CIE Mar 2017 v2 Paper 5 Q1

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The enthalpy change of reaction, Hr, for the decomposition of sodium hydrogencarbonate, NaHCO3(s), cannot be measured directly. PMT 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) A student must carry out two separate experiments and use the results of these experiments to determine the enthalpy change of reaction for the decomposition of sodium hydrogencarbonate. (a) Suggest why the enthalpy change of reaction, Hr, sodium hydrogencarbonate cannot be measured directly. for the decomposition of[1] In both experiments the student used a weighing boat. A weighing boat is a small vessel used to contain solid samples when they are weighed. Experiment 1 Reaction between sodium carbonate, Na2CO3(s), and dilute hydrochloric acid, HCl (aq) step 1 The student added approximately 3 g of Na2CO3(s) to a weighing boat and accurately measured the combined mass of the weighing boat and Na2CO3(s). This mass was recorded in Table 1.1. step 2 The student used a measuring cylinder to measure 50 cm3 of 2 mol dm3 HCl (aq). step 3 The experiment was carried out and the results were recorded in Table 1.2. step 4 The student reweighed the empty weighing boat and recorded the mass in Table 1.1. Table 1.1 mass results from Experiment 1 mass of weighing boat and Na2CO3(s) / g mass of empty weighing boat after addition of Na2CO3(s) to HCl (aq) / g 4.15 0.97 mass of Na2CO3(s) added / g Table 1.2 temperature results from Experiment 1 10 24.6 24.7 24.5 24.3 24.1 23.9 time / minutes temperature of mixture / C 20.0 19.8 19.8 19.8UCLES 2017 9701/52/F/M/17<br /> PMT (b) Outline how the student carried out step 3 of the experiment. You may find it helpful to write your answer as a series of smaller steps.Draw a labelled diagram of the apparatus. [3]UCLES 2017 9701/52/F/M/17 [Turn over<br /> The student plotted a graph of the results and drew two lines of best fit which were both extrapolated as shown. PMT 27 26 25 24 temperature of mixture / C 23 22 21 20 19 4 6 time / minutes 10 (c) Use the graph to determine the theoretical temperature increase at 4 minutes. theoretical temperature increase =C [1]UCLES 2017 9701/52/F/M/17<br /> PMT (d) Use Table 1.1 on page 2 to determine the mass of Na2CO3(s) which was added to the HCl (aq). Use this value and your answer to (c) to determine the enthalpy change, H1, for the reaction shown. Na2CO3(s) + 2HCl (aq) 2NaCl (aq) + H2O(l) + CO2(g) Give your answer to three significant figures. [Assume that the specific heat capacity of the solution is 4.18 J g1 K1.] [Ar: Na, 23.0; C, 12.0; O, 16.0] H1 =kJ mol1 [3] (e) (i) Explain why the student did not add the Na2CO3(s) to the HCl (aq) at 0 minutes.[1] (ii) Suggest why the temperature measured at 5 minutes was lower than the temperature measured at 6 minutes.[1]UCLES 2017 9701/52/F/M/17 [Turn over<br /> PMT Experiment 2 Reaction between sodium hydrogencarbonate, NaHCO3(s), and dilute hydrochloric acid, HCl (aq) step 1 step 2 The student weighed an empty weighing boat and recorded the mass in Table 1.3. The student added exactly 4.20 g of NaHCO3(s) to the weighing boat and recorded the mass in Table 1.3. step 3 The student carried out the same experimental procedure as in steps 2 and 3 of Experiment 1. Table 1.3 mass results from Experiment 2 mass of empty weighing boat / g mass of weighing boat and NaHCO3(s) / g 0.95 5.15 mass of NaHCO3(s) added / g (f) Explain why the method of determining the mass of solid added in Experiment 2 is less accurate than the method of determining the mass of solid added in Experiment 1.[1] (g) (i) In Experiment 2 a 50 cm3 measuring cylinder was used to measure the 50 cm3 of HCl (aq). The 50 cm3 measuring cylinder had 1 cm3 graduations. Calculate the maximum percentage error in measuring 50 cm3 of HCl (aq) with this 50 cm3 measuring cylinder. maximum percentage error =% [1] (ii) Explain why measuring the concentration of the 2 mol dm3 HCl more precisely would not affect the result of the experiment.[1] (iii) Suggest what the student should change to reduce the percentage error associated with the temperature readings without changing the apparatus.[1]UCLES 2017 9701/52/F/M/17<br /> (h) The student used the results from Experiment 2 and correctly determined the enthalpy change for the reaction between NaHCO3(s) and HCl (aq), H2, to be +24.2 kJ mol1. NaHCO3(s) + HCl (aq) NaCl (aq) + H2O(l) + CO2(g) H2 = +24.2 kJ mol1 Use the axes to draw a sketch graph of the expected results of Experiment 2. PMT temperature of mixture / C 4 time / minutes [2]UCLES 2017 9701/52/F/M/17 [Turn over<br /> (i) Use H1 from (d) and H2 from (h) to determine the enthalpy change of reaction, Hr, for the decomposition of NaHCO3(s). PMT 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) An energy cycle has been drawn for you. 2NaHCO3(s) + 2HCl (aq) Hr Na2CO3(s) + H2O(l) + CO2(g) + 2HCl (aq) 2NaCl (aq) + 2H2O(l) + 2CO2(g) If you were unable to calculate H1 in (d), assume H1 = 26.7 kJ mol1. This is not the correct value of H1. Hr =kJ mol1 [2] [Total: 18]UCLES 2017 9701/52/F/M/17<br />

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