Chemistry 9701/52 October/November 2019: question paper and mark scheme

Question

Many organic reactions performed in the laboratory require heating under reflux. 4‑nitromethylbenzene can be converted directly to 4‑aminobenzaldehyde by heating it under reflux with an excess of solution A. Solution A is an aqueous alkaline mixture of Na2S•9H2O and S8. Ethanol is used as a solvent for the reaction. 4-nitromethylbenzene 4-aminobenzaldehyde CH3 CHO solution A O2N H 2N The apparatus for this reaction is shown in the diagram. reflux condenser tap funnel solution A round-bottomed flask 4-nitromethylbenzene dissolved in ethanol (a) (i) Show, using a labelled arrow, where the cooling water enters the reflux condenser. [1] (ii) After the complete addition of solution A to the solution in the round-bottomed flask, a vigorous exothermic reaction takes place. What should be done to control the vigorous exothermic reaction during the addition of solution A? . …… . …… [1] (iii) The mixture is heated to increase the rate of reaction. This is carried out under reflux. Other than to increase the rate of reaction, explain why a reflux process is used for heating the mixture. . …… . …… [1] (iv) A student suggests that placing a bung in the top of the condenser would improve the experiment. Comment on whether the student is correct. Explain your answer. . …… . …… . …… [1] (b) Once the reaction is complete, the round-bottomed flask is removed from the set‑up on page 2 and included in a new set-up for distillation. The distillation process separates the substances in the mixture in the flask. (i) Complete and label the diagram to show how the apparatus could be set up for distillation. [2] (ii) The mixture in the round-bottomed flask is heated until all of the first substance distils off. The heating is then stopped. After distillation a hot solution remains in the round-bottomed flask. The table shows some data relevant to the experiment. substance melting point / °C boiling point / °C CHO 72 138 H 2N H2O 0 100 CH3CH2OH –114 78 Use the data to identify the liquid that is first distilled off. Explain your answer. . …… . …… [1] (iii) 4‑aminobenzaldehyde is soluble in hot water, but not in cold water. Starting with the hot solution in the round‑bottomed flask, suggest how pure crystals of 4‑aminobenzaldehyde can be obtained. . …… . …… . …… [2] (c) Under acidic conditions, 4‑aminobenzaldehyde (C7H7NO) forms a solid compound with molecular formula C14H12N2O. 2C7H7NO C14H12N2O + H2O A student carries out two experiments according to the method on page 2 to make C7H7NO. In Experiment 1, the student obtains 6.00 g of solid. In Experiment 2, the student uses the same method as in Experiment 1 but adds acid to neutralise the alkali in the mixture in the flask after heating under reflux. The same quantities of reagents are used in Experiment 1 and Experiment 2. Suggest how the mass of solid obtained in Experiment 2 compares to that of Experiment 1. Explain your answer. …… [2] (d) 4‑aminobenzaldehyde can be reacted with a halogenoalkane to form 4-(dimethylamino)benzaldehyde, compound D. 4-aminobenzaldehyde compound D CHO CHO halogenoalkane H 2N (CH3)2N Some data is given that is relevant to this reaction. carbon‑halogen boiling point halogenoalkane bond energy / °C / kJ mol–1 CH3Cl –97 340 CH3Br 4 280 CH3I 43 240 Give two reasons why CH3I is the preferred halogenoalkane in the conversion of 4‑aminobenzaldehyde to compound D. 1 …… 2 …… [2] [Total: 13]

