Chemistry 9701/51 May/June 2020: question paper and mark scheme

Question

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 Cl C H •• O C 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]

Question

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 –1 T/K T /K Kp log Kp 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] 1.00 0.80 0.60 0.40 0.20 0.00 log Kp –0.20 –0.40 –0.60 –0.80 –1.00 –1.20 –1.40 0.00260 0.00270 0.00280 0.00290 0.00300 0.00310 0.00320 0.00330 0.00340 1 –1 /K T (d) (i) State and explain what your graph shows about the accuracy of the experimental results. . …… . …… . …… [1] (ii) Suggest a reason for your answer in (d)(i). . …… . …… [1] (iii) Suggest what the student could do to improve the accuracy of the experiment. . …… . …… [1] (e) (i) Use the graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be on your line of best fit. Give your answer to three significant figures. coordinates 1 …… coordinates 2 …… gradient = …… K [2] 1 (ii) The relationship between log Kp and is given by the equation shown. T log Kp = (–∆H / 2.303 RT ) + constant R = 8.31 J K–1 mol–1 Use the gradient determined to calculate a value for ∆H. If you were unable to determine a value for the gradient, use the value 2800 K. This is not the correct value. ∆H = …… kJ mol–1 [2] (f) (i) With reference to the results obtained in this experiment, state and explain how Kp varies with temperature. . …… . …… [2] (ii) With reference to (b) and the data for Kp in the table in (c)(ii) suggest how the colour of the equilibrium mixture at 370 K will differ, if at all, from the colour at room temperature. Explain your answer. difference in colour of mixture …… explanation …… . …… . …… [2] [Total: 19]