OCR A Jun 2015 Paper 5 Q6

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Three redox systems, C, D and E are shown in Table 6.1. 18 C Ag(NH3)2 D +(aq) + e Ag+(aq) + e (aq) + e Ag(CN)2 Ag(s) + 2NH3(aq) Ag(s) Ag(s) + 2CN(aq) Table 6.1 The two cells below were set up in an experiment to compare the standard electrode potentials of redox systems C, D and E. The signs on each electrode are shown. Ag salt bridge Ag Ag salt bridge Ag NH3(aq) Ag(NH3)2(aq) )+ CN(aq) Ag(CN)2(aq) ) NH3(aq) Ag(NH3)2(aq) )+ Ag+(aq) (a) List the three redox systems in order by adding the labels C, D and E to the table below. redox system E oo Most negative Least negative [1]OCR 2015<br />
 (b) A standard cell is set up between redox system D in Table 6.1 and a standard hydrogen half- cell. The standard cell potential of redox system D is +0.34 V. 19 The cell delivers a current for a length of time. The pH of the solution in the standard hydrogen half-cell decreases. (i) What is the pH of the solution in a standard hydrogen half-cell? (ii) Explain, in terms of electrode potentials and equilibrium, why the pH of the solution in the hydrogen half-cell decreases as this cell delivers current. pH =[1][2] (iii) Write the equation for the overall cell reaction that takes place in this cell.[1] (c) The CN ion is the conjugate base of a very toxic weak acid. In aqueous solutions of CN ions, an acidbase equilibrium is set up. (i) Complete the equation for this equilibrium and label the conjugate acidbase pairs. CN H2O+[1] (ii) Explain, in terms of equilibrium, why acidic conditions should not be used with cells containing CN(aq) ions.[1]OCR 2015 Turn over<br />
 20 (d) Direct-ethanol fuel cells (DEFCs) are being developed in which the fuel is ethanol rather than hydrogen. The half-equation for the reaction at the ethanol electrode of the DEFC is shown below: C2H5OH + 3H2O 2CO2 + 12H+ + 12e (i) State one important difference between a fuel cell and a modern storage cell.[1] (ii) Suggest one advantage of using ethanol, rather than hydrogen, in a fuel cell for vehicles.[1] (iii) The overall reaction in a DEFC is the same as for the complete combustion of ethanol. Write the equation for the overall reaction in a DEFC.[1] (iv) Deduce the half-equation for the reaction at the oxygen electrode in a DEFC.[1] (v) Using oxidation numbers, show that oxidation and reduction take place in a DEFC. Oxidation:Reduction:[2] [Total: 13]OCR 2015<br />
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