OCR A Jun 2015 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2015 Paper 2 Q1

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Answer all the questions. This question is about different alkanes present in crude oil. (a) Compound A, shown below, is one of the structural isomers of C7H16. (i) What is meant by the term structural isomers? Compound A[1] (ii) Name compound A.[1] (b) The structural isomers of C5H12 have different boiling points. Draw the skeletal formula of the structural isomer of C5H12 with the highest boiling point. (c) A molecule of an alkane has 24 carbon atoms. State the empirical formulae of this alkane.[1] [1]OCR 2015<br />
(d) Alkanes are used as fuels. (i) Construct an equation for the complete combustion of octane C8H18.[1] (ii) Combustion of 36.48 g of octane produced 2.50 mol of carbon dioxide. Show that this combustion was incomplete. [2] (e) Alkanes in crude oil can be used to manufacture ethene. Two stages are required. (i) Name the two stages.[1] (ii) Write an equation for the preparation of ethene from an alkane.[1] [Total: 9]OCR 2015 Turn over<br />

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F322 Mark Scheme June 2015 Question (a) (i) Answer (compounds or molecules having the) same molecular formula but different structural formulae (cid:57)(cid:3) (ii) 2,2,3trimethylbutane (cid:57)(cid:3) (b) (c) (d) C12H25 (cid:57)(cid:3) (cid:57)(cid:3) (i) C8H18 + 12O28CO 2 + 9H 2O (cid:57)(cid:3) Mark 1 1 1 Guidance ALLOW different structure OR different displayed formula OR different skeletal formula for structure DO NOT ALLOW any reference to spatial/space Same formula is not sufficient (no reference to molecular) Different arrangement of atoms is not sufficient (no reference to structure/structural) ALLOW trimethylbutane as the ONLY alternative response DO NOT ALLOW molecular formulae OR structural formula OR displayed formula OR mixture of the above IGNORE C24H50 ALLOW multiples e.g. 2C8H18 + 25O216CO 2 + 18H 2O IGNORE state symbols<br />
F322 Question (ii) Mark Scheme June 2015 Mark 2 Answer (n(C8H18) burned) = 0.32 (mol) (cid:57)(cid:3) (n(CO2) from complete combustion) = 2.56 or 2.6 mol OR (ratio nCO2/nC8H18 ) = 7.8(125) OR (n C8H18 produce 2.5 mol CO2) = 0.31(25) (cid:57)(cid:3) Guidance DO NOT ALLOW ECF from an incorrect moles of octane DO NOT ALLOW ECF from incorrect ratio from equation in (i) ALLOW the following alternate methodsMethod 1 (mass CO2 produced) = 110 g (cid:57)(cid:3) (mass CO2 from complete combustion) = 80.3244 = 112.64 or 112.6 or 113 g(cid:57)(cid:3)Method 2 (n C8H18 to produce 2.5 mol CO2) = 0.31(25) (cid:57)(cid:3) (mass of octane required to produce 2.50 mol CO2) = 35.6 OR 35.63 OR 35.625 g (cid:57)(cid:3)<br />
F322 Question (e) (i) Fractional distillation AND cracking (cid:57)(cid:3) Answer Mark Scheme June 2015 Mark ALLOW either order Guidance ALLOW any correct equation with correct formulae to show cracking forming C2H4 of the type: alkaneshorter alkane(s) + alkene, e.g. C10H22C8H18 + C2H4 10H22C6H14 + 2C2H4 C ALLOW C2H6C2H4 + H2 ALLOW correct molecular formulae OR structural OR displayed OR skeletal OR mixture of the above. IGNORE state symbols (ii) Correct equation showing cracking of an alkane to form ethene (cid:57)(cid:3) Total<br />

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