Cambridge A Level Chemistry 9701 — 2020 Oct/Nov Paper 5 · Variant 2

1 Calcium hydroxide, Ca(OH)2, is slightly soluble in water. Aqueous calcium hydroxide is also known as limewater. A student measures the concentration of saturated aqueous calcium hydroxide using the following method. step 1 Add a spatula measure of solid calcium hydroxide to 250 cm3 of distilled water in a conical flask. Stopper the flask and shake the flask thoroughly. step 2 Leave the flask stoppered for 24 hours. step 3 Filter the aqueous calcium hydroxide into a clean, dry beaker. step 4 Transfer 25.00 cm3 of the aqueous calcium hydroxide to a clean conical flask. step 5 Carry out a titration with 0.100 mol dm–3 hydrochloric acid. step 6 Record the volume of hydrochloric acid required to react with the aqueous calcium hydroxide. (a) (i) Suggest why it is necessary for the flask to remain stoppered in step 2. ....................................................................................................................................... [1] (ii) Suggest why the student leaves the solution for 24 hours in step 2. ............................................................................................................................................. ....................................................................................................................................... [1] (b) (i) The student finds that the mean titre is 10.65 cm3. The equation for the reaction is shown. Ca(OH)2(aq) + 2HCl (aq) → CaCl 2(aq) + 2H2O(l) Calculate the number of moles of calcium hydroxide that reacted. moles of Ca(OH)2 = .............................. mol [1] (ii) Calculate the concentration of the aqueous calcium hydroxide in g dm–3. [Ar: Ca, 40.1; O, 16.0; H; 1.0] concentration of Ca(OH)2(aq) = .............................. g dm–3 [2] (iii) In a chemistry data book, the student found the value for the concentration of a saturated solution of calcium hydroxide at 293 K to be higher than the value calculated in (b)(ii). Give two reasons why the data book value is higher than the value calculated in (b)(ii). (Assume the student recorded all volumes correctly, carried out the titration accurately and recorded concordant titres.) ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (c) A second student carried out the experiment but did not filter their solution correctly in step 3, leaving some undissolved calcium hydroxide suspended in the solution. Predict and explain how this would affect the titre obtained in their experiment. .................................................................................................................................................... .............................................................................................................................................. [1] (d) Describe how to prepare exactly 250.0 cm3 of 0.100 mol dm–3 hydrochloric acid from 1.00 mol dm–3 hydrochloric acid. Include the names and capacities of any suitable apparatus used. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 10]

Mark scheme: 1(a)(i) To prevent carbon dioxide / CO2 from entering the flask 1 1(a)(ii) To ensure that the solution is saturated 1 1(b)(i) (10.65 / 1000 × 0.100) / 2 = 0.0005325 OR 5.325 × 10–4 (mol) 1 1(b)(ii) M1: Conversion of mol to g (multiplication by 74.1) 0.0005325 × 74.1 = 0.03945825 (mol) 2 M2: Scaling up to 1000 cm3 (multiplication by 1000 / 25) = 0.03945825 × 1000 / 25 = 1.57833 (g dm–3) 1(b)(iii) M1: The temperature (of solution) was different (from 293 K) OR Temperature (of solution) was unknown M2: 24 hours was insufficient time (to reach saturation) 2 1(c) Increase (in titre volume) AND greater number of moles of (calcium) hydroxide present in flask 1 1(d) M1: Pipette AND 25(.00) cm3 (1.0 mol dm–3) HCl (Into…) M2: 250 cm3 volumetric flask AND top up to mark with distilled water 2

