OCR A Jun 2011 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2011 Paper 2 Q1

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Answer all the questions. 1 Many organisms use the aerobic respiration of glucose, C6H12O6, to release useful energy. (a) The overall equation for aerobic respiration is the same as for the complete combustion of C6H12O6. (i) Write the equation for the aerobic respiration of C6H12O6.[1] (ii) Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.[2] (b) The table shows some enthalpy changes of combustion, Hc. substance Hc / kJ mol1 C(s) H2(g) C6H12O6(s) 394 286 2801 (i) What is meant by the term enthalpy change of combustion, Hc?[2]OCR 2011<br />
(ii) The enthalpy change of formation, Hf , of glucose, C6H12O6, cannot be determined directly. The equation for this enthalpy change is shown below. 6C(s) + 6H2(g) + 3O2(g) C6H12O6(s) Suggest why the enthalpy change of formation of C6H12O6 cannot be determined directly.[1] (iii) Use the Hc values in the table to calculate the enthalpy change of formation of C6H12O6. Hf =kJ mol1 [3] [Total: 9]OCR 2011 Turn over<br />

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Mark Scheme June 2011 F322 Question (a) (i) (ii) Answer C6H12O6 + 6O2 (cid:198) 6CO2 + 6H2O (cid:57) Bond breaking absorbs energy AND bond forming releases energy (cid:57) More energy released than absorbed (cid:57) Mark Guidance 1 2 IGNORE state symbols ALLOW bond breaking is endothermic AND bond forming is exothermic DO NOT ALLOW bond forming requires energy The second marking point is dependent on the correct identification of the energy changes during bond breaking and bond making ALLOW exothermic change transfers more energy than endothermic change OR bond forming transfers more energy than bond breaking OR (the sum of the) bond enthalpies in the products is greater than the (sum of the) bond enthalpies in the reactants OR (the sum of the) bond enthalpies of the bonds made is greater than (the sum of) the bond enthalpies of the bonds broken IGNORE reference to strong and weak bonds IGNORE reference to number of bonds broken or made IGNORE enthalpy of products is less than enthalpy of reactants ALLOW energy released DO NOT ALLOW energy required ALLOW element OR compound OR molecule DO NOT ALLOW one mole of atoms ALLOW reacts fully with oxygen ALLOW will make other compounds (containing carbon and hydrogen or carbon, oxygen and hydrogen) ALLOW reaction cannot be carried out experimentally IGNORE heat is lost to the surroundings (b) (i) (Enthalpy change) when one mole of a substance (cid:57) is completely combusted OR burns in excess oxygen (cid:57) (ii) Would make carbon dioxide and water instead OR activation energy (too) high OR rate is (too) slow OR do not react together (cid:57)<br />
F322 Question (b) (iii) Answer (+)2801 (cid:57) + ()3946 + ()2866 OR ()4080 (cid:57) 1279 (cid:57) Mark Scheme June 2011 Total Mark Guidance IGNORE sign IGNORE sign ALLOW full marks for 1279 with no working out (cid:57)(cid:57)(cid:57) Unit not needed ALLOW ECF enthalpy change of combustion of carbon dioxide and waterenthalpy of combustion of glucose ALLOW for 2 marks: +1279 cycle wrong way around OR +151 OR +691 one value not6 OR 6881 OR +6881 wrong sign for 2801 or 4080 OR +2121 (cid:57)(cid:57) correct cycle but not6 ALLOW for 1 mark: 151 OR 691 cycle wrong way around and one value not6 OR 2121 cycle wrong way around and not6 OR 3481 OR +3481 (cid:57) wrong sign and not6 Note: There may be other possibilities<br />

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