8 (2410U20-1) 10. (a) The enthalpy of neutralisation of an acid is defined as the enthalpy change when 1 mol of aqueous H+ ions is neutralised by aqueous OH– ions according to the equation shown. The reaction is exothermic. © WJEC CBAC Ltd. 08 OH–(aq) H+(aq) H2O(I) + Some students followed the instructions below to determine the enthalpy change of neutralisation of methanoic acid, HCOOH. 1. Weigh 24.7 g of methanoic acid and mix with water to make 250 cm3 of solution. Record the temperature of the solution. 2. Transfer eight 25.0 cm3 portions of this solution into eight insulated cups. 3. Using a burette add 5.0 cm3 of aqueous sodium hydroxide to the solution in the first cup. Stir and record the maximum temperature reached. 4. Add the following volumes of aqueous sodium hydroxide to each of the remaining cups in turn: 10.0 cm3 15.0 cm3 20.0 cm3 25.0 cm3 30.0 cm3 35.0 cm3 40.0 cm3 Stir and record the maximum temperature reached in each cup. 5. Plot a graph of maximum temperature reached against volume of sodium hydroxide added. Their results are plotted in the graph. 20 25 30 35 40 0 10 20 30 40 50 Maximum temperature / °C Volume of NaOH added / cm3 (2410U20-1) Turn over. 2410 U201 0 9 9 Examiner only (i) Name the apparatus used to transfer exactly 25.0 cm3 of methanoic acid solution into the insulated cups. [1] ..................................................................................... (ii) State why a higher maximum temperature is recorded when increasing volumes of sodium hydroxide are added. [1] (iii) Explain why the maximum temperature recorded decreases when more than 30 cm3 of sodium hydroxide is added. [2] (iv) On the graph, draw one straight line through the points that show an increase in maximum temperature and another straight line through the points that show a decrease in maximum temperature. [1] (v) From the graph, deduce the volume of sodium hydroxide needed to neutralise 25.0 cm3 of the methanoic acid solution and the temperature increase at that point. Assume that the initial temperature of every 25.0 cm3 of methanoic acid solution is 22.0 °C. [2] Volume = ........................................................ cm3 Temperature increase = ........................................................ °C © WJEC CBAC Ltd. 09 10 (2410U20-1) Examiner only (vi) Use your answers to part (v) to calculate the amount of heat released by the neutralisation reaction. [2] Heat released = ........................................................ J (vii) Calculate the number of moles of methanoic acid in 25.0 cm3 of the solution and hence the enthalpy change of neutralisation of methanoic acid. [3] Enthalpy change of neutralisation = ........................................................ kJ mol–1 © WJEC CBAC Ltd. 10 (2410U20-1) Turn over. 2410 U201 11 11 Examiner only (b) The experiment is repeated using hydrochloric acid instead of methanoic acid and a more negative value of the enthalpy change of neutralisation is calculated. Suggest and explain a reason for this difference. [2] (c) (i) Write the equation for the reaction that occurs when solid copper(II) carbonate is added to aqueous methanoic acid to form aqueous copper(ll) methanoate, (HCOO)2Cu. Include state symbols. [2] (ii) State what is observed during the reaction in part (i). [2] © WJEC CBAC Ltd. 11 18 12 (2410U20-1) 11. (a) Compound X contains carbon, hydrogen and oxygen only. It has no reaction with acidified potassium dichromate. Simplified versions of its mass spectrum, IR spectrum and 13C NMR spectrum are shown. © WJEC CBAC Ltd. 12 0 20 40 60 80 Intensity Mass spectrum m/z 500 1000 1500 2000 3000 4000 Wavenumber / cm–1 IR spectrum Transmittance 200 180 160 140 120 100 80 60 40 20 0 δ / ppm 13C NMR spectrum Intensity (2410U20-1) Turn over. 13 Examiner only Identify compound X. You must use information from all the sources given and explain how you used it. [8] Compound X © WJEC CBAC Ltd. 13 14 (2410U20-1) Examiner only (b) On the axes below, complete the low resolution 1H NMR spectrum you would expect for the compound you identified in part (a). You should indicate where you would expect to see peaks and the relative intensities of the peaks. [2] © WJEC CBAC Ltd. 14 10 11 10 9 8 7 6 5 4 3 2 1 0 δ / ppm Intensity (2410U20-1) Turn over. 15 © WJEC CBAC Ltd. 15 BLANK PAGE PLEASE DO NOT WRITE ON THIS PAGE