Chemistry 9701/35 May/June 2024: question paper and mark scheme

Question

Iodide ions in aqueous solution are oxidised to iodine by a variety of oxidising agents. One of these is the peroxodisulfate ion, S2O82−, which reacts as shown. 2I−(aq) + S2O82−(aq) I2(aq) + 2SO42−(aq) Sodium thiosulfate is added to the reaction mixture to react with iodine as it is produced. When all of the thiosulfate has reacted, further iodine produced reacts with starch indicator to give a dark colour. You will carry out two experiments to investigate how the rate of this reaction is affected by changing the concentration of the peroxodisulfate ion. FA 1 is 0.0200 mol dm−3 potassium peroxodisulfate, K2S2O8. FA 2 is 0.00500 mol dm−3 sodium thiosulfate, Na2S2O3. FA 3 is 1.00 mol dm−3 potassium iodide, KI. FA 4 is starch indicator. (a) Method Experiment 1 ● Label one of the 100 cm3 beakers A and the other 100 cm3 beaker B. ● Fill one burette with FA 1. Label this burette FA 1. ● Run 20.00 cm3 of FA 1 from the burette into beaker A. ● Fill the second burette with FA 2. Label this burette FA 2. ● Run 10.00 cm3 of FA 2 from the burette into beaker B. ● Use the measuring cylinder to add 20.0 cm3 of FA 3 to beaker B. ● Add 10 drops of FA 4 to beaker B. ● Add the contents of beaker A to beaker B and start timing immediately. ● Stir the mixture once and place the beaker on the white tile. ● Stop timing as soon as the solution turns a dark colour. ● Record this time to the nearest second in the space for results. ● Wash out both beakers and dry them using paper towel. Experiment 2 ● Run 10.00 cm3 of FA 1 from the burette into beaker A. ● Run 10.00 cm3 of FA 2 from the burette into beaker B. ● Use the measuring cylinder to add 20.0 cm3 of FA 3 into beaker B. ● Use the same measuring cylinder to add 10.0 cm3 of distilled water to beaker B. ● Add 10 drops of FA 4 to beaker B. ● Add the contents of beaker A to beaker B and start timing immediately. ● Stir the mixture once and place the beaker on the white tile. ● Stop timing as soon as the solution turns a dark colour. ● Record this time to the nearest second. Record all your data in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the rate of reaction for both experiments. Use the following formula to calculate the rate of reaction. 1000 rate = reaction time Results I II III IV V VI [6] (b) (i) Explain why the concentration of potassium peroxodisulfate used in each experiment is proportional to the volume of FA 1 used. …… [1] (ii) A student thinks that the rate of reaction is proportional to the concentration of FA 1. Complete Table 1.1 to suggest volumes of reactants that could be used in a further experiment to confirm whether the student is correct. Do not carry out this experiment. Table 1.1 volume / cm3 volume FA 1 FA 2 FA 3 distilled water FA 4 10 drops [2] (c) A student correctly carried out the method in (a) but had been given a more concentrated solution of sodium thiosulfate. State how you would expect the student’s times to differ from yours. Explain your answer. …… [1] (d) The potassium iodide is in a large excess in Experiments 1 and 2. Suggest why a large excess of iodide ions is needed in these experiments. …… [1] [Total: 11]

Question

You will carry out an experiment to determine the enthalpy change, ΔH, when one mole of ammonium chloride dissolves in water. FA 5 is ammonium chloride, NH4Cl. (a) Method ● Weigh the container with FA 5. Record the mass in the space for results. ● Support the cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 25.0 cm3 of distilled water into the cup. ● Place the thermometer in the water and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of the water at time t = 0. ● Start the stop-clock and leave it running for the whole experiment. ● Measure and record the temperature of the water in the cup every half minute for 2 minutes. ● At t = 2½ minutes, tip all the FA 5 into the cup. Stir the contents of the cup. ● Measure and record the temperature of the contents of the cup at t = 3 minutes and then every half minute up to and including t = 8 minutes. ● Weigh the container with any residual FA 5. Record the mass. ● Calculate and record the mass of FA 5 added. Results I II III IV V [5] (b) Plot a graph of temperature (y-axis) against time (x-axis) on the grid. You should choose a scale that allows you to plot 2 °C below the minimum temperature reached. Label any points you consider to be anomalous. Draw two straight lines of best fit. One line is for the temperature before adding FA 5 and the other line is for the warming of the solution once the minimum temperature has been reached. Extrapolate both these lines to t = 2½ minutes. I II III IV [4] (c) (i) Use your graph to determine the temperature change, ΔT, at t = 2½ minutes. ΔT at t = 2½ minutes = …… °C [1] (ii) Calculate the energy change, in J, in the reaction. energy change = …… J [1] (iii) Calculate the amount, in mol, of ammonium chloride used. amount of NH4Cl = …… mol [1] (iv) Calculate the enthalpy change, ΔH, in kJ mol−1, when one mole of ammonium chloride dissolves in water. ΔH = …… …… kJ mol−1 [1] sign value (d) Use your results in (a) to calculate the maximum percentage error for the temperature change from 0 to 4 minutes. Assume that the maximum uncertainty in a single thermometer reading is ±0.5 °C. Show your working. maximum percentage error = …… % [1] [Total: 14] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Question

(a) FA 6 is a salt containing a Group 1 ion and an anion that consists of a transition metal and a non-metal element. (i) Transfer FA 6 into a hard-glass test-tube. Heat the tube gently at first and then strongly. Record all your observations and identify the gas produced. The residue is FA 7. You will use FA 7 in (a)(ii). observations …… gas produced …… [3] Allow FA 7 to cool before starting (a)(ii). While FA 7 is cooling you may wish to continue with (b)(i). (ii) Put a 2 cm depth of acidified aqueous potassium manganate(VII) in a test-tube. Add the same depth of aqueous sodium hydroxide. Then add FA 7 and stir using the glass rod for about 30 seconds. Filter the mixture and collect the filtrate. Record your observations. Put a 1 cm depth of the filtrate in a test-tube. Add sulfuric acid until in excess. Record your observations. [2] (b) (i) FA 8 and FA 9 are both aqueous solutions of salts. FA 8 contains one cation and one anion. FA 9 contains two cations and one anion. One of the cations and both anions are listed in the Qualitative analysis notes. Carry out the following tests and record your observations in Table 3.1. For each test use a 1 cm depth of FA 8 or FA 9 in a test-tube. Table 3.1 observations test FA 8 FA 9 Test 1 Add sulfuric acid. Test 2 Add aqueous sodium hydroxide, then transfer the mixture into a boiling tube and warm. Test 3 Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add nitric acid. Test 4 Add FA 8 with shaking until in excess. [7] (ii) Deduce the identity of the three ions listed in the Qualitative analysis notes that are present in FA 8 and FA 9. Suggest the identity of one other cation. Give the formula of each ion. If you cannot identify an ion write ‘unknown’. FA 8 contains …… and …… . FA 9 contains …… and …… and …… . [2] (iii) Write an ionic equation for one reaction that occurred in Test 2 in Table 3.1. Include state symbols. …… [1] [Total: 15] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+ 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint 4 Tests for elements element test and test result iodine, I2 gives blue-black colour on addition of starch solution 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 138.9 140.1 140.9 144.2 – 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. 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