OCR A Jan 2013 Paper 2 Q6 (with explained solutions) |

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OCR A Jan 2013 Paper 2 Q6

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6 Many reactions are catalysed by biological catalysts called enzymes. 14 Ethanol is manufactured by the enzyme-catalysed fermentation of glucose. C6H12O6 2C2H5OH + 2CO2 (a) Fermentation is an exothermic reaction. Explain why. Use ideas about the enthalpy changes associated with bond breaking and bond making in your answer.[2] (b) Carbon dioxide is a greenhouse gas that has been linked to climate change. (i) What happens within a carbon dioxide molecule when it absorbs infrared radiation?[1] (ii) Chemists are developing methods for carbon capture and storage to minimise climate change. Give two examples of carbon capture and storage. 12[2] (c) One benefit of using an enzyme to catalyse a reaction is that there is a large increase in the rate of reaction without the enzyme being used up. State two other benefits, other than cost, of using enzymes to catalyse reactions. 12[2]OCR 2013<br />
(d) Catalysts, such as enzymes, increase the rate of a reaction. 15 Explain why. Use a labelled Boltzmann distribution of molecular energies in your answer.[5] [Total: 12]OCR 2013 Turn over<br />

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F322/01 Question 6 (a) Marks Answer Bond breaking absorbs energy AND bond forming releases energy (cid:57) More energy released than absorbed (cid:57) The second marking point is dependent on the correct identification of the energy changes during bond breaking and bond making (b) (i) (C=O) bond vibrates (more) OR bond bends (more) OR bond stretches (more) (cid:57) 19 Mark Scheme January 2013 Guidance ALLOW bond breaking is endothermic AND bond forming is exothermic DO NOT ALLOW bond forming requires energy ALLOW more energy is released when the bond in the products are formed than is required to break the bonds in the reactants 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 OR more energy associated with bond making than with bond breaking 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 IGNORE molecule vibrates/rotates It refers to the molecule and is insufficient DO NOT ALLOW any reference to bond breaking. DO NOT ALLOW a stated bond if not present in CO2 eg CO, CH<br />
F322/01 Mark Scheme January 2013 Answer Question (ii) Any two from: (injected) deep into the oceans / sea (cid:57) (Stored) in geological formations OR (stored) deep in rocks OR (stored) in old mines OR (stored) in old oil wells OR old gas fields (cid:57) (Stored) by reaction with metal oxides OR reaction to form (solid) carbonates OR (stored) as a carbonate OR equation to show formation of metal carbonate (cid:57) Marks Guidance DO NOT ALLOW reference to carbon being storedthe answer must either refer to carbon dioxide or not mention the name of the stored substance. Assume it refers to CO2 DO NOT ALLOW dumping waste at the bottom of the sea ALLOW on the sea-bed DO NOT ALLOW dissolve CO2 in the sea OR (stored) in ocean DO NOT ALLOW geographical formations ALLOW stored under the sea (bed) ALLOW pumped into oil wells to force last bit of oil out DO NOT ALLOW buried underground DO NOT ALLOW react with metals to form carbonates IGNORE mineral storage 20<br />
F322/01 Question (c) Mark Scheme January 2013 Answer Any two from: Energy demand Low(er) temperature (can be used) OR reduces CO2 emissions (from burning fossil fuels) (cid:57) Specificity enzymes have a great deal of specificity (cid:57) Atom economy greater atom economy OR less waste (cid:57) Toxicity can reduce use of toxic solvents OR reduces use of toxic catalysts OR reduces the use of toxic reactants (cid:57) Marks Guidance ALLOW allows use of room temperature OR allows use of a lower pressure OR uses less fuel IGNORE lower energy demand OR lower activation energy IGNORE cheaper IGNORE less greenhouse gases OR reduces global warming ALLOW making specific isomers / enantiomers ALLOW for making pure products ALLOW generating specified products ALLOW increases atom economy ALLOW reduce use of hazardous/toxic/harmful/poisonous chemicals ALLOW enzymes are non toxic IGNORE can be reused 21<br />
F322/01 Question (d) Mark Scheme January 2013 Answer Ea cat Ea energy (number of) molecule Catalyst lowers the activation energy (because of a different reaction pathway) (cid:57) Diagram of Boltzmann distribution (cid:57) axes labelled (number of) molecules and energy (cid:57) Greater proportion of molecules with energy above activation energy with catalyst (cid:57) more effective collisions OR more successful collisions OR increased frequency of successful collisions (cid:57) extra molecules with energy above Ea Total 22 Marks Guidance Can be scored from the diagram by correctly labelling Ea cat closer to the origin than Ea Boltzmann distribution must start at origin AND must not touch x-axis at high energy DO NOT ALLOW Boltzmann distribution mark if two curves drawn DO NOT ALLOW Boltzmann distribution curve bending upwards at higher energy ALLOW particles instead of molecules DO NOT ALLOW the first use of atoms but credit atoms if used in a subsequent marking point DO NOT ALLOW enthalpy on x-axis instead of energy ALLOW more molecules with energy above activation energy (with a catalyst) OR more molecules overcome the activation energy (with a catalyst) OR more molecules have enough energy to react (with a catalyst) OR more molecules are able to react at lower energies More collisions OR more frequent collisions are not sufficient 12<br />

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