Question

Naphthalene, C10H8, is an aromatic organic compound that is a solid at room temperature and pressure. The enthalpy change of fusion, ΔHfusion, of naphthalene is the enthalpy change that occurs when naphthalene melts. It cannot be directly measured. C10H8(s) C10H8(l) The melting point of naphthalene depends on its purity. When other compounds dissolve in liquid naphthalene its melting point is lowered. Diphenylamine, (C6H5)2NH, dissolves in liquid naphthalene. By varying the amount of diphenylamine and measuring the melting points of the different mixtures, the enthalpy change of fusion of naphthalene can be calculated. (a) The equation that links the melting point of a naphthalene-diphenylamine mixture with its enthalpy change of fusion, ΔHfusion, is shown. ΔHfusion log Y = A – 2.30 × RTm Y = mole fraction of naphthalene R = molar gas constant, 8.31 J K–1 mol–1 Tm = melting point of naphthalene in K A is a constant (i) A series of experiments is performed using the same amount, 0.100 mol, of naphthalene each time. Calculate the mass of naphthalene, C10H8, that should be used for each of these experiments. [Ar: C, 12.0; H, 1.0] mass of naphthalene = …… g [1] (ii) The melting point and freezing point of a substance are the same. The melting point, Tm, of a substance can be found by recording the temperature at which the substance freezes, measured when crystals first start to appear on cooling. The results of a series of experiments using 0.100 mol of naphthalene and different masses of diphenylamine are shown. Process the results to complete the table. Record all your data to three significant figures. The mole fraction of naphthalene, Y, is calculated as shown. nN Y= nN + nD nN = amount in moles of naphthalene, C10H8, = 0.100 nD = amount in moles of diphenylamine, (C6H5)2NH temperature at amount of mole fraction which crystals (C6H5)2NH, nD log Y of C10H8, Y appear, Tm / mol / 10–3 K–1 /K 0.00 1.00 353 2.83 0.00 0.00888 349 0.0178 345 0.0266 341 0.0355 338 0.0444 334 0.0533 331 0.0621 329 0.0769 325 [3] (b) Plot a graph on the grid to show the relationship between and log Y. Use a cross (×) to plot each data point. Draw the straight line of best fit. 0.000 –0.050 –0.100 log Y –0.150 –0.200 –0.250 2.80 2.85 2.90 2.95 3.00 3.05 3.10 1 Tm / 10 K –3 –1 [2] (c) (i) Use the graph to determine the gradient of the line of best fit. State the co‑ordinates of both points you used in your calculation. co-ordinates 1 …… co-ordinates 2 …… gradient = …… K [2] (ii) Use your answer to (c)(i) to determine the value of the enthalpy change of fusion of naphthalene, ΔHfusion, in kJ mol–1. ΔHfusion = …… kJ mol–1 [1] (d) (i) Do you consider the results obtained to be reliable? Explain your answer. . …… . …… . …… [1] (ii) Different literature values for the enthalpy change of fusion of naphthalene suggest that 10.00 g of naphthalene require between 1.45 kJ and 1.47 kJ to melt. Use this information to calculate the range of ΔHfusion values of naphthalene, C10H8, given in literature. Use your values to comment on the accuracy of the experimental procedure. [Ar: C, 12.0; H, 1.0] If you were not able to calculate ΔHfusion in (c)(ii), you may use 18.4 kJ mol–1, but this may not be the correct answer. range …… kJ mol–1 comment . …… . …… [2] (e) The enthalpy change calculated in this reaction is actually ΔH1, shown in the Hess’ cycle. ΔHfusion C10H8(s) C10H8(l) ΔHmixing ΔH1 C10H8(l) mixed with diphenylamine It is assumed that the enthalpy change when C10H8(l) and diphenylamine are mixed, ΔHmixing, is zero, and therefore ΔH1 = ΔHfusion. State how the value of ΔH1 compares to the value of ΔHfusion if the mixing of naphthalene and diphenylamine is endothermic. Explain your answer. …… [2] (f) A student incorrectly uses a value for the Mr of diphenylamine that is too low. This produces incorrect values for the mole fraction, Y. (i) Predict the effect this will have on the calculated values of Y. Explain your answer. nN Y= nN + nD nN = amount in moles of naphthalene nD = amount in moles of diphenylamine . …… . …… . …… [1] (ii) The student uses the incorrectly calculated value of Y from (f)(i) in the determination of ΔHfusion. ΔHfusion log Y = A – 2.30 × RTm Predict how the student’s calculated value of ΔHfusion is different from the actual value. Explain your answer. . …… . …… . …… [2] [Total: 17]