Edexcel Jan 2013 Paper 3 Q3 (with explained solutions) |

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Edexcel Jan 2013 Paper 3 Q3

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3 Weak acids such as ethanoic acid cannot be titrated with weak bases such as ammonia using an indicator since there is never any distinct colour change. An alternative technique is to use thermometric titration as follows. 1. 30.0 cm3 of dilute ethanoic acid is placed in a polystyrene cup and its temperature measured. 2. 5.00 cm3 of ammonia solution of concentration 1.05 mol dm3 is then added to the acid, the mixture stirred and the temperature measured again. 3. Further 5.00 cm3 portions of ammonia are added, followed by measurement of the temperature, until a total of 35.0 cm3 has been added. The results of this experiment are tabulated below. Volume of NH3(aq) added /cm3 0.00 5.00 10.0 15.0 20.0 25.0 30.0 35.0 Temperature /oC 20.7 22.4 24.0 25.7 26.4 25.3 24.0 22.7 (a) (i) Plot these data on the axes below. Draw two straight lines through the points on your graph. Extrapolate the lines until they intersect, to enable you to determine the end-point volume. (2) Temperature / oC 2827262524232221200 10 15 20 25 30 35 Volume of ammonia / cm3 *P41216A01116* 11 Turn over<br />
(ii) State the volume of the ammonia solution at the end-point. (2)(iii) Explain why the temperature rises until the end-point is reached. (1)(iv) Explain why the temperature falls when more ammonia solution is added after the end-point. (2)(b) In a similar experiment, 25.0 cm3 of ethanoic acid of concentration 2.00 mol dm3 was reacted with 25.0 cm3 of 2.00 mol dm3 aqueous ammonia. The initial temperature was 20.6 C and the temperature at the end-point was 29.8 C. 12 (i) Use the expression below to calculate the heat energy evolved in this reaction. (Assume that the density of the reaction mixture is 1.00 g cm3 and that the specific heat capacity of the mixture is 4.18 J g1 C1.) energy transferred = massspecific heat capacitytemperature change in joules (2) *P41216A01216*<br />
(ii) Calculate the number of moles of ethanoic acid used in this reaction. (iii) The reaction that occurs is CH3COOH(aq) + NH3(aq)CH3COONH4(aq) Use your values from (b)(i) and (ii) to calculate the enthalpy change per mole for this reaction. Include a sign and units in your answer. Give your answer to three significant figures. (1) (3) (Total for Question 3 = 13 marks) *P41216A01316* 13 Turn over<br />

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Mark Mark Mark Mark Question Number 3a(i) Acceptable answers Reject All points correctly plotted, 1 small square (allow one slip) (1) two best-fit straight lines which include all points(1) Acceptable Answers 18.50.5 (cm3)(2)1.5 (cm3)(1) further out scores zero Acceptable answers Reaction is exothermic OR heat is given out (until reaction is complete ) Reject Reject Acceptable Answers Reject No further reaction occurring / no further heat produced/all the ethanoic acid is used up. (1) the ammonia solution is cool/the volume increases. (1) Mark Independently. Just there is no ethanoic acid. Just heat is lost to the surroundings. Question Number 3a(ii) Question Number 3a(iii) Question Number 3a(iv)<br />
Reject Mark Reject Reject Mark Mark Total = 13 Marks Question Number 3b(i) Question Number 3b(ii) Question Number 3b(iii) Acceptable Answers q = 50 x 4.18 x (29.820.6) (J) (1) = 1922.8 (J) / 1923 (J )(1) Ignore SF except 1 SF OR 1.923 kJ Answer alone scores 2. TE for second mark if incorrect mass or temperature change e.g Use of 25 gives 961.4 (J) and scores 1 Ignore any sign and units if given Acceptable Answers (amount = (2.00 x 25/1000) =) 0.050/0.0500. Acceptable Answers H = - 1922.80.05 J mol1 = - 38456 (J mol1) (1) = - 38.5 (1) kJ mol1 (1) OR H = - 19230.05 J mol1 = - 38460 (J mol1) (1) = - 38.5 (1) kJ mol1 (1) If 1920 J quoted above the answer is38.4 kJ mol1 second mark is for number to 3 SF third mark is for sign and units<br />

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