Chemistry 9701/31 May/June 2023: question paper and mark scheme

Question

The thiosulfate ion, S2O32–, decomposes when an acid is added. S2O32–(aq) + 2H+(aq) S(s) + SO2(aq) + H2O(l) The rate of this reaction can be investigated by measuring how long it takes the solution to produce enough sulfur so that it cannot be seen through. You will investigate how the concentration of the thiosulfate ion affects the rate of the reaction. Note: A small amount of sulfur dioxide gas may be formed in the experiment. It is very important that you avoid inhaling any fumes. As soon as each experiment is complete, empty the reaction mixture into the quenching bath and rinse the beaker thoroughly. FA 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is 2.00 mol dm–3 hydrochloric acid, HCl. (a) Method Experiment 1 • Fill a burette with FA 1. • Run 40.00 cm3 of FA 1 into the 100 cm3 beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the FA 1 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 the space for results on page 4. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in Experiment 2. Experiment 2 • Refill the burette with FA 1. • Fill the other burette with distilled water. • Run 20.00 cm3 of FA 1 into the 100 cm3 beaker. • Run 20.00 cm3 of distilled water into the same beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the solution 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. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in the next experiment. Experiments 3–5 • Carry out three further experiments to investigate how using different volumes of FA 1 affects the reaction time. Note that the combined volumes of FA 1 and distilled water must always be 40.00 cm3. Do not use a volume of FA 1 that is less than 15.00 cm3. Record all your results in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. The rate of reaction can be calculated using the following formula. 1000 rate = reaction time Results I II III IV V VI VII VIII [8] (b) On the grid, plot the rate (y-axis) against the volume of FA 1 (x-axis). Start each axis at the origin (0,0). Ring any anomalous points. Draw a line of best fit. I II III IV [4] (c) In these experiments, the volume of FA 1 is a measure of the concentration of thiosulfate ions. A student suggested that the graph shows that the rate of reaction is directly proportional to the concentration of thiosulfate ions. Explain, using your graph, whether you agree with this student. …… [1] (d) Use your graph to calculate the time you would expect to record if you had used 12.50 cm3 of FA 1 and followed the same method. Show clearly on the graph how you calculated this time. time = …… s [2] (e) Another student broke the beaker and decided to use a Petri dish instead. beaker Petri dish Fig. 1.1 What effect, if any, would this have on the times measured in the experiment in (a)? Explain your answer. …… [2] [Total: 17]

Question

Many salts contain water of crystallisation which can be removed by heating to form the anhydrous salt. You will determine the enthalpy change of dehydration for hydrated magnesium sulfate. MgSO4•7H2O(s) MgSO4(s) + H2O(l) You will determine this enthalpy change by measuring the changes in temperature when samples of hydrated magnesium sulfate and anhydrous magnesium sulfate are dissolved separately in excess water. FA 3 is hydrated magnesium sulfate, MgSO4•7H2O. FA 4 is anhydrous magnesium sulfate, MgSO4. (a) Method Experiment 1 • Weigh the container with FA 3. Record the mass in the space below. • Support the cup in the 250 cm3 beaker. • Rinse the 25 cm3 measuring cylinder with distilled water. 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. • Tip all of the FA 3 into the water in the cup. Stir the mixture thoroughly. • Record the highest or lowest temperature of the mixture. • Calculate the change in temperature. Record this change. • Weigh the container with any remaining FA 3. Record the mass. • Calculate the mass of FA 3 used. Record this mass. Experiment 2 • Repeat the method using FA 4 in place of FA 3. Use the second cup. Results I II III IV V [5] (b) Calculations (i) Calculate the heat energy transferred, in J, in each experiment. Assume that 4.18 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C. Experiment 1 with FA 3 Experiment 2 with FA 4 heat energy = …… J heat energy = …… J [1] (ii) Calculate the enthalpy change, ∆H, in kJ mol–1, when 1.0 mol of solid dissolves in water in each experiment. Experiment 1 with FA 3 Experiment 2 with FA 4 ∆H1 = …… …… kJ mol–1 ∆H2 = …… …… kJ mol–1 sign value sign value [3] (iii) Use your answers to (b)(ii) to calculate the enthalpy change when 1.0 mol of hydrated magnesium sulfate is dehydrated to form 1.0 mol of anhydrous magnesium sulfate. MgSO4•7H2O(s) MgSO4(s) + 7H2O(l) Show clearly, by a Hess’s energy cycle or any other suitable means, how you calculated your answer. If you were unable to complete the calculation in (b)(ii) then assume that the enthalpy change in Experiment 1, ∆H1, is +8.7 kJ mol–1 and that the enthalpy change in Experiment 2, ∆H2, is –15.5 kJ mol–1. These may not be the correct values. enthalpy change of dehydration of MgSO4•7H2O = …… …… kJ mol–1 [2] sign value [Total: 11] 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

(a) (i) A student finds a container of a compound which is thought to be FA 3. The student labels the container FA 5. Carry out tests to determine whether your sample of FA 5 contains magnesium ions, sulfate ions and water of crystallisation. Record your tests and observations in a suitable form in the space below. If any solid is heated, a hard-glass test-tube must be used. [4] (ii) Complete Table 3.1 to indicate the contents of FA 5. Put a tick (3) in one box in each row. Table 3.1 yes no FA 5 contains magnesium ions FA 5 contains sulfate ions FA 5 contains water of crystallisation [1] (b) FA 6 contains one cation and one anion, both of which are listed in the Qualitative analysis notes. (i) In a hard-glass test-tube heat a spatula measure of FA 6. Heat gently at first and then heat strongly. Record all your observations. …… [2] (ii) Carry out the following tests and record your observations. Table 3.2 test observations Test 1 To a small spatula measure of FA 6 in a test-tube, add 5 cm depth of dilute hydrochloric acid. Keep the solution for Test 2. Test 2 Put 1 cm depth of the solution from Test 1 in a test-tube. Add aqueous ammonia dropwise at first and then until it is in excess. [2] (iii) Use your observations from (b)(i) and (ii) to identify the ions present in FA 6. cation …… anion …… [1] (iv) Write the ionic equation for the reaction that occurs in Test 2 before excess ammonia is added. Include state symbols. If you were unable to identify the cation in the solution then use the symbol M2+. …… [2] [Total: 12] 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 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 138.9 140.1 140.9 144.4 – 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