OCR A Jun 2013 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2013 Paper 2 Q1

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Answer all the questions. 1 Crude oil is a complex mixture of many hydrocarbons. Crude oil is processed by the petroleum industry to make fuels and petrochemicals. (a) The straight-chain alkane, A, is present in crude oil. A has molecules with ten carbon atoms. (i) What is the molecular formula of A?[1] (ii) B is a branched-chain isomer of A. Draw the skeletal formula of a possible structure for B. Name your structure. name[2] (iii) The branched-chain isomer B has a lower boiling point than the straight chain alkane A. Explain why.[2] (b) A chemist heats a pure sample of C15H32 in the presence of a catalyst. A reaction called cracking happens. (i) Construct an equation to show the cracking of C15H32.[1] (ii) When cracking takes place, a large number of products are formed. Suggest why a large number of products are formed.[1]OCR 2013<br />
(c) The petroleum industry processes straight-chain alkanes into cyclic hydrocarbons. For example, octane can be processed into a cyclic hydrocarbon and hydrogen. (i) Suggest the structure of this cyclic hydrocarbon. (ii) Why does the petroleum industry process straight-chain alkanes into cyclic hydrocarbons?[1] [Total: 9] [1]OCR 2013 Turn over<br />

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F322/01 Mark Scheme June 2013 (i) C10H22 (cid:57) (ii) Correct skeletal formula (cid:57) Question 1 (a) Answer Correct name for structure drawn providing the structure is a branched chain isomer of C10H22 (cid:57) Marks IGNORE the name decane Guidance DO NOT ALLOW structural formula OR displayed formula Examples of skeletal formulae: ALLOW name even if structural or displayed formula drawn DO NOT ALLOW incorrect nomenclature eg 2-ethyloctane, 6- methylnonane, 2-methnonane, 2-methylnonan, 2-methynonane There are many more isomers that can be drawn<br />
F322/01 Mark Scheme June 2013 Question 1 (a) (iii) B has less surface (area of) contact OR ORA Answer AND B has fewer van der Waals forces OR B has weaker van der Waals forces OR ORA (cid:57) So less energy needed to break the intermolecular forces in B OR ORA (cid:57) Marks Guidance Both answers need to be comparisons Assume it refers to B ALLOW B has less points of contact AND fewer VDW DO NOT ALLOW less points of contact between atoms Reference to just surface area or closeness of molecules is not sufficient. IGNORE if not qualified IGNORE B more compact OR B has a shorter chain DO NOT ALLOW B is a smaller molecule DO NOT ALLOW B has fewer electrons Intermolecular forces is not sufficient for the first marking point must refer to van der Waals ALLOW ORA throughout in terms of A if specified ALLOW in B it takes less energy to overcome the intermolecular forces ALLOW it is easier to overcome the intermolecular forces DO NOT ALLOW so less energy is needed to break bonds DO NOT ALLOW intermolecular bonds<br />
F322/01 Question 1 (b) Answer (i) Correct equation for the cracking of C15H32 (cid:57) eg C15H32 (cid:198) C13H28 + C2H4 Mark Scheme Marks (ii) (idea that) any carboncarbon bond (in the chain) can break (cid:57) June 2013 Guidance ALLOW molecular formula OR correct structural OR displayed OR skeletal formula OR mixture of the above (as long as unambiguous) ALLOW any correct equation that has an alkane and alkene(s) (and hydrogen) as products OR has alkenes and hydrogen as products e.g. C15H32 (cid:198) C11H24 + 2C2H4 C15H32 (cid:198) C6H12 + C9H18 + H2 IGNORE state symbols ALLOW carbon chain can break in many different places ALLOW the position of breakdown of the carbon chain is random ALLOW the carbon chain can break in many different places ALLOW carbon chain can split in many different places Carbon chain is cracked in many places is not sufficient Molecule can break anywhere is not sufficient / cannot control where the molecule breaks is not sufficient Molecule can form many different chain lengths is not sufficient<br />
F322/01 Mark Scheme June 2013 Question Answer (c) (i) Any cyclic hydrocarbon with eight carbon atoms in all (cid:57) eg Marks Guidance ALLOW correct structural OR displayed OR skeletal formula OR mixture of the above (as long as unambiguous) ALLOW equation with the correct product DO NOT ALLOW if any other extra structure is included which is incorrect DO NOT ALLOW aromatic cyclooctatetraene but ALLOW this as a normal structural formula IGNORE hydrogen as an extra product IGNORE any name given<br />
F322/01 Mark Scheme June 2013 Guidance The answer must relate to combustion or burning ALLOW cyclic hydrocarbons allow smoother burning OR cyclic hydrocarbons increase octane number OR cyclic hydrocarbons reduce knocking OR cyclic hydrocarbons are less likely to produce pre-ignition OR cyclic hydrocarbons are more efficient fuels OR cyclic hydrocarbons burn better OR easier to burn OR cyclic hydrocarbon combust more easily OR improves combustion DO NOT ALLOW cyclic hydrocarbons ignite more easily ALLOW ora for straight chain hydrocarbons IGNORE cyclic hydrocarbons increase volatility of fuel IGNORE cyclic hydrocarbons have a lower boiling point Cyclic hydrocarbons are a better fuel on their own is NOT sufficient Cyclic hydrocarbons burn more cleanly on their own is NOT sufficient Question 1 (c) (ii) Cyclic hydrocarbons promote efficient combustion (cid:57) Answer Marks Total<br />

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