Chemistry 9701/33 May/June 2023: question paper and mark scheme

Question

A redox reaction takes place between hydroxylamine, NH2OH, and the iron(III) ion, Fe3+, in acidic conditions. The iron(III) ion is reduced to an iron(II) ion, Fe2+. The reaction is slow at room temperature but is complete in a few minutes at 100 °C. The reaction is shown by one of the following equations. equation 1 NH2OH(aq) + Fe3+(aq) Fe2+(aq) + H+(aq) + ½N2(g) + H2O(l) equation 2 NH2OH(aq) + 2Fe3+(aq) 2Fe2+(aq) + ½N2O(g) + 2H+(aq) + ½H2O(l) equation 3 NH2OH(aq) + 3Fe3+(aq) 3Fe2+(aq) + NO(g) + 3H+(aq) You will carry out a titration to determine which of equations 1, 2 or 3 best represents the reaction. The iron(II) ions formed in the reaction with the hydroxylamine are oxidised by manganate(VII) ions. MnO4–(aq) + 5Fe2+(aq) + 8H+(aq) Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a solution prepared by boiling a 1.00 dm3 aqueous mixture containing 3.30 g of hydroxylamine hydrochloride, NH2OH•HCl , excess iron(III) chloride, FeCl 3, and excess sulfuric acid. Any water lost by evaporation was replaced after cooling. FA 3 is dilute sulfuric acid. Assume that one mole of hydroxylamine hydrochloride gives one mole of hydroxylamine in solution. (a) Method • Fill the burette with FA 1. • Pipette 25.0 cm3 of FA 2 into a conical flask. • Use the 25 cm3 measuring cylinder to add 10 cm3 of FA 3 into the same conical flask. • 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, all your burette readings and the volume of FA 1 added in each accurate titration. Keep FA 3 for use in Question 2(a). Results 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 FA 2 required …… cm3 of FA 1. [1] (c) Calculations (i) Calculate the amount, in mol, of potassium manganate(VII) present in the volume of FA 1 in (b). amount of KMnO4 = …… mol [1] (ii) Use your answer to (c)(i) to calculate the amount, in mol, of iron(II) ions in 25.0 cm3 of solution FA 2. amount of Fe2+ = …… mol [1] (iii) Calculate the amount, in mol, of hydroxylamine hydrochloride that has reacted in the FA 2 pipetted into the conical flask. Show your working. amount of NH2OH•HCl = …… mol [2] (iv) Use your answer to (c)(iii) to deduce which of the three suggested equations corresponds to your results. Show your working. The correct equation number is …… . [1] [Total: 13]

Question

The reaction between thiosulfate ions and hydrogen ions produces a precipitate of sulfur. When the concentration of hydrogen ions, [H+(aq)], is kept constant, the rate is proportional to one of the following: • the concentration of thiosulfate ions, [S2O32–(aq)] • the square of the concentration of thiosulfate ions, [S2O32–(aq)]2. You will determine which relationship is correct by mixing solutions of sodium thiosulfate and sulfuric acid. You will measure the time taken for a fixed amount of sulfur to be precipitated. Throughout these experiments care must be taken to avoid inhaling any SO2 gas that is produced. It is very important that as soon as each experiment is complete, the contents of the beaker are emptied into the quenching bath and the beaker is rinsed thoroughly. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. FA 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. (a) Method Experiment 1 • Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FA 4 into the 100 cm3 beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 3. • Add FA 3 to FA 4 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in Table 2.1. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready for use in Experiment 2. Experiment 2 • Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 4 into the 100 cm3 beaker. • Use the same measuring cylinder to add 20.0 cm3 of distilled water to the same beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 3. • Add FA 3 to the mixture of FA 4 and distilled water in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in Table 2.1. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready for use in Experiment 3. Experiment 3 • Carry out one further experiment to investigate how the reaction time changes with a different volume of FA 4. Do not use a volume of FA 4 that is less than 20.0 cm3. Record your results in Table 2.1. Results Table 2.1 volume / cm3 experiment reaction time / s FA 3 FA 4 distilled water 1 10.0 50.0 0 I 2 10.0 30.0 20.0 II 3 III IV [4] (b) Calculations (i) Complete Table 2.2. Use the following expression to calculate the rate. 1000 rate = reaction time Table 2.2 concentration of experiment rate / s–1 FA 4 / mol dm–3 1 2 3 [2] (ii) Use data from two of your experiments to deduce which relationship for the rate is correct. Place a tick next to the statement in Table 2.3 which most accurately fits your results. Show your working. Table 2.3 The rate is proportional to [S2O32–(aq)]. The rate is proportional to [S2O32–(aq)]2. [2] (c) The uncertainty in a 50 cm3 measuring cylinder is ±0.5 cm3. Calculate the maximum percentage error in the volume of FA 4 in Experiment 2. maximum percentage error in volume of FA 4 = ……% [1] (d) Suggest improvements to your method and the processing of the results that would allow you to make a more reliable conclusion in (b)(ii). Do not suggest changes to the apparatus or to the concentrations of FA 3 and FA 4. Do not carry out any of your suggestions. …… [3] [Total: 12] 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. Rinse and reuse test‑tubes and boiling tubes where possible. No additional tests should be attempted.

Question

FA 5 is a solid mixture of two salts and contains two different anions. None of the ions present contains nitrogen. (a) (i) Place a small spatula measure of FA 5 in a hard‑glass test‑tube. Heat the test‑tube, gently at first, then more strongly, for a total of approximately 2 minutes. Then allow the test‑tube to cool. Record your observations. …… [2] (ii) Place a large spatula measure of FA 5 in a boiling tube. Add an approximately 3 cm depth of distilled water and shake the tube and contents. Record your observations. Keep the contents of the test-tube for use in (a)(iii). …… [2] (iii) Put 1 cm depth of the solution from (a)(ii) in a test‑tube. Add aqueous sodium hydroxide. Record your observations. …… [1] (b) (i) Put 3 cm depth of dilute nitric acid in a test‑tube. Carefully add a spatula measure of FA 5. Record your observations. Keep the contents of the test-tube for use in (b)(iii). …… [2] (ii) Suggest what your observations in (a)(ii) tell you about the identity of one of the cations in FA 5. …… [1] (iii) Select reagents for further tests to identify any anion present in FA 5. Carry out your tests and record your reagents, conditions and observations in the space below. [2] (iv) From your observations in (b)(i) and (b)(iii), deduce the formulae of the two anions present in FA 5. anions [1] (v) Write an ionic equation for one reaction occurring in (b)(i) or (b)(iii). Include state symbols. …… [1] (c) FA 6 is a solution of a salt. One of the two ions contains a nitrogen atom. Both ions are listed in the Qualitative analysis notes. Select reagents to identify the ion containing the nitrogen atom. Use a 1 cm depth of FA 6 in a boiling tube to carry out your tests. Record your tests, results and conclusion in the space below. [3] [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), off‑white ppt. rapidly turning brown off‑white ppt. rapidly turning brown Mn2+(aq) 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 at www.cambridgeinternational.org after the live examination series. 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 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. 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