Chemistry 9701/32 May/June 2024: question paper and mark scheme

Question

Students are told to plan and carry out an experiment to determine the enthalpy change, ΔH1, when one mole of anhydrous sodium thiosulfate, Na2S2O3, is hydrated. Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s) One student suggested adding five moles of water to one mole of anhydrous sodium thiosulfate and measuring the temperature change. Their teacher said that method would not work and suggested another method using Hess’s law. You will carry out the teacher’s method to determine the enthalpy change, ΔH2, when one mole of hydrated sodium thiosulfate, Na2S2O3•5H2O, is dissolved in water. Na2S2O3•5H2O(s) + water Na2S2O3(aq) You will do this by adding a known mass of hydrated sodium thiosulfate to a known volume of water and measuring the temperature change when the solid dissolves. (a) Explain why the student’s suggestion to add five moles of water to one mole of anhydrous sodium thiosulfate would not be a suitable method to determine ΔH1. …… [1] (b) Teacher’s method FB 1 is hydrated sodium thiosulfate, Na2S2O3•5H2O. ● Support the cup in the 250 cm3 beaker. ● Use the 25 cm3 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 in the space for results. ● Weigh the container with FB 1. Record the mass. ● Add all of the FB 1 to the water in the cup. ● Stir the mixture. Measure and record the minimum temperature reached. ● Reweigh the container and any residual FB 1. Record the mass. ● Calculate and record the mass of FB 1 added. ● Calculate and record the change in temperature. Results I II III IV [4] (c) Calculations (i) Calculate the energy change, in J, when FB 1 is added to water. energy change = …… J [1] (ii) Calculate the enthalpy change, ΔH2, in kJ mol–1, when one mole of hydrated sodium thiosulfate, FB 1, dissolves in water. ΔH2 = …… …… kJ mol–1 [2] sign value (iii) The enthalpy change, ΔH3, when one mole of anhydrous sodium thiosulfate is dissolved in water is –8.1 kJ mol–1. Na2S2O3(s) + water Na2S2O3(aq) ΔH3 = –8.1 kJ mol–1 Use your answer to (c)(ii) and the information given to construct a Hess’s cycle to calculate ΔH1 in kJ mol–1. Show clearly how you used the data. (If you were unable to calculate an answer in (c)(ii), assume a value of 31.6 kJ mol–1. Note this may not be the correct value and the sign has been omitted.) Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s) ΔH1 ΔH1 = …… …… kJ mol–1 [2] sign value (d) A sample of FB 1 was contaminated with anhydrous sodium thiosulfate. State what effect this would have on the temperature change in (b). Explain your answer. …… [1] [Total: 11]

Question

Iodate ions contain iodine and oxygen. They have the formula IOx– where x is an integer. In this experiment you will determine the value of x in an iodate. You will first react IOx– with an excess of iodide ions, I–, to form iodine, I2. The amount of iodine produced is then determined by titration with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) FB 2 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 3 is a solution containing 0.0140 mol dm–3 IOx– ions. FB 4 is dilute sulfuric acid, H2SO4. FB 5 is 0.500 mol dm–3 potassium iodide, KI. FB 6 is starch indicator. (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 10 cm3 of FB 4 to the conical flask. ● Use the same measuring cylinder to add 10 cm3 of FB 5 to the conical flask. ● Add FB 2 from the burette until the solution turns yellow. ● Add 10–15 drops of FB 6 to the solution in the conical flask. ● Continue to add more FB 2 from the burette until the blue-black colour just disappears. ● Perform a rough titration and record your burette readings in the space below. The rough titre is …… cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure any recorded results show the precision of your practical work. ● Record, in a suitable form in the space below, all your burette readings and the volume of FB 2 added in each accurate titration. Keep the remaining FB 2 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FB 3 required …… cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Use your answer to (b) and the information given to calculate the amount, in mol, of iodine formed when 25.0 cm3 of FB 3 reacts with 10 cm3 of FB 5. amount of I2 = …… mol [1] (iii) Calculate the amount, in mol, of IOx– ions in 25.0 cm3 of FB 3. amount of IOx– ions = …… mol [1] (iv) An unbalanced equation for the reaction of IOx– ions with iodide ions, I–, and hydrogen ions, H+, is shown. IO…. – + ..…… I– + ..…… H+ …….. I2 + …… H2O Use the ratio of your answers to (c)(ii) and (c)(iii) to balance this equation and determine the value of x. Show your working. ratio IOx– : I2 = 1 : …… x = …… [2] (d) A student carries out the same experiment as in (a) but uses 0.0140 mol dm–3 IO2– ions in place of FB 3. Tick the correct box in Table 2.1. Explain your answer. (If you were unable to determine the value of x in (c)(iv) or you determined the value of x to be 2, assume x = 4. Note that this may not be the correct value). Table 2.1 Volume of FB 2 will be smaller. Volume of FB 2 will be unchanged. Volume of FB 2 will be larger. [2] [Total: 15] 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

FB 7 is an aqueous solution of a salt containing one cation and one anion, both of which are listed in the Qualitative analysis notes. (a) (i) Carry out the following tests using a 1 cm depth of FB 7 in a test-tube for each test. Record your observations in Table 3.1. Table 3.1 test observations Test 1 Add aqueous barium chloride or aqueous barium nitrate. Test 2 Add aqueous sodium hydroxide, then transfer the mixture to a boiling tube and warm gently, then add a small piece of aluminium foil. Test 3 Add aqueous ammonia. [4] (ii) The results of the tests in Table 3.1 allow you to deduce one cation and two possible anions in FB 7. Deduce which ions may be present in FB 7. Give the formulae of the ions. cation …… anion …… or …… [2] (iii) Describe a test to identify which of the possible anions is present in FB 7. Carry out your test. Record your observations and conclusion. test …… observations …… conclusion …… [2] Question 3 continues on page 12. (b) (i) FB 8 and FB 9 are solutions of Group 1 salts. Each contains one anion, both of which are listed in the Qualitative analysis notes. One of the anions contains oxygen but not nitrogen. The other anion is a halide. Carry out tests to identify the two anions. Record your tests and observations in a suitable form in the space below. [4] (ii) Use your observations in (b)(i) to complete Table 3.2 by identifying the formulae of the anions in FB 8 and FB 9. Table 3.2 FB 8 FB 9 anion [1] (iii) Write an ionic equation for a reaction observed in (b)(i) for one of the anions tested. Include state symbols. …… [1] [Total: 14] 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 16 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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