Chemistry 9701/33 May/June 2024: question paper and mark scheme

Question

When ammonium compounds are heated with sodium hydroxide or calcium hydroxide, ammonia is liberated. The ionic equation for the reaction is shown. NH4+ + OH− NH3 + H2O FA 1 is prepared by heating 10.0 cm3 of aqueous ammonium chloride, NH4Cl (aq), with 25.0 cm3 of 2.00 mol dm−3 sodium hydroxide, NaOH. The sodium hydroxide is in excess. The reaction mixture is cooled and diluted to 250 cm3 with distilled water. You will determine the concentration of the aqueous solution of ammonium chloride by titrating the remaining sodium hydroxide from the preparation of FA 1 with a known concentration of sulfuric acid. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) FA 2 is 0.0520 mol dm−3 sulfuric acid, H2SO4. FA 3 is bromophenol blue indicator. (a) Method ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 1 into a conical flask. ● Add a few drops of FA 3 to the 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 in the space below, all your burette readings and the volume of FA 2 added in each accurate titration. 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 1 required …… cm3 of FA 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sulfuric acid present in the volume of FA 2 in (b). amount of H2SO4 = …… mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of sodium hydroxide in 25.0 cm3 of FA 1. amount of NaOH = …… mol [1] (iv) Use your answer to (c)(iii) and the information given to calculate the amount, in mol, of sodium hydroxide that reacted with the ammonium chloride when FA 1 was prepared. amount of NaOH that reacted with NH4Cl = …… mol [2] (v) Use your answer to (c)(iv) and the information given to calculate the concentration, in mol dm−3, of the ammonium chloride solution used to prepare FA 1. concentration of NH4Cl = …… mol dm−3 [1] (d) When ammonia gas is prepared by heating one of its salts with calcium hydroxide, it can be dried by passing it through a drying agent. Drying agents for gases include calcium oxide, calcium sulfate, concentrated sulfuric acid and phosphorus(V) oxide. Select one of these drying agents and suggest why it is not suitable for drying ammonia gas. …… is not suitable because …… …… [1] [Total: 15]

Question

A sample of ammonium bromide has been accidentally mixed with another salt. A student suggests that the percentage purity of the ammonium bromide can be determined by measuring the change in temperature when a solution is prepared. The enthalpy change when the sample dissolves in water, ΔH, can then be calculated and compared with the literature value for ammonium bromide. You will carry out the student’s experiment. FA 4 is ammonium bromide, NH4Br, mixed with another salt. (a) Method ● 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 FA 4. Record the mass. ● Add all of FA 4 to the water in the cup. ● Stir the mixture gently so that FA 4 dissolves. ● Measure and record the minimum temperature reached. ● Calculate and record the change in temperature. ● Weigh the container with any residual FA 4. Record the mass. ● Calculate and record the mass of FA 4 added to the cup. Results I II III IV [4] (b) (i) Calculate the energy change, in J, for the reaction. energy change = …… J [1] (ii) When one mole of ammonium bromide dissolves in water the enthalpy change, ΔH, is +16.8 kJ mol−1. Use this value to calculate the amount, in mol, of ammonium bromide in the 25.0 cm3 of solution FA 4 used in your experiment. Assume that the enthalpy change when the other salt dissolves in water is 0 kJ mol−1. amount of NH4Br in 25.0 cm3 = …… mol [1] (iii) Calculate the mass, in g, of ammonium bromide in your sample. (If you were unable to calculate the amount of ammonium bromide in (b)(ii), then assume it is 2.06 × 10−2 mol. This may not be the correct answer.) mass of NH4Br in sample = …… g [1] (iv) Hence calculate the percentage by mass of ammonium bromide in your sample. % by mass of NH4Br = …… % [1] (c) Another student states that this method is not accurate as the enthalpy change when the other salt dissolves in water could be exothermic. Suggest how the calculated value in (b)(iv) would change if the other salt dissolves in water exothermically. Explain your answer. …… [1] (d) The maximum uncertainty in a single thermometer reading is ±0.5 ºC. Calculate the maximum percentage error in your temperature change recorded in (a). Show your working. maximum percentage error = …… % [1] [Total: 10] 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 5 is a mixture containing two cations and two anions. Three of the ions are listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Table 3.1. Table 3.1 test observations Test 1 Heat a spatula measure of FA 5 in a hard-glass test-tube. Test 2 Put a 2 cm depth of dilute nitric acid in a boiling tube. Add a spatula measure of FA 5. Dilute the solution from Test 2 with an equal volume of distilled water. Keep the resulting solution for use in (a)(ii). This solution is FA 6. [3] (ii) Carry out the following tests using a 1 cm depth of FA 6 in a test-tube for each test. Record your observations in Table 3.2. Table 3.2 test observations Test 1 Add a 1 cm depth of sodium chlorate(I), then add aqueous sodium thiosulfate dropwise. Test 2 Add aqueous sodium hydroxide. Test 3 Add aqueous ammonia. Test 4 Add a 1 cm depth of aqueous copper(II) sulfate, then add aqueous sodium thiosulfate dropwise until no further change occurs. Test 5 Add a few drops of aqueous silver nitrate, then add aqueous ammonia until no further change occurs. [4] (iii) FA 5 reacts with nitric acid to make FA 6 in Test 2 in Table 3.1. State a change you would see in one of the observations in Table 3.2 if hydrochloric acid had been used to prepare FA 6. …… [1] (iv) Use your observations in (a)(i) and (a)(ii) to identify the formulae of three of the ions present in FA 5. Formulae of ions present are …… and …… and …… . [2] (b) FA 7, FA 8 and FA 9 are each known to be propan-1-ol, propanal, propanoic acid or propanone. No two liquids are identical. (i) You will carry out tests to identify samples of FA 7, FA 8 and FA 9. Test 1 has been carried out for you. Carry out Test 2 and record your observations in Table 3.3. Table 3.3 observations test FA 7 FA 8 FA 9 Test 1 orange precipitate orange precipitate no change Put a 1 cm depth in a test- tube. Add a few drops of 2,4-dinitrophenylhydrazine (2,4-DNPH reagent). Test 2 Put a 1 cm depth in a test- tube. Add a small spatula measure of solid sodium carbonate. [1] (ii) State what you can deduce from Tests 1 and 2 in Table 3.3 about the functional group present in each of FA 7, FA 8 and FA 9. …… [2] (iii) Describe a test to identify FA 7 and FA 8 using the reagents provided. Carry out your test. Record your observations and conclusions. test …… observations …… FA 7 is …… . FA 8 is …… . [2] [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 138.9 140.1 140.9 144.2 – 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. 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