Chemistry 9701/52 October/November 2021: question paper and mark scheme

Question

It is possible to measure the enthalpy change of combustion, ∆Hc, of ethanol, C2H5OH, using the following apparatus. thermometer metal can water lid tripod wick spirit burner containing ethanol wooden blocks A student carries out an experiment to determine the value for ∆Hc of ethanol using the following instructions: ● Weigh the spirit burner with ethanol and lid, record the starting mass to two decimal places. ● easure 100.00 cm3 of water and place it into the metal can. M ● Place a thermometer, with 0.1 °C graduations, into the water and stir it, wait for 2 minutes. ● Record the temperature of the water. ● Light the wick and allow the flame to heat the water. ● Continue to stir the water using the thermometer. ● A fter the temperature has risen by approximately 20 °C place the lid on the flame to extinguish it. ● Record the maximum temperature of the water. ● Weigh the spirit burner and record the final mass. The student obtained the following results. initial maximum change in mass of initial mass of final mass of temperature temperature temperature of ethanol spirit burner / g spirit burner / g of water / °C of water / °C water, ∆T / °C burned / g 18.1 38.2 153.29 152.76 (a) Complete the table. Record your answers to the correct number of decimal places. [1] (b) Calculate the number of moles of ethanol burned. Give your answer to three significant figures. [Ar: C, 12.0; H, 1.0; O, 16.0] moles of ethanol = …… [1] (c) Use the formula q = mc∆T to determine the energy change, q, that took place during the experiment. Use q and your answer to (b) to calculate the enthalpy change of combustion of ethanol, ∆Hc, in kJ mol–1. Include a sign in your answer. 1.00 cm3 of water has a mass of 1.00 g c = 4.18 J g–1 K–1 ∆Hc = …… kJ mol–1 [2] (d) Calculate the percentage error of the temperature change recorded in the table in (a). Show your working. percentage error = …… [1] (e) State the effect, if any at all, on the accuracy of the experiment if the spirit burner was allowed to burn for longer. Explain your answer. . …… . …… . …… . …… [1] (f) The flame was extinguished, but the lid of the spirit burner was not replaced immediately. Predict how this would affect the value of ∆Hc. Explain your answer. . …… . …… . …… . …… [2] (g) The value for ∆Hc of ethanol under standard conditions is –1367 kJ mol–1. (i) Other than the reaction not being carried out under standard conditions, suggest two reasons why the value the student obtained in (c) is different from the actual value. 1 . …… . …… 2 . …… . …… [2] (ii) It is possible to calculate ∆Hc of ethanol using average bond enthalpies and the chemical equation for the reaction. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) Using average bond enthalpies, ∆Hc of ethanol is –1297 kJ mol–1. Explain why this value is different from the actual value for ∆Hc of ethanol under standard conditions. . …… . …… [1] [Total: 11]

Question

Halogenoalkanes undergo hydrolysis with aqueous sodium hydroxide to form alcohols. e.g. CH3CH2CH2CH2Br(l) + NaOH(aq) → CH3CH2CH2CH2OH(l) + NaBr(aq) A student carried out an experiment to compare the rate at which three halogenoalkanes, 1-chlorobutane, CH3CH2CH2CH2Cl, 1-bromobutane, CH3CH2CH2CH2Br, and 1-iodobutane, CH3CH2CH2CH2I, undergo hydrolysis. The method used was as follows: ● Place a 5 cm3 sample of each halogenoalkane into separate test-tubes. ● Add 1 cm3 of organic solvent to each test-tube. ● dd 2 cm3 aqueous sodium hydroxide to each test-tube. A ● Add 3 drops of acid–base indicator to the mixture. ● Heat the test-tubes in a thermostatically controlled, electrically heated water bath. ● Record the time taken for the indicator to change colour. (a) Give two reasons why the experiment was carried out using an electrically heated water bath. 1 . …… . …… 2 . …… . …… [2] (b) Sodium hydroxide is corrosive. Apart from wearing safety glasses and a lab coat, state one safety precaution which must be taken when handling sodium hydroxide. . …… [1] (c) Suggest why an organic solvent must be used in this experiment. . …… . …… [1] (d) Why is acid–base indicator added to the reaction mixture? . …… . …… [1] (e) The student obtained the following results. time taken for indicator 1 halogenoalkane / s–1 to change colour / s time 1-chlorobutane 417 1-bromobutane 238 1-iodobutane 135 1 (i) Complete the table to show .[1] time 1 (ii) can be used to represent rate of reaction. time 1 Suggest what the values tell you about the trend in carbon-halogen bond enthalpies. time . …… . …… [1] (f) Identify one additional variable that must be controlled in this experiment. . …… [1] Question 2 continues on the next page. (g) The student decided to investigate the order of reaction with respect to aqueous sodium hydroxide. CH3CH2CH2CH2Cl (l) + NaOH(aq) → CH3CH2CH2CH2OH(l) + NaCl (aq) step 1 An excess of 1-chlorobutane was mixed with 1.00 mol dm–3 NaOH(aq), at room temperature. step 2 A stop-clock was immediately started. step 3 At intervals of 60 seconds the student took 10.00 cm3 samples from the reaction mixture, for 11 minutes. step 4 Each 10.00 cm3 sample was immediately added to ice in a conical flask. step 5 The concentration of NaOH(aq) in each sample was determined by titration. The results are shown. concentration of time / s NaOH(aq) / mol dm–3 0 1.00 60 0.75 120 0.62 180 0.51 240 0.39 300 0.31 360 0.24 420 0.19 480 0.12 540 0.13 600 0.11 660 0.09 (i) Plot a graph of concentration of NaOH(aq) (y-axis) against time (x-axis). Use a cross (×) to plot each data point. Draw a curved line of best fit. 1.00 0.90 0.80 0.70 0.60 NaOH(aq) / mol dm–3 0.50 0.40 0.30 0.20 0.10 0 0 100 200 300 400 500 600 700 time / s [2] (ii) Circle the point which you consider to be most anomalous. [1] (iii) Suggest one reason for this anomalous point. . …… . …… [1] (iv) Draw construction lines on the graph to calculate two consecutive half-lives for this reaction. first half-life = …… s second half-life = …… s [2] (v) State whether you consider this to be a first-order reaction with respect to NaOH. Explain your answer. . …… . …… [1] (h) The total volume of the reaction mixture at the start of the experiment was 250 cm3. (i) Name a piece of apparatus that could be used to remove 10.00 cm3 samples from the reaction mixture. . …… [1] (ii) Suggest why the student did not remove 25.00 cm3 samples for titration from the reaction mixture. . …… . …… [1] (iii) Explain why each sample is added to ice in step 4. . …… . …… [1] (i) Another method for following the rate of a reaction is to measure changes in electrical conductivity. Explain why this would not be a suitable method for following this reaction. . …… . …… [1] [Total: 19]