Chemistry 9701/33 February/March 2020: question paper and mark scheme

Question

The concentrations of solutions of hydrogen peroxide are often represented in terms of ‘volume strength’. In this experiment you will determine the ‘volume strength’ of a solution of hydrogen peroxide by titration with acidified potassium manganate(VII). 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) FA 1 is 0.0300 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous hydrogen peroxide, H2O2. (a) Method Dilution of FA 3 ●● Pipette 25.0 cm3 of FA 3 into the 250 cm3 volumetric flask. ●● Make the solution up to the mark using distilled water. ●● Shake the flask thoroughly. ●● Label this diluted solution of hydrogen peroxide FA 4. Titration ●● Fill the burette with FA 1. ●● Rinse the pipette thoroughly with distilled water and then with a little FA 4. ●● Pipette 25.0 cm3 of FA 4 into a conical flask. ●● Use the 25 cm3 measuring cylinder to add 20 cm3 of FA 2 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 all of your burette readings and the volume of FA 1 added in each accurate titration. Keep FA 1, FA 2 and FA 3 for use in Questions 2 and 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required …… cm3 of FA 1. [1] (c) Calculations (i) Give your answers to (ii), (iii), (iv) and (v) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of potassium manganate(VII) present in the volume calculated in (b). moles of KMnO4 = …… mol [1] (iii) The equation for the reaction of potassium manganate(VII) with hydrogen peroxide is shown. 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) Use your answer to (c)(ii) to calculate the number of moles of hydrogen peroxide used in each titration. moles of H2O2 = …… mol Hence calculate the concentration of H2O2 in FA 4, in mol dm–3. concentration of H2O2 in FA 4 = …… mol dm–3 [1] (iv) Calculate the concentration of H2O2 in FA 3, in mol dm–3. concentration of H2O2 in FA 3 = …… mol dm–3 [1] (v) When hydrogen peroxide decomposes in the presence of a catalyst, oxygen is produced. 1 H2O2(aq) H2O(l) + 2 O2(g) The ‘volume strength’ of hydrogen peroxide is equal to the volume of oxygen, in dm3, produced under room conditions, when 1.00 dm3 of the solution decomposes. Use your answer to (c)(iv) and the equation above to calculate the volume, in dm3, of oxygen produced when 1.00 dm3 of FA 3 decomposes. This is the ‘volume strength’, in vol, of FA 3. (Under room conditions 1.00 mol of gas occupies a volume of 24.0 dm3. If you were unable to calculate the concentration of H2O2 in FA 3, assume that it is 1.02 mol dm–3. This may not be the correct value.) ‘volume strength’ of FA 3 = …… vol [2] (d) The maximum error in reading a 25.0 cm3 pipette is ±0.06 cm3. Show by calculation that the pipette is more accurate than a burette for measuring 25.0 cm3 of solution. . …… . …… [1] [Total: 15]

