OCR A Jan 2013 Paper 5 Q6 (with explained solutions) |

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OCR A Jan 2013 Paper 5 Q6

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6 Storage cells and fuel cells are types of electrochemical cell used as sources of energy. 14 Information about five redox systems that could be used in electrochemical cells is shown below. You may need to use this information throughout this question. redox system 1 2 3 4 5 Fe2+(aq) + 2e 2H2O(l) + 2e 2H+(aq) + 2e O2(g) + 2H2O(l) + 4e O2(g) + 4H+(aq) + 4e Fe(s) 2OH(aq) + H2(g) H2(g) 4OH(aq) 2H2O(l) E o / V 0.44 0.83 0.00 +0.40 +1.23 (a) The standard electrode potential of redox system 1 can be measured by constructing an electrochemical cell. Draw a diagram below to show how the standard electrode potential could be measured for redox system 1. State the conditions needed to measure this standard electrode potential. conditions:[4] (b) When an alkaline hydrogenoxygen fuel cell is being used to produce electrical energy, chemical changes take place within the cell. (i) Write half-equations for the changes that take place at each electrode. oxygen electrode:hydrogen electrode:[2] (ii) Write the overall equation for the cell reaction.[1]OCR 2013<br />
(iii) What is the standard cell potential of this fuel cell? 15 standard cell potential = V [1] (c) State one important difference between a fuel cell and a conventional storage cell.[1] (d) People often assume that hydrogenoxygen fuel cells are a source of energy that is carbon neutral, i.e. there is no net increase in carbon dioxide from using the fuel cell. Suggest one reason why this assumption may not be correct.[1] (e) A student constructs a cell as follows.A half-cell is made from a strip of chromium metal and a solution of aqueous chromium(III) sulfate. A second half-cell is made from a strip of a metal X and a solution of XSO4(aq). The two half-cells are connected together and a current is allowed to pass for a length of time. The chromium electrode gains 1.456 g in mass. The electrode made of metal X loses 1.021 g in mass. Determine the identity of metal X. Show all your working.OCR 2013 X =[4] [Total: 14] Turn over<br />

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F325 Question 6 (a) Marks Answer Circuit: complete circuit with voltmeter and salt bridge linking two half-cells (cid:57) Half cells: Pt AND H+/HCl (solution) AND H2 gas (introduced via enclosed container around Pt) (cid:57) Fe AND Fe2+ (solution) (cid:57) Conditions: 1 mol dm3 solutions AND 298 K / 25 C AND 1 atm/100 kPa/101 kPa/1 bar pressure (cid:57) (b) (i) oxygen electrode: O2(g) + 2H2O(l) + 4e (cid:111) 4OH(aq) (cid:57) hydrogen electrode: H2(g) + 2OH(aq) (cid:111) 2H2O(l) + 2e (cid:57) (ii) 2H2(g) + O2(g) (cid:127)(cid:111) 2H2O(l) (cid:57) (iii) 1.23 (V) (cid:57) 16 Mark Scheme January 2013 Guidance Voltmeter must be shown AND salt bridge must be labelled ALLOW any correct circuit for a cell ALL labels required In H2 half cell, DO NOT ALLOW just acid ALL conditions required ALLOW if 1 mol dm3/1M mentioned for just one solution Look also on diagram in addition to answer lines DO NOT ALLOW 1 mol for concentration ALLOW multiples for each equation State symbols NOT requiredIGNORE even if wrong If oxygen and hydrogen equations are written on the wrong lines ALLOW 1 mark if both correct ALLOWsign shown instead of arrow as long as equation is shown the right way around ALLOW one mark if both acid equations are given i.e. oxygen electrode: O2(g) + 4H+(aq) + 4e (cid:111) 2H2O(l) AND hydrogen electrode: H2(g) (cid:111) 2H+(aq) + 2e ALLOW multiples, e.g. H2 + O2 (cid:111) H2O IGNORE state symbols DO NOT ALLOW if H2O OR OH OR e are shown on both sides This is the ONLY correct answer<br />
F325 Mark Scheme January 2013 Question (c) A fuel cell reacts a fuel/H2 with oxygen to produce a voltage/electrical energy (cid:57) Answer Marks (d) (e) Fossil fuels used to make hydrogen OR fossil fuels required to make fuel cell (cid:57) Correctly calculates amount of Cr = 1.456/52.0 = 0.028(0) (cid:57) NOTE: The remaining marks are ONLY available if a 3:2 molar ratio has been used 3 mol X reacts with 2 mol Cr3+ OR 3 mol X (cid:127)(cid:111) 2 mol Cr (cid:57) Correctly calculates amount of X = amount of Cr x 1.5 = 0.028(0) x 1.5 = 0.042(0) (cid:57) Correctly calculates Molar mass/Ar of X = 1.021/0.042(0) = 24.3 (g mol1) AND X identified as Mg (cid:57) Total 14 17 Guidance ALLOW a fuel cell requires constant supply of a fuel/H2 (and oxygen)/reactants OR operates continuously as long as a fuel/H2 (and oxygen) are added DO NOT ALLOW storage cells can be recharged (Not all storage cells can be recharged) Response requires link between fossil fuels / carbon- containing compounds and manufacture of the fuels cell or H2 i.e. energy required to make H2 is not sufficient FULL ANNOTATIONS MUST BE USED --------------------------------------------------------------------------- ALLOW equation: 2Cr3+ + 3X (cid:127)(cid:111) 3X2+ + 2Cr Note: 3rd marking point subsumes the 2nd marking point ALLOW magnesium OR Mg2+ Mg with no evidence of how 24.3 had been calculated does not score this mark ALLOW ECF from incorrect amount of Cr for 2nd, 3rd and 4th marks -------------------------------- Common error 3:2 ratio inverted between 2nd and 3rd marks: 3 marks: 3rd mark ECF: 0.028(0)1.5 = 0.0187 (mol) (cid:57) Molar mass of X = 54.7 (g mol1) AND X = Mn (cid:57)<br />

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