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Edexcel Jun 2013 Paper 4 Q17
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![SECTION B Answer ALL the questions. Write your answers in the spaces provided. 17 The equation for the combustion of hydrogen is H2(g) + O2(g) o H2O(l) (3) (3) calculate the standard entropy change of the system (S 9 (a) Use the standard molar entropies on page 2 and page 25 of the data booklet to system) for this reaction. Note that the standard molar entropies of the elements are given per atom so that the standard molar entropy of oxygen, S9[O2(g)] = +102.5 J mol1 K1. (b) The standard enthalpy change for the combustion of hydrogen is 285.8 kJ mol1. Use this value to calculate the entropy change of the surroundings for the combustion of hydrogen at 298 K. Give your answer to 3 significant figures and include a sign and units. 10 *P41572A01024*<br />
(c) Use your answers to (a) and (b) to calculate the total entropy change (S 9 total) for the combustion of 1 mol of hydrogen. Include a sign and units in your answer. *(d) By considering both the thermodynamic stability and the kinetic inertness of a mixture of hydrogen and oxygen, explain why hydrogen does not react with oxygen unless ignited. (2) (2)(Total for Question 17 = 10 marks) *P41572A01124* 11 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2013_QP_-_Unit_4_Edexcel_Chemistry_A-level_17-0.jpg)
![Section B Question Number 17(a) Question Number 17(b) Acceptable Answers Reject Mark Units are not required in (a) or (c) but if used should be correct. Penalise incorrect units in (a), (b) & (c) once only IGNORE case of J and K order of units First mark: 65.3/ 130.6 and 69.9 (J mol1 K1) (1) Second mark: S = 69.9(130.6 +102.5) (1) Third mark: S = 163.2 = 163 (J mol1 K1) (1) Correct answer with no working scores 3 Ignore SF except 1 SF TE at each stage If 65.3 used instead of 130.6 penalize once (answer is then S = 97.9 (J mol1 K1) +163 or any positive answer Acceptable Answers Reject Mark Ssurroundings =H / T or just numbers (1) = +285800/298 = +959.06 = +959 J mol1 K1 / +0.959 kJ mol1 K1 Correct value to 3SF (1) Correct units and positive sign (1) Correct answer with no working scores 3 answer with no sign 6CH04011306<br />
Question Number 17(c) Question Number 17(d) Acceptable Answers Reject Mark Stotal = Ssystem + Ssurroundings (1) Allow Sreaction for Ssystem Stotal = answer (a) + answer (b) = 163.2 + 959 = (+)795.8 = (+)796 (J mol1 K1) If Ssurroundings=+959.06 then Stotal = +795.9 (1) Correct answer with no working scores 2 Ignore SF except 1 SF TE on values in (a) & (b) no TE on incorrect equation If answer to (a) = -97.9 (J mol1 K1) Stotal = (+)861.1 (J mol1 K1) Acceptable Answers Reject Mark Reference to the stability of individual elements A mixture of hydrogen and oxygen is thermodynamically unstable because Stotal is positive OR Reaction between hydrogen and oxygen is thermodynamically feasible because Stotal is positive ALLOW S for Stotal (1) No TE on negative Stotal from (c) The mixture is kinetically inert /stable or reaction is (very) slow because the activation energy is (very) high (1) Mixture / reaction is kinetically inert / stable but thermodynamically unstable / feasible scores 1 mark IGNORE References to spark / flame providing the (activation) energy for reaction Total for Question 17 = 10 Marks 6CH04011306<br />
Question Number Acceptable Answers Reject Mark Reject Mark Ka = [H+]2 / [HC2O4 [H+][A]/ [HA] ] 2(aq) + H3O+(aq) (aq) + H2O(l)C2O4 18(a)(i) HC2O4 (or ) ALLOW H2O(aq) Equation (1) states (1) ALLOW for 1 mark (aq)C2O4 HC2O4 States mark is not stand alone but can be awarded if the equation has a minor error e.g. an incorrect charge 2(aq) + H+(aq) Acceptable Answers Question Number 18(a)(ii) Ka = [C2O4 2] [H3O+] / [HC2O4 ] OR Ka = [C2O4 No TE on incorrect equation in (a)(i) Penalise incorrect charges in (i) and (ii) once only 2] [H+] / [HC2O4 ] Question Number 18 (a) (iii) Reject Mark Acceptable Answers ] No TE on (a)(ii) Ka = 104.28 OR 5.24807 x 10-5 (mol dm3) (1) Ka = [H+]2 / [HC2O4 Ka = [H+]2 / 0.050 [H+] = (0.05 x 104.28) = 1.61988 x103 (mol dm3) (1) TE on incorrect Ka value pH = log 1.61988 x 103 = 2.7905 = 2.8 (1) For final mark TE on algebraic / arithmetical errors providing pH1.3 Correct answer with no working scores 3 Ignore SF except 1 SF 6CH04011306<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_MS_-_Unit_4_Edexcel_Chemistry_A-level_17-0.jpg)
