OCR A Jun 2012 Paper 2 Q5 (with explained solutions) |

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OCR A Jun 2012 Paper 2 Q5

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5 Methane and ethane are important fuels. 15 (a) Methane could be manufactured by the reaction between carbon dioxide and hydrogen. CO2(g) + 4H2(g) CH4(g) + 2H2O(g) Using the table of bond enthalpies, calculate the enthalpy change of reaction for this manufacture of methane. bond CH HH C=O OH average bond enthalpy / kJ mol1 +415 +436 +805 +464 enthalpy change of reaction =kJ mol1 [3]OCR 2012 Turn over<br />
16 (b) Methane is a greenhouse gas. Scientists are concerned that the concentration of methane in the atmosphere is slowly increasing. (i) Explain how atmospheric methane molecules can contribute to global warming.[2] (ii) One way that scientists hope to minimise global warming is by developing Carbon Capture and Storage, CCS, techniques. Describe two of these CCS techniques.[2]OCR 2012<br />
(c) Ethane reacts with bromine in the presence of ultraviolet radiation to form many organic products. 17 (i) Two of these products are bromoethane and hydrogen bromide. Describe the mechanism of the reaction between ethane and bromine that forms bromoethane and hydrogen bromide. Include in your answerthe type of bond fission that occurs equations for each step of the reaction the name of each step of the reaction. Your answer needs to be clear and well organised using the correct terminology.[7] (ii) Give two reasons why there are many organic products of the reaction between bromine and ethane.[2]OCR 2012 [Total: 16] Turn over<br />

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F322 Question 5 (a) Answer FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 162 (kJ mol-1) award 3 marks Energy associated with bond breaking = 3354 OR (2805) + (4436) (cid:57) Energy associated with bond making = 3516 OR (4415) + (4464) (cid:57) Enthalpy change = 162 (cid:57) (b) (i) Absorbs IR radiation (cid:57) Bonds vibrate (cid:57) Mark Scheme June 2012 Marks Guidance IF there is an alternative answer, check to see if there is any ECF credit possible using working below. IF ECF, ANNOTATE WITH TICKS AND CROSSES, etc IGNORE sign IGNORE sign ALLOW ECF from wrong additions of energy associated with bond breaking and/or from bond making ALLOW two marks for (+)162, (+)6870, 6870 or (+)766 ALLOW one mark for 766 IGNORE absorbs heat ALLOW IR re-radiated DO NOT ALLOW absorbs UV radiation DO NOT ALLOW blocks IR radiation ALLOW bonds stretch OR bonds bend IGNORE molecule vibrates/rotates DO NOT ALLOW bonds break 22<br />
F322 Question 5 (b) (ii) Any two from: Answer (liquid) injected deep into the oceans (cid:57) Stored in (old) geological formations OR stored underground in rocks OR stored in (old) mines OR stored in (old) oil wells (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) Mark Scheme June 2012 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 ALLOW store deep in the oceans OR on the sea-bed (cid:57) ALLOW stored deep under the sea DO NOT ALLOW dissolve CO2 in the sea OR stored in ocean ALLOW stored under the sea bed ALLOW pumped into oil wells to force last bit of oil out IGNORE mineral storage 23<br />
F322 Question 5 (i) (c) Mark Scheme June 2012 Answer Homolytic (cid:57) Br2 (cid:127)(cid:111) 2Br (cid:57) Br + C2H6 (cid:127)(cid:111) HBr + C2H5 (cid:57) C2H5 + Br2 (cid:127)(cid:111) C2H5Br + Br (cid:57) Br + C2H5 (cid:127)(cid:111) C2H5Br OR Br + Br (cid:127)(cid:111) Br2 OR C2H5 + C2H5 (cid:127)(cid:111) C4H10 (cid:57) Two names of steps linked to appropriate equations (cid:57) OR three names of steps linked to appropriate equations (cid:57)(cid:57) Marks Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC IGNORE dot for radical and any state symbols for all equations If more than one termination step is written they must all be correct to be awarded the mark DO NOT ALLOW termination steps with H initiation step linked to correct equation propagation step linked to one equation in which there is a radical on the left and a radical on the right termination step linked to equation involving two radicals: If no equations are given to link the names of the step then award one mark for mention of all three steps If halogen other than bromine do not give equation mark for initiation and only give one mark for all three terms linked to appropriate equations If hydrocarbons other than ethane are used DO NOT ALLOW any marks for the equations in the propagation steps 24<br />
F322 Question 5 (c) (ii) Any two from: Answer More than one CH bond can be substituted OR multi- substitution can occur OR more than one substitution can happen (cid:57) Lots of termination steps (cid:57) termination steps can give products that will also react with (bromine) radicals (cid:57) Total Mark Scheme June 2012 Marks Guidance ALLOW equations or examples of multi substitution ALLOW an equation to illustrate formation of other products eg butane ALLOW examples of other products that can be formed in termination steps eg bromobutane ALLOW examples of products eg butane reacting with bromine radicals to give bromobutane 16 25<br />

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