Cambridge A Level Chemistry 9701 — 2020 May/June Paper 5 · Variant 3

1 Trichloromethane and propanone are both organic liquids. The molecules within each liquid are attracted to each other by relatively weak permanent dipole-dipole interactions. When trichloromethane is mixed with propanone a strong electrostatic attraction forms between the two different molecules. trichloromethane propanone Cl CH3 O C Cl C H •• Cl CH3 electrostatic attraction A student plans to perform an experiment to investigate the strength of this electrostatic attraction by finding the temperature change when equal volumes of trichloromethane and propanone are mixed together. (a) (i) State and explain your prediction for the temperature change for this experiment. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) The student is given only the following equipment and chemicals for the experiment. 1 × 25 cm3 beaker 2 × thermometers 2 × 25 cm3 measuring cylinders 50 cm3 trichloromethane 50 cm3 propanone Outline the method the student should use in this one experiment to find the temperature change when trichloromethane is mixed with propanone. Give details of the volumes of liquids used and any readings taken. volumes used ...................................................................................................................... ............................................................................................................................................. readings taken ..................................................................................................................... ............................................................................................................................................. ............................................................................................................................................. method used ....................................................................................................................... ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. [3] (b) The apparatus used leads to significant heat loss. State one improvement the student could make to the apparatus to reduce heat loss. .................................................................................................................................................... .............................................................................................................................................. [1] (c) Trichloromethane and propanone are both volatile and flammable. State one relevant precaution that should be taken when carrying out this experiment. .................................................................................................................................................... .............................................................................................................................................. [1] (d) State one change, apart from reducing heat loss, that could be made to improve the accuracy of this experiment. .................................................................................................................................................... .............................................................................................................................................. [1] (e) In another experiment, a student uses 37.50 g of trichloromethane and 19.75 g of propanone and determines that the energy released is 1.67 kJ. (i) Calculate the number of moles of each compound in this mixture. Mr trichloromethane = 119.5 Mr propanone = 58.0 moles of trichloromethane = .............................. mol moles of propanone = .............................. mol [1] (ii) Calculate the enthalpy change, ∆H, of the electrostatic attraction formed between trichloromethane and propanone. You must include a sign in your answer. ∆H = .............................. kJ mol–1 [2] (f) Suggest an experiment the student could carry out to test whether the number of moles of trichloromethane affects the temperature change. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 11]

Mark scheme: 1(a)(i) AND Electrostatic attraction between the different molecules is stronger electrostatic attractions within each type of molecule. 1(a)(ii) Two equal volumes and a total volume < 25 cm3 1 Measure initial temperature of liquids (in measuring cylinders) – leave to equilibrate 1 Mix two liquids in the beaker, monitor temperature every minute until temp starts to fall / read the maximum temperature 1 1(b) one of: • Use a polystyrene cup instead of the glass beaker / insulate the glass beaker / use a dewar flask • Cover the beaker with a lid • Insulate the glass beaker 1 1(c) The experiment must not be performed near any naked flames OR Use an electric heater 1 1(d) Use a burette / graduated pipette instead of a measuring cylinder OR Use larger volumes of liquids OR Use a more accurate thermometer 1 1(e)(i) 37.50 / 119.5 = 0.314 AND 19.75 / 58 = 0.341 1 1(e)(ii) M1 Use of limiting factor (0.314) for number of moles M2 ∆H = –1.67/0.314 = –5.32 kJmol-1 2 1(f) keep volume of trichloromethane same and vary volume of propanone and record temperature change 1

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2 Nitrogen dioxide can be prepared by strongly heating anhydrous lead nitrate, Pb(NO3)2(s). The thermal decomposition occurs according to the equation shown. 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g) The nitrogen dioxide, NO2, can be separated from the oxygen by cooling the gas mixture produced until the NO2 condenses and the oxygen does not. melting point / K boiling point / K nitrogen dioxide 262 294 oxygen 54 90 (a) Draw a labelled diagram of the laboratory apparatus (assembled) that could be used to prepare liquid nitrogen dioxide from the thermal decomposition of anhydrous lead nitrate. [2] (b) At room temperature, nitrogen dioxide exists in equilibrium with dinitrogen tetroxide according to the equation shown. 2NO2(g) N2O4(g) brown colourless For this equilibrium, Kp = pN2O4 / p2NO2. pN2O4 and pNO2 are measured in kPa. State the units of Kp. .............................................................................................................................................. [1] A student plans to investigate the variation of Kp with temperature. (c) (i) A sample of the mixture of nitrogen oxides is introduced into a gas syringe at 295 K and the gas syringe is sealed so that it is both airtight and watertight. The volume occupied by the mixture is measured at different temperatures. The Kp value is calculated at each temperature. Name the apparatus you would use to heat the gas syringe at different temperatures between 295 K and 370 K so that a volume reading of the gas syringe could be easily taken. ....................................................................................................................................... [1] (ii) The results obtained are shown in the table. 1 Complete the table by calculating values for and log Kp. T 1 Record the value of to three significant figures. T Record the value of log Kp to two decimal places. 1 T / K / K–1 Kp log Kp T 377 0.076 361 0.122 344 0.257 330 0.741 315 1.506 312 3.490 295 9.025 [2] 1 (iii) Plot a graph on the grid of log Kp against . T Draw a line of best fit through the plotted points. [2] 0.0 1 T

Mark scheme: 2(a) M1 Heating a tube containing lead nitrate. Labels: Lead nitrate AND Heat A Bunsen for heat M2 Use of iced water for cooling an unsealed collection vessel Labels: Iced water A Cold water 2(b) kPa–1 1 2(c)(i) Thermostatically controlled water bath 1 Question Answer Marks 2(c)(ii) T / K 1 / T / K–1 Kp Log10Kp 377 0.00265 0.076 –1.12 361 0.00277 0.122 –0.91 344 0.00291 0.257 –0.59 330 0.00303 0.741 –0.13 315 0.00317 1.506 0.18 312 0.00320 3.490 0.54 295 0.00339 9.025 0.96 M1 Column 2 to 3 SF M2 Column 4 to 2 DP 2 2(c)(iii) M1 7 points plotted correctly 1 M2 Line of best fit drawn 1 2(d)(i) Not accurate as there is a lot of scatter about the line of best fit 1 2(d)(ii) Temperature of each experiment is difficult to record accurately 1 2(d)(iii) Repeat and take an average 1 2(e)(i) M1 Co-ordinates read and recorded correctly 1 M2 Gradient calculated to 3 SF 1 Question Answer Marks 2(e)(ii) M1 Rearrangement to show –∆H/2.303R = Gradient 1 M2 Gradient x 2.303 x 8.31 correctly calculated 1 2(f)(i) M1 Kp is inversely proportional to temperature 1 M2 Because ∆H has a negative value. 1 2(f)(ii) M1 Darker brown 1 M2 Equilibrium shifts to left hand side (at higher temperatures). 1

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