Chemistry 9701/52 May/June 2022: question paper and mark scheme

Question

A student plans an experiment to find a value for the molar volume, Vm, of hydrogen gas at room conditions. Hydrogen gas is formed when magnesium reacts with dilute hydrochloric acid, HCl (aq). Mg(s) + 2HCl (aq) → MgCl 2(aq) + H2(g) The student is provided with the following materials: ● a piece of magnesium ribbon ● 0.50 mol dm–3 HCl (aq) ● a water trough ● a side-arm conical flask ● a 250 cm3 measuring cylinder with 2 cm3 graduations for the collection of gas ● a 50 cm3 measuring cylinder ● a balance that measures to 2 decimal places ● access to any necessary laboratory equipment, except gas syringes. The student plans the following procedure. Step 1 Prepare the piece of magnesium ribbon for use in the experiment. Step 2 Measure 30 cm3 of HCl (aq) and pour into a side-arm conical flask. Step 3 Attach the conical flask to a collection system for the hydrogen gas. Step 4 Place the magnesium ribbon in the conical flask. Step 5 Stopper the flask. Step 6 Wait until the final volume of gas collected is constant. Step 7 Wait for an additional 2 minutes, then measure and record the final volume of gas collected. (a) Complete Fig. 1.1 to show how the apparatus should be assembled for the collection and measurement of gas. Label your diagram. Fig. 1.1 [2] (b) The surface of the magnesium ribbon has an oxide layer. (i) State how the student should prepare the piece of magnesium ribbon before it is used in this experiment. . …… [1] (ii) State what additional information about the magnesium is required before the experiment is performed. . …… [1] (c) (i) Show by calculation that a volume of 30 cm3 of 0.50 mol dm–3 HCl (aq) is enough to react with 0.16 g of magnesium ribbon. Show your working. Mg(s) + 2HCl (aq) → MgCl 2(aq) + H2(g) [2] (ii) State why it is not necessary to use a burette to measure 30 cm3 of 0.50 mol dm–3 HCl (aq). . …… . …… [1] (d) The student waits for 2 minutes before taking a reading of the volume. Suggest why the student waits for 2 minutes before measuring the volume of gas in step 7. . …… . …… [1] (e) The student collects 146 cm3 of hydrogen gas during the experiment. (i) Calculate the percentage error in collecting the hydrogen gas. Show your working. percentage error = …… [1] (ii) Calculate the molar volume of hydrogen gas using the student’s results from this experiment. molar volume = …… cm3 [1] (f) The student’s experimental value for the molar volume of hydrogen is lower than the value quoted in the table of important values, constants and standards on page 11. Suggest one experimental weakness that might have led to this outcome. Explain how the method could be improved to overcome the weakness you have noted. experimental weakness . …… . …… improvement . …… . …… . …… [2] [Total: 12]

Question

In a neutral solution, aqueous potassium iodide acts as a catalyst for the decomposition of aqueous hydrogen peroxide. 2H2O2(aq) → 2H2O(l) + O2(g) A student plans to carry out an investigation to find how temperature affects the initial rate of the decomposition of aqueous hydrogen peroxide, H2O2(aq), in the presence of aqueous potassium iodide, KI(aq). The student knows that the initial rate of the reaction can be measured by timing the production of the oxygen. The student carries out a series of experiments. In experiment 1 the student notes the temperature of the H2O2(aq) and KI(aq) under room conditions. The solutions are mixed in apparatus designed to collect the oxygen produced. A stop-watch is started at the beginning of the reaction. The volume of oxygen is noted at regular time intervals. In experiments 2–8 the solutions are heated to different temperatures before mixing and measurement of the oxygen produced. The data collected is used to determine a value for the activation energy of the decomposition of H2O2(aq) in the presence of KI(aq). (a) State the independent variable. . …… [1] (b) State two variables that need to be controlled. 1 . …… 2 . …… [2] (c) (i) State how the student should prepare 250.0 cm3 of 0.100 mol dm–3 H2O2(aq) from 0.500 mol dm–3 H2O2(aq). Calculate the minimum volume of 0.500 mol dm–3 H2O2(aq) required for preparation of the 0.100 mol dm–3 H2O2 solution. Give the name and capacity of any key apparatus which should be used. Write your answer as a series of numbered steps. . …… . …… . …… . …… . …… . …… [3] (ii) Hydrogen peroxide causes eye and skin irritation. State what precaution should be taken when preparing the solution in (c)(i) other than wearing goggles. . …… [1] (d) (i) The student performs experiments 1–8 using a range of temperatures. The results are shown in Table 2.1. 1 Complete the table and record the values of to three significant figures and the values T of log initial rate to three significant figures. Table 2.1 experiment temperature temperature 1 –1 initial rate number / °C /K T /K / mol s–1 log initial rate 1 20 293 5.75 × 10–6 2 25 298 7.94 × 10–6 3 30 303 1.17 × 10–5 4 35 308 1.45 × 10–5 5 39 312 2.19 × 10–5 6 46 319 3.72 × 10–5 7 52 325 5.25 × 10–5 8 55 328 6.31 × 10–5 [2] 1 (ii) Plot a graph on the grid to show the relationship between log initial rate and . T Use a cross (×) to plot each data point. Draw a line of best fit. [2] (iii) Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure. . …… . …… [2] –4.2 –4.3 –4.4 –4.5 –4.6 –4.7 log initial rate –4.8 –4.9 –5.0 –5.1 –5.2 –5.3 0.00305 0.00310 0.00320 0.00330 0.00340 0.00350 1 –1 T /K (iv) Determine the gradient of your 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] 1 (v) The relationship between log initial rate and is given by the expression: T Ea log initial rate = constant – 2.303 RT Use the gradient calculated in (d)(iv) to calculate a value for the activation energy, Ea. (If you were unable to obtain an answer to (d)(iv) you may use the value –3100 K. This is not the correct value.) Ea = …… kJ mol–1 [2] (e) It is not possible to repeat the investigation. State whether the data from the investigation is reliable. Justify your answer. . …… . …… [1] [Total: 18] Important values, constants and standards molar gas constant R = 8.31 J K–1 mol–1 Faraday constant F = 9.65 × 104 C mol–1 Avogadro constant L = 6.022 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at s.t.p. (101 kPa and 273 K) Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1) The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 12 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.4 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge