OCR A Jun 2017 Paper 2 Q3 (with explained solutions) |

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OCR A Jun 2017 Paper 2 Q3

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3 Alkanes are saturated hydrocarbons with the general formula CnH2n+2. (a) A student carries out an experiment to measure the enthalpy change of combustion, cH, of hexane. The student finds that combustion of 1.29 g of hexane changes the temperature of 200 g of water from 20.5 C to 65.5 C. (i) Calculate the enthalpy change of combustion, cH, of hexane, in kJ mol1. Give your final answer to an appropriate number of significant figures. cHkJ mol1 [4] (ii) The calculated value of cH for hexane from this experiment is different from the data book value. Suggest two reasons for this difference. 12[2]<br />
(b) Data book values for the standard enthalpy changes of combustion, cH , of the first four alkanes are shown in the table. Alkane cH / kJ mol1 methane 890 ethane 1560 propane 2219 butane 2877 (i) The values for the first three alkanes are plotted on the graph below. Plot the value for butane on the graph. relative molecular mass 20 40 60 80 100 1000 2000 cH / kJ mol1 3000 4000 5000 [1]<br />
(ii) Use the graph to estimate the energy released during complete combustion of 1.80 g of pentane. Show relevant working below and on the graph. energy released =kJ [3] (c) The equation for the complete combustion of cyclohexane is shown below. C6H12(l) + 9O2(g) 6CO2(g) + 6H2O(l) Standard enthalpy changes of formation, fH , are shown in the table. Substance fH / kJ mol1 C6H12(l) 156.3 CO2(g) 393.5 H2O(l) 285.8 Calculate the standard enthalpy change of combustion, cH , of cyclohexane. cH =kJ mol1 [3]<br />

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H032/02 Question Answer (a) (i) FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 2510 (kJ mol1) award 4 marks IF answer = 2508 / 2507 (kJ mol1) award 3 marks (not rounded to 3SF, ignore sign) IF answer = + 2510 (kJ mol1) award 3 marks (incorrect sign) IF answer = 2510000 (kJ mol1) award 3 marks (used J instead of kJ) ----------------------------------------------------------------------------- Moles n(C6H14) = 0.0150 molEnergy q calculated correctly = 37620 (J) OR 37.620 (kJ)Calculating H Correctly calculates H in kJ mol1 AND to 3 or more SFRounding AND sign calculated value of H rounded to 3 SF AND sign 10 Mark scheme June 2017 Marks Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC moles = 1.29/86.0 IGNORE trailing zeros q = 2004.1845.0 ALLOW correctly rounded to 3 sig figs: 37.6 kJ ALLOW ECF from incorrect q ALLOW ECF from incorrect molar mass or incorrect moles of hexane to 3 SF or more correctly rounded IGNORE sign at this intermediate stage IGNORE working H = 37.62/0.015 = 2508 (kJ mol1) H = 37.6/0.015 = 2507 (kJ mol1) H = 2510 (kJ mol1) Final answer must havesign and 3 SF<br />
H032/02 Question (ii) Any two from the following: AnswerHeat released to the surroundings Incomplete combustionNon-standard conditionsJune 2017 Mark scheme Marks Guidance ALLOW heat loss ALLOW incomplete reaction OR not everything burns IGNORE reference to evaporation (b) (i) Value for butane plotted accurately on the graphrelative molecular mass = 58 cH o = 2877 kJ mol1 Check accuracy:There must be a visible pointVertically: touching the 58 lineHorizontally: between 2800 and 2900 relative molecular mass = 72 (ii) FIRST, CHECK THE ANSWER ON ANSWER LINE 11<br />
H032/02 Question Mark scheme June 2017 Answer Marks Guidance cH o = 3509 kJ mol1 Expected value within range: ()3500 to ()3600 (kJ mol1) moles = 1.80/72.0 IGNORE sign ALLOW ECF from incorrectly calculated moles of pentane OR incorrectly estimated cH o IF energy released = 87.5 (minimum) to 90 (maximum) AND line is extrapolated to 72 (molar mass) award 3 marks IF energy released <87.5 OR > 90.0 check the estimated value of cH o from the graph Estimation of cH o extrapolated (straight) line of best fit (see graph) AND correctly estimated value cH o from graphCalculation of energy released n(C5H12) = 0.0250 molenergy released = 0.0250correctly estimated value of cH o 12<br />
H032/02 Question (c) Answer FIRST, CHECK THE ANSWER ON ANSWER LINE IF enthalpy change = 3919.5 (kJ mol1) award 3 marks IF enthalpy change = (+)3919.5 (kJ mol1) award 2 marks ------------------------------------------------------------------------------ Working for CO2 AND H2O seen anywhere (1 mark) Marks 6()393.5 AND 6()285.8 OR ()2361 AND ()1714.8 OR ()4075.8Calculates cH A further 2 marks for correct answer AND correct sign = (6393.5) + (6285.8)(156.3) =3919.5 (kJ mol1)Total 13 13 Mark scheme June 2017 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC IF there is an alternative answer, check to see if there is any ECF credit possible ALLOW 3 marks for cH = 3920 FINAL answer rounded to 3 SF Common incorrect answers are shown below ALLOW 2 marks for cH = 3924 From cH = (6394 + 6286)(156) Data rounded to 3 sig figs ALLOW 2 marks for cH = 4232.1 All data added together (6393.5) + (6285.8) + (156.3) ALLOW 1 mark for cH = (+)4232.1<br />

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