Chemistry 9701/35 October/November 2024: question paper and mark scheme

Question

The neutralisation of an acid by an alkali is an exothermic reaction. The concentration of an acid can be found by measuring the temperature change when the acid reacts with an alkali. You will determine the concentration of sulfuric acid by adding aqueous sodium hydroxide of known concentration to the sulfuric acid and measuring the temperature change. FA 1 is 2.01 mol dm–3 sodium hydroxide, NaOH. FA 2 is sulfuric acid, H2SO4. (a) Method ● Support the cup in the 250 cm3 beaker. ● Pipette 25.0 cm3 of FA 1 into the cup. ● Place the thermometer into the FA 1 in the cup. Tilt the cup if necessary to ensure the bulb of the thermometer is fully covered. Record the temperature of FA 1 in Table 1.1. This is the temperature when the volume of FA 2 is 0.00 cm3. ● Fill the burette with FA 2. ● Run 5.00 cm3 of FA 2 into the cup containing FA 1. ● Stir the mixture and record the maximum temperature in Table 1.1. ● Run further 5.00 cm3 portions of FA 2 into the same cup. ● After each addition of FA 2 stir the contents of the cup. Record the maximum temperature for each addition. Table 1.1 total volume of 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 FA 2 / cm3 temperature / °C I II III IV [4] Keep the rest of FA 2 for use in Question 2. (b) Plot a graph of temperature of solution (y-axis) against total volume of FA 2 added (x-axis) on the grid. Select a scale for the y-axis to include a value 3 °C above your maximum temperature reading. Label any points you consider to be anomalous. Draw two lines of best fit through the points on your graph. Draw the first line for the increase in temperature and the second line after the maximum temperature was reached. Extrapolate the lines so they intersect. This intersection corresponds to the volume of FA 2 needed to neutralise the FA 1 in your experiment in (a). I II III IV V 25.0 cm3 of FA 1 required …… cm3 of FA 2. [5] (c) (i) Calculate the amount, in mol, of sodium hydroxide, FA 1, pipetted into the cup. amount of NaOH = …… mol [1] (ii) The equation for this neutralisation reaction is shown. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) Calculate the concentration, in mol dm–3, of sulfuric acid in FA 2. Show your working. concentration of H2SO4 = …… mol dm–3 [2] [Total: 12]

Question

Acids react with carbonates to produce carbon dioxide gas. Na2CO3(s) + H2SO4(aq) Na2SO4(aq) + CO2(g) + H2O(l) This reaction can be used to determine the concentration of acid, using the mass of carbon dioxide released. You will determine the concentration of sulfuric acid in FA 2. FA 2 is the solution used in Question 1. FA 3 is sodium carbonate, Na2CO3. (a) Method ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of FA 2 into the conical flask. ● Weigh the flask with the acid. Record the mass. ● Weigh the container with FA 3. Record the mass. ● Carefully tip all of FA 3 into the acid in the conical flask. Swirl the contents of the flask and leave the flask to stand with occasional swirling until the fizzing stops. ● Weigh the container with any residual FA 3. Record the mass. ● Calculate and record the mass of FA 3 added to the flask. ● Calculate and record the total mass of flask + acid + FA 3. ● Weigh the flask and contents when the fizzing has stopped. Record the mass. ● Calculate and record the mass of carbon dioxide given off during the experiment. Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide given off in the reaction. amount of CO2 = …… mol [1] (ii) The sodium carbonate, FA 3, was in excess in the reaction with sulfuric acid. Show by calculation that the sodium carbonate was in excess. Use your answer to (b)(i). [2] (iii) Calculate the concentration, in mol dm–3, of sulfuric acid in FA 2. concentration of H2SO4 = …… mol dm–3 [1] (c) (i) A student does not have a conical flask and uses a small beaker for the reaction. Explain why a conical flask is better. …… [1] (ii) Two students made suggestions of how they thought the experiment in (a) could be adapted to determine the concentration of sulfuric acid, FA 2, by using other reactions. In each case their teacher told them that this method was not suitable. Explain, in each case, why the method is not suitable. Do not consider factors based on quantities of any reagent. Student 1 suggested using magnesium in place of sodium carbonate. Student 2 suggested using calcium carbonate in place of sodium carbonate. [3] (d) State the uncertainty in a single reading of your balance. uncertainty = ± …… g Calculate the maximum percentage error in the mass of FA 3 that you weighed out in (a). 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

FA 4, FA 5 and FA 6 are compounds of the same metal in different oxidation states. (a) (i) Place a small spatula measure of FA 4 in a hard-glass test-tube. Heat the tube gently at first and then more strongly. Record all your observations. Leave the tube and contents to cool and keep for use in (a)(iii). …… [2] (ii) Dissolve a small spatula measure of FA 4 in approximately 5 cm depth of distilled water in a boiling tube and add approximately 1 cm depth of dilute sulfuric acid. Carry out the following tests and record your observations in Table 3.1. Table 3.1 test observations Test 1 To a 1 cm depth of aqueous FA 4 in a test-tube add aqueous iron(II) sulfate. Test 2 To a 1 cm depth of aqueous FA 4 in a test-tube add hydrogen peroxide. [2] (iii) To the cooled test-tube in (a)(i) add a 5 cm depth of distilled water. Observe and record the colour formed. …… [1] (b) Dissolve a small spatula measure of FA 5 in a boiling tube half-filled with distilled water. Warming may be needed to dissolve the FA 5. Carry out the following tests and record your observations in Table 3.2. For each of the tests use a 1 cm depth of this FA 5 solution in a test-tube. Table 3.2 test observations Test 1 Add dilute nitric acid, then add aqueous silver nitrate. Test 2 Add aqueous barium chloride or barium nitrate, then add dilute nitric acid. Test 3 Add aqueous sodium hydroxide, then add hydrogen peroxide. [3] (c) Carry out the following tests and record your observations in Table 3.3. Identify any gases produced. For each of the tests use a small spatula measure of FA 6 in a test-tube. Table 3.3 test observations and gases produced Test 1 Add a 1 cm depth of dilute nitric acid. Test 2 Add a few drops of concentrated hydrochloric acid. CARE Hydrochloric acid is corrosive. Fill the test-tube with water as soon as you have made your observation. Test 3 Add a 1 cm depth of hydrogen peroxide. [3] (d) (i) Use your observations from the tests on FA 4, FA 5 and FA 6 to suggest the identity of the metal present in all 3 compounds. Metal identity …… [1] (ii) Identify the oxidation state of the metal in each compound. FA 4 …… FA 5 …… FA 6 …… [2] [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 3 4 5 6 7 8 9 10 11 12 23.0 24.3 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. 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