a 0.10 mol dm3 solution of copper(II) sulfate, starting from the 1.0 mol dm3 copper(II) sulfate solution. (3)(ii) The student was advised that the results would be more accurate if the measurements were made starting from the most dilute solution and working in order to the most concentrated. Explain why working from high to low concentration would be less accurate. (1)10 *P48386A01016*<br />
(iii) Plot a graph of electrode potential, E, (on the vertical axis) against log10[Cu2+]. Use appropriate scales, and label the axes of the graph. (3) (iv) State the relationship between the electrode potential of the Cu2+(aq)|Cu half-cell and the concentration of the copper(II) ions. (1)(Total for Question 3 = 18 marks) *P48386A01116* 11 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2017_IAL_QP_-_Unit_6_Edexcel_Chemistry_A-level_3-0.jpg)
![Question Number 3(a)(i) Question Number Acceptable Answer Reject (High resistance) voltmeter OR Potentiometer ALLOW high resistivity voltmeter Low resistance voltmeter Galvanometer Voltmeter cell Mark (1) Acceptable Answer Reject Mark Cu2+ / Cu and any other metal (1) Reject Mark (1) Reject Mark Just paper solids (2) 3(a)(ii) Copper / Cu Acceptable Answer Question Number 3(a)(iii) Platinum / Pt Question Number 3(a)(iv) Filter paper (1) Acceptable Answer IGNORE salt bridge (soaked in saturated solution of) potassium nitrate / KNO3 ALLOW NaNO3 / KCl / NaCl (1)<br />
Question Number 3(a)(v) Solution containing soluble iron(II) Acceptable Answer Reject Mark (2) and iron(III) compounds identified by name or formula e.g. iron(II) sulfate / FeSO4 and iron(III) chloride / FeCl3 ALLOW Solution containing Fe2+ and Fe3+ (1) Both solutions 1 mol dm3 in iron ions ALLOW Solutions equimolar in iron ions (1) Name or formula of soluble iron compounds with 1 mole of Fex+ per mole of both compounds e.g. 0.5 mol dm3 Fe2(SO4)3 and 1.0 mol dm3 FeSO4 scores (2) marks If no other mark scored 1.0 mol dm3 Fe2+ / Fe3+ OR 0.5 mol dm3 Fe2(SO4)3 OR 1.0 mol dm3 FeSO4 scores (1) marks Question Number 3(b)(i) Acceptable Answer Reject Mark 2Fe3+ + Cu2Fe2+ + Cu2+ OR Multiples ORforIGNORE State symbols even if incorrect. (1) Reverse reaction Equations with uncancelled electrons<br />
Question Number 3(b)(ii) Acceptable Answer Reject Mark cell = 0.770.34 = (+)0.43 (2) cell = 0.43 22.9 % 20 % (literature value) Eo (V) TE on reverse reaction in (b)(i) (Eo (V)) (1) % error = 100 x (0.430.35)0.43 = 18.6 % TE on incorrect calculation of Eo value must be the denominator (1) IGNORE SF except 1 SF cell but literature Acceptable Answer Reject Mark Question Number 3(c)(i) MP1 (3) Measuring cylinder / beaker / syringe No mention of appropriate volume Use a pipette to measure 25.0 (or 10.0) cm3 of the 1.0 mol dm3 copper(II) sulfate solution ALLOW Burette (1) MP2 Transfer this to a 250.0 (or 100.0) cm3 volumetric / graduated / standard flask (1) MP3 Make solution up to the mark with (distilled) water and then mix / shake / invert (1) If MP1 and MP2 not awarded, mention of pipette and volumetric flask scores 1 Acceptable Answer Reject Mark Increases the possibility of contamination of Cu2+ / CuSO4 due to residues from earlier experiments in beakers or on salt bridge / electrodes ALLOW Reverse explanation i.e. low concentration to high reduces contamination risk (1) Question Number 3(c)(ii)<br />
Question Number 3(c)(iii) See below for example Acceptable Answer Reject Mark (3) Non- linear scale scores (0) Choice of scale to cover at least half the grid in both directions and labelled axes with Any units units on y axis, which may be labelled E/V. x axis may be log10[Cu2+(aq)] or log10[Cu2+] on x-axis or log10[CuSO4(aq)] or log10[CuSO4] (1) log scale reversed All points given in table correctly plotted TE on axes used (1) Any sensible smooth best fit straight line point to point line (1)<br />
Reject Mark E[Cu2+(aq)] (1) Acceptable Answer Question Number 3(c)(iv) Electrode potential / E is proportional to log10 (concentration of copper((II)) ions) / log10[Cu2+((aq))] OR Elog10[Cu2+((aq))] / log10[CuSO4((aq))] ALLOW log / lg for log10 IGNORE directly/ reference to exponential relationships No TE on incorrectly plotted graph (Total for Question 3 = 18 mark)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_IAL_MS_-_Unit_6_Edexcel_Chemistry_A-level_3-0.jpg)
