OCR A Jun 2012 Paper 5 Q6

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6 Standard electrode potentials for eight redox systems are shown in Table 6.1. You will need to use this information throughout this question. 14 redox system half-equation 1 2 3 4 5 6 7 8 Cr2O7 2H+(aq) + 2e Fe3+(aq) + e 2(aq) + 14H+(aq) + 6e O2(g) + 4H+(aq) + 4e Cu2+(aq) + 2e CO2(g) + 2H+(aq) + 2e HCOOH(aq) + 2H+(aq) + 2e Cr3+(aq) + 3e H2(g) Fe2+(aq) 2Cr3+(aq) + 7H2O(l) 2H2O(l) Cu(s) HCOOH(aq) HCHO(aq) + H2O(l) Cr(s) E o / V 0.00 +0.77 +1.33 +1.23 +0.34 0.22 +0.06 0.74 Table 6.1 (a) A student sets up a standard cell in the laboratory based on redox systems 2 and 8. His circuit allows him to measure the standard cell potential. (i) Draw a labelled diagram to show how the student could have set up this cell to measure its standard cell potential. (ii) Write down the overall cell reaction.[1] (iii) Write down the standard cell potential. [3] standard cell potential =V [1] (b) Select from Table 6.1, the strongest oxidising agent.[1]OCR 2012<br />
 15 (c) Using the redox systems in Table 6.1, construct an equation for a reaction between acidified dichromate(VI) ions and methanoic acid, HCOOH. Rather than using [O] or [H], your equation must show the actual reactants and products. [2] (d) A student added some chromium metal to an acidified solution containing copper(II) ions. A reaction took place. The student concluded that chromium is more reactive than copper. (i) Explain, in terms of their electrode potentials, why chromium is more reactive than copper in this reaction.[2] (ii) When this experiment was carried out, the student observed some bubbles of a gas. Suggest an explanation for this observation.[1] (e) Methanoic acid, HCOOH, has the common name of formic acid. Direct-Formic Acid Fuel Cells (DFAFCs) are being developed for use in small, portable electronics such as phones and laptop computers. In this fuel cell, methanoic acid (the fuel) reacts with oxygen to generate a cell potential. (i) Predict the standard cell potential of a DFAFC. standard cell potential =V [1] (ii) Suggest two advantages of using methanoic acid as the fuel in a fuel cell rather than hydrogen.[2]OCR 2012 [Total: 14] Turn over<br />

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