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2 Sodium thiosulfate reacts with excess dilute hydrochloric acid according to the following equation. Na2S2O3(aq) + 2HCl (aq) → 2NaCl (aq) + S(s) + SO2(g) + H2O(l) The reaction produces sulfur dioxide which is corrosive and toxic by inhalation. The effect of temperature on the rate of this reaction can be investigated by the ‘disappearing cross experiment’ as follows. step 1 Measure 10.00 cm3 of 0.100 mol dm–3 Na2S2O3(aq) and transfer into a boiling tube. step 2 Into a second boiling tube, transfer 10 cm3 of 0.500 mol dm–3 HCl (aq). step 3 Heat both tubes in the same water-bath. step 4 Place a conical flask on a piece of paper, on which is drawn a large X. step 5 When the solutions reach the required temperature, remove them from the water-bath, pour the contents of both tubes into the conical flask and immediately start a timer. step 6 The X on the paper should be viewed from above the flask as shown in the diagram. step 7 When the X can no longer be seen, due to formation of solid sulfur, stop the timer. step 8 Repeat the experiment using the same volumes and concentrations of solution at different temperatures. view from above the flask conical flask dilute hydrochloric acid and sodium thiosulfate solution paper with X (a) State one precaution that should be taken to reduce a hazard caused by one of the products of this reaction. .................................................................................................................................................... .............................................................................................................................................. [1] (b) Suggest appropriate equipment for measuring the required volumes of Na2S2O3(aq) and HCl (aq) stated in the method. Na2S2O3(aq) ................................................................................................................................ HCl (aq) ...................................................................................................................................... [1] The student recorded their data in the table. A B C D E F time, t, for rate oftemperature temperature, T 1 1 cross to reaction / K–1 2.303 log / °C / K 1 T t disappear / s / s–1 t 60 333 7.39 0.135 –2.00 50 323 13.46 0.0743 –2.60 41 314 21.12 0.0473 –3.05 36 309 40.45 0.0247 –3.70 27 300 49.40 0.0202 –3.90 21 294 70.11 0.0143 –4.25 (c) Complete column E in the table, giving your values to three significant figures. [1] (d) The activation energy, Ea, of the reaction between sodium thiosulfate and hydrochloric acid can be calculated using the equation shown. 1 2.303 log = –Ea / RT t R = 8.31 J K–1 mol–1 1 1 (i) Plot a graph of 2.303 log on the y-axis and on the x-axis. Draw a line of best fit. t T –1.00 –1.50 –2.00 –2.50 12.303 log t –3.00 –3.50 –4.00 –4.50 –5.00 0.00290 0.00300 0.00310 0.00320 0.00330 0.00340 0.00350 1 / K–1 T [2] (ii) Circle the point on the graph you consider to be most anomalous. Suggest two possible errors in how the procedure was carried out which could have caused this anomalous point. 1 .......................................................................................................................................... ............................................................................................................................................. 2 .......................................................................................................................................... ............................................................................................................................................. [2] (iii) 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 selected from your line of best fit. Give the gradient to three significant figures. coordinates 1 ................................................ coordinates 2 .............................................. gradient = .............................. K [2] (iv) Use the gradient you have calculated and the equation to calculate Ea, in kJ mol–1, for the reaction. Give your answer to two significant figures. Include a sign. Ea = .............................. kJ mol–1 [3] (e) A student repeats the experiment at 60 °C but with a different concentration of Na2S2O3(aq). The new solution is named solution B. The time taken until X can no longer be seen is 11.52 s. Assume no error was made in the timing. State whether the concentration of solution B is more or less than 0.10 mol dm–3. Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (f) It is more appropriate for the values in column C of the table on page 6 to be recorded to the nearest second. Suggest one reason for this. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 14]

Mark scheme: 2(a)(i) use a fume cupboard / fume hood 1 2(b) burette AND measuring cylinder 1 Question Answer Marks 2(c) 1/T(K–1) 0.00300 0.00310 0.00318 0.00324 0.00333 0.00340 All values to 3SF 1 2(d)(i) M1: 6 points plotted correctly M2: straight line of best fit drawn which must extend to cover all data points 2 2(d)(ii) Assuming point is below the line: M1: Only one most anomalous point indicated AND the timer was stopped after the reaction had finished OR the timer started before the reaction had started M2: The temperature (recorded / causing the anomaly) was higher than actual temperature (of the mixture) OR The actual temperature (of the mixture) was lower than (recorded / causing the anomaly) temperature 2 2(d)(iii) M1: Co-ordinates read and recorded correctly M2: Gradient calculated to 3 SF 2 2(d)(iv) M1: Ea = – (gradient × R) = –2(d)(iii) × 8.31 J mol–1 M2: = M1 / 1000 (correct conversion from J to kJ) M3: = ‘+’ sign and 2SF 3 Question Answer Marks 2(e) concentration is less AND (so) time is higher 1 2(f) reaction time of the observer is (too) slow (to measure to 1 / 100 of a second) 1

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3 A student investigates the reaction between magnesium and hydrochloric acid to determine the order of reaction with respect to hydrochloric acid. The student is supplied with 2.00 mol dm–3 hydrochloric acid and six identical, clean pieces of magnesium ribbon. In experiment 1, the student places 50 cm3 of 2.00 mol dm–3 hydrochloric acid in a 100 cm3 beaker and adds a piece of magnesium ribbon to the acid in the beaker. The student records how long it takes for the magnesium ribbon to disappear. The student carries out five further experiments, diluting the 2.00 mol dm–3 hydrochloric acid each time to provide results from five different concentrations of acid. The student carries out each experiment in a 100 cm3 beaker. (a) (i) Complete the table so the student can use it to record the reaction time for each experiment. The table should include suggested volumes of acid and water for each experiment. Columns should be suitably labelled. experiment number 1 2 3 4 5 6 [2] (ii) Identify the dependent variable in this experiment. ....................................................................................................................................... [1] (iii) Apart from the magnesium ribbon, identify one variable that should be controlled in these experiments. ....................................................................................................................................... [1] (b) Explain why the student’s proposed method of determining the reaction time may lead to inaccuracies in the recorded results. .................................................................................................................................................... .............................................................................................................................................. [1] (c) Suggest one other variable that could be measured to determine the order of the reaction. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 6]

Mark scheme: 3(a)(i) M1: Labels and units for column headings Vol(ume) HCl / cm3 Vol(ume) H2O / cm3 Time / s M2: Experiment 1 has 50(.00) and 0(.00) AND Other sets of whole number volumes showing differing concentrations of HCl but total volume must not be above 100 cm3 3(a)(ii) time taken (for reaction to finish) 1 3(a)(iii) temperature 1 3(b) hard to tell when magnesium has dissolved 1 3(c) volume of gas (evolved) OR mass loss (of reaction mixture) 1

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