Question

In this experiment you will determine the enthalpy change, ΔH, for the catalytic decomposition of hydrogen peroxide into water and oxygen. 1 H2O2(aq) H2O(l) + 2 O2(g) FA 3 is aqueous hydrogen peroxide, H2O2. FA 5 is manganese(IV) oxide, MnO2. (a) Method Experiment 1 ●● Support one of the plastic cups inside the 250 cm3 beaker. ●● Use the 50 cm3 measuring cylinder to add 30 cm3 of FA 3 into the plastic cup. ●● Measure and record the initial temperature of the solution. ●● Add a heaped spatula measure of FA 5 to the solution in the plastic cup. ●● Stir constantly until the maximum temperature is reached and record this temperature. ●● Calculate and record the temperature rise. ●● Rinse and dry the thermometer. Experiment 2 ●● Support the second plastic cup inside the 250 cm3 beaker. ●● Use the 50 cm3 measuring cylinder to add 40 cm3 of FA 3 into the plastic cup. ●● Measure and record the initial temperature of the solution. ●● Add a heaped spatula measure of FA 5 to the solution in the plastic cup. ●● Stir constantly until the maximum temperature is reached and record this temperature. ●● Calculate and record the temperature rise. I II III IV V [5] (b) Calculation (i) Calculate the energy released in Experiment 1. [Assume that 4.2 J changes the temperature of 1.0 cm3 of solution by 1.0 °C.] energy released = …… J [1] (ii) Use your answer to 1(c)(iv) to calculate the number of moles of hydrogen peroxide used in Experiment 1. (If you were unable to calculate the concentration of H2O2 in FA 3, assume that it is 1.02 mol dm–3. This may not be the correct value.) moles of H2O2 = …… mol [1] (iii) Calculate the enthalpy change, in kJ mol–1, for the decomposition of 1 mole of hydrogen peroxide into water and oxygen. enthalpy change = …… …… kJ mol–1 [1] sign value (c) (i) A student suggested that the experiment would be more accurate if the same mass of FA 5, manganese(IV) oxide, had been weighed out for each experiment. State and explain whether you agree with the student’s suggestion. . …… . …… . …… . …… [2] (ii) The student also suggested that Experiments 1 and 2 should give the same temperature rise, even though a greater volume of FA 3 was used in Experiment 2. State and explain whether you agree with the student’s suggestion. . …… . …… . …… [1] [Total: 11] Qualitative analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen ● the formation of any precipitate and its solubility in an excess of the reagent added ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Question

(a) FA 3 is aqueous hydrogen peroxide, H2O2. FA 6 is a solution containing two cations and one anion from those listed in the Qualitative analysis notes. (i) To a 1 cm depth of FA 6 in a boiling tube, add aqueous sodium hydroxide until it is in excess. Then heat the tube, gently and carefully. Keep the mixture obtained in the boiling tube for the test in (a)(ii). Record all your observations. Identify the cations in FA 6. observations …… . …… . …… . …… . …… FA 6: cations are …… and . …… [4] (ii) To the mixture obtained from (a)(i), carefully add a 1 cm depth of FA 3. Record your observations. . …… . …… [1] (iii) One reaction taking place in (a)(ii) involves oxidation of one of the cations in FA 6. Give the half-equation to show this oxidation reaction. State symbols are not required. . …… [1] FA 1 is aqueous potassium manganate(VII). (b) FA 2 is dilute sulfuric acid. FA 7 and FA 8 are solutions, each containing one cation and one anion. (i) Carry out the following tests and record your observations in the table. observations test FA 7 FA 8 Test 1 To a 1 cm depth of solution in a test‑tube, add a small spatula measure of solid sodium carbonate. Test 2 To a 1 cm depth of solution in a test‑tube, add an equal volume of FA 2 and a few drops of FA 1, then add a few drops of aqueous starch. Test 3 To a 1 cm depth of solution in a test‑tube, add a few drops of aqueous silver nitrate, then add aqueous ammonia. [5] (ii) Identify the anion in FA 7. anion . …… [1] (iii) Identify FA 8. FA 8 is . …… [1] (iv) Carry out one further test to confirm the identity of the cation in FA 8. State the name of the reagent you used and record the observation(s) you made. reagent …… observation(s) …… . …… [1] [Total: 14] Qualitative analysis notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. pale blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess 2 Reactions of anions ion reaction carbonate, CO2 liberated by dilute acids CO32– chloride, gives white ppt. with Ag+(aq) (soluble in NH3(aq)) Cl –(aq) bromide, gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)) Br –(aq) iodide, gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) I (aq) – nitrate, NH3 liberated on heating with OH–(aq) and Al foil NO3 (aq)– nitrite, NH3 liberated on heating with OH–(aq) and Al foil NO2 (aq)– sulfate, gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids) SO4 (aq)2– sulfite, gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) SO32–(aq) 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 (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint 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 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 89–103 104 105 106 107 108 109 110 111 112 114 116 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Fl Lv francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium flerovium livermorium – – – – – – – – – – – – – 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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