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. [Table] melting point/K boiling point/K nitrogen dioxide 262 294 oxygen 54 90 A student plans to investigate the variation of Kₚ with temperature.

1. (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]
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]
3. (c)(i)A sample of the mixture of nitrogen oxides is introduced into a gas syringe at 295K 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 295K and 370K so that a volume reading of the gas syringe could be easily taken.[1]
4. (c)(ii)The results obtained are shown in the table. Complete the table by calculating values for 1/T and log Kₚ. Record the value of 1/T to three significant figures. Record the value of log Kₚ to two decimal places. [Table] T/K 1/T / K⁻¹ 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]
5. (c)(iii)Plot a graph on the grid of log Kₚ against 1/T. Draw a line of best fit through the plotted points.[2]
6. (d)(i)State and explain what your graph shows about the accuracy of the experimental results.[1]
7. (d)(ii)Suggest a reason for your answer in (d)(i).[1]
8. (d)(iii)Suggest what the student could do to improve the accuracy of the experiment.[1]
9. (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]
10. (e)(ii)The relationship between log Kₚ and 1/T is given by the equation shown. log Kₚ = (−∆H/2.303RT) + 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]
11. (f)(i)With reference to the results obtained in this experiment, state and explain how Kₚ varies with temperature.[2]
12. (f)(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 370K will differ, if at all, from the colour at room temperature. Explain your answer. difference in colour of mixture .......................... explanation ..........................[2]

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