Edexcel Jun 2014 (R) Paper 1 Q21 (with explained solutions) |

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Edexcel Jun 2014 (R) Paper 1 Q21

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21 Propane is a saturated hydrocarbon with molecular formula C3H8. *(a) Explain the meaning of the terms saturated and hydrocarbon. (2) SaturatedHydrocarbon(b) Propane is sold in small cylinders for use as a fuel in camping stoves. The enthalpy change of combustion of propane can be measured by experiment using one of these cylinders. A known mass of propane is burned to heat a container of water, and the temperature rise of the water is measured. The results of the experiment are shown below. Mass of propane burned Temperature of water at start Final temperature of water Mass of water in container 0.33 g 18.0 C 45.1 C 100 g (i) How would the mass of propane which was burned be measured? (1)(ii) Calculate the energy transferred in the experiment, using the results above and the following expression. Energy transferred (J) = massspecific heat capacitytemperature change The specific heat capacity of water is 4.18 J g1 C1. (1) *P42990A01928* 19 Turn over<br />
(iii) Calculate the enthalpy change of combustion of propane, (cid:168)Hc, in kJ mol1. Give your answer to three significant figures and include a sign. (iv) The results of this experiment are inaccurate due to heat loss. Suggest one other source of error, other than measurement errors and limitations of the equipment. (3) (1)(c) Another way of calculating the enthalpy change of combustion for propane is to use mean bond enthalpy data. (i) Complete the equations in the Hess cycle below. The enthalpy change of +6490 kJ mol1 is the total energy required to break the bonds in propane and in oxygen. (1) C3H8(g) +O2(g) (cid:168)Hx 3CO2(g) + 4H2O(g) + 6490 kJ mol1 Z kJ mol1 3C() + 8H(g) + O(g) 20 *P42990A02028*<br />
(ii) Use the data in the table to calculate the enthalpy change, Z, in kJ mol1. Bond C O H O Mean bond enthalpy / kJ mol1 805 464 (iii) Use the cycle in (c)(i), and your answer to (c)(ii), to calculate the enthalpy change, (cid:168)Hx, in kJ mol1, for the combustion of propane. (iv) The data book value for the standard enthalpy change of combustion, c , for propane is 2219.2 kJ mol1. This value is more exothermic than that (cid:168)H (cid:57) calculated using mean bond enthalpy data. Give one reason for this. (1) (1) (1)(Total for Question 21 = 12 marks) *P42990A02128* 21 Turn over<br />

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Question Number 21(a) Acceptable Answers Reject Mixture of carbon and hydrogen only Reject Reject Reject Mark Mark Mark Mark (Contains) only (CC) single bonds/ onlybond(s) OR (Contains) no (C=C) double bond(s)/no triple bond(s) OR Cannot undergo addition (reactions) ALLOW Has maximum number of hydrogen atoms / has maximum amount of hydrogen /can form no more bonds / no pi-bonds. IGNORE references to alkanes (1) (Compound of) carbon and hydrogen ONLY/ENTIRELY/PURELY (1) Question Number 21(b)(i) Measure mass (of cylinder) before and after Acceptable Answers (burning) Question Number 21(b)(ii) Energy transferred = (100 x 4.18 x 27.1 =) Acceptable Answers 11327.8 (J) / 11.328 kJ Ignore SF except 1 SF Acceptable Answers Question Number 21(b)(iii) Mol propane = 0.33/ 44 = 0.0075 (1) Hc = ( 11.3278/ 0.0075) = ( 1510.4) = 1510 (kJ mol1) (1) Sign and 3SF (1) Allow TE from b(ii) 6CH0101R 1406<br />
Question Number 21(b)(iv) Incomplete combustion Acceptable Answers Mark Reject Evaporation of water Transfer losses Not under standard conditions Not all the fuel burns Reject Mark Question Number 21(c)(i) Question Number 21(c)(ii) Question Number 21(c)(iii) Allow carbon monoxide forms soot forms Ignore references to specific heat capacity of the apparatus or evaporation of propane Acceptable Answers C3H8(g) + 5O2(g)3CO2(g) + 4H2O(g) + 6490 kJ mol1 3C(g) + 8H(g) + 10 O (g) Balancing and state symbol required Acceptable Answers Z = (6x C=O + 8xO-H = 4830 + 3712) = (+)8542 (kJ mol1) Acceptable Answers HX = 6490 - 8542 = -2052 (kJ mol1) Allow TE from 21(c)(ii) Reject Reject Mark Mark 6CH0101R 1406<br />
Question Number 21(c)(iv) Bond energy calculation based on H2O(g) Acceptable Answers based on H2O(l) OR o Hc Allow Bond energy varies with environment/ mean bond energies do not equal actual bond energies for these reactants Ignore reference to standard conditions Reject Mark Total for Q21 = 12 marks 6CH0101R 1406<br />
Acceptable Answers UV light/ ultraviolet light/ (sun) light / UV radiation IGNORE References to heat and or pressure. Reject Mark Acceptable Answers Reject Mark Single electron Species/ particle with unpaired electron Allow atom Question Number 22(c)(i) Cl-Cl bond is weaker than a C-H bond / Acceptable Answers breaks more easily than a C-H bond OR Reverse argument Acceptable Answers CHCl3 + ClCCl3 + HCl (1) CCl3 + Cl2CCl4 + Cl (1) Max (1) if 2 equations based on methane. Acceptable Answers Question Number 22(c)(iii) CCl3 + ClCCl4 Question Number 22(d) Acceptable Answers 100% as only one product / 100% as no by product(s) / 100% as no waste product (formed) Reject Reject Reject Reject Question Number 22(a) Question Number 22(b) Question Number 22(c)(ii) Mark Mark Mark Mark Just atom economy is high(er) / no mention of 100% Total for Q22 = 7 marks 6CH0101R 1406<br />

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