Chemistry 9701/34 October/November 2019: question paper and mark scheme

Question

Hydrogen peroxide decomposes in a reaction catalysed by manganese(IV) oxide. 2H2O2(aq) 2H2O(l) + O2(g) You will investigate this decomposition by measuring the volume of oxygen collected over a period of time. You will also use the volume of oxygen collected to calculate the concentration of the aqueous hydrogen peroxide. FB 1 is aqueous hydrogen peroxide, H2O2. FB 2 is manganese(IV) oxide, MnO2. (a) Method ● Fill the tub with water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so that the open end is in the water just above the base of the tub. ● Use the 50 cm3 measuring cylinder to place 30 cm3 of FB 1 and 20 cm3 of distilled water into the reaction flask labelled X. ● Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the 250 cm3 measuring cylinder. ● Remove the bung from the neck of flask X. Add all of the FB 2 into the hydrogen peroxide in the flask and replace the bung immediately. Start the stop-clock and leave it running until the end of the experiment. ● Remove the flask from the clamp and swirl to mix the contents, then replace the flask in the clamp. ● After 1 minute measure the volume of gas collected. ● After 4 minutes from the start of the experiment measure the volume of gas collected. Keep FB 1 for use in Question 3. Results [3] (b) Calculation (i) Use the volume of gas that you collected at 4 minutes to calculate the number of moles of hydrogen peroxide which had decomposed at this time. (Assume 1 mol of gas occupies 24.0 dm3 at this temperature.) moles of H2O2 = …… mol [1] (ii) Assume all the H2O2 had decomposed in 4 minutes. Calculate the initial concentration of H2O2, in mol dm3, in FB 1. initial concentration of H2O2 = …… mol dm–3 [2] (c) A student missed taking a reading at 1 minute so took a reading at 2 minutes instead. This student stated that after 2 minutes: volume of gas collected rate of reaction = cm3 minute–1 2 Is the student correct? Explain your answer. . …… . …… . …… . …… [2] (d) Another student carried out the experiment in (a) but used twice the mass of manganese(IV) oxide. State and explain what effect this would have on the results obtained. . …… . …… [1] [Total: 9]

Question

In Question 1 you determined the concentration of a sample of aqueous hydrogen peroxide, FB 1, by measuring the volume of oxygen produced when it decomposed. In Question 2 you will determine the concentration of a different sample of aqueous hydrogen peroxide by titration with acidified manganate(VII) ions. The equation for the reaction is shown. 2MnO4–(aq) + 6H+(aq) + 5H2O2(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) FB 3 is aqueous hydrogen peroxide, H2O2. FB 4 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FB 5 is 1 mol dm–3 sulfuric acid, H2SO4. (a) Method ● Fill the burette with FB 4. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Rinse the 50 cm3 measuring cylinder with distilled water. ● Use this measuring cylinder to add 20 cm3 of FB 5 into 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 certain any recorded results show the precision of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FB 4 added in each accurate titration. Keep FB 4 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FB 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 3 required …… cm3 of FB 4. [1] (c) Calculations (i) Calculate the number of moles of manganate(VII) ions present in the volume of FB 4 recorded in (b). moles of MnO4– = …… mol [1] (ii) Use your answer to (c)(i) and the equation on page 4 to determine the number of moles of hydrogen peroxide present in 25.0 cm3 of FB 3. moles of H2O2 = …… mol [1] (iii) Calculate the concentration, in mol dm–3, of hydrogen peroxide in FB 3. concentration of H2O2 = …… mol dm–3 [1] (d) In Question 1 and in Question 2 you determined the concentration of aqueous hydrogen peroxide using different methods. The method used in Question 2 is the more accurate. Identify two sources of error in the determination of the concentration in Question 1 and suggest how these errors could be minimised. error 1 …… minimised by . …… error 2 …… minimised by . …… [2] (e) A student suggested one source of error in the method used in Question 2 was that the sulfuric acid was measured using a measuring cylinder and that a pipette should be used. Explain whether this suggestion is correct. . …… . …… [1] (f) (i) Another student was given a sample of aqueous hydrogen peroxide that was labelled as ‘10 vol’. The theoretical concentration of this sample of H2O2(aq) is 0.833 mol dm–3. The student used a titration method to find the actual concentration of this sample and found it to be 0.796 mol dm–3. Calculate the percentage difference, based on the theoretical concentration, between the actual and theoretical concentrations. percentage difference = …… % [1] (ii) When determining the concentration of hydrogen peroxide in a school or college laboratory, the value is nearly always much lower than the theoretical value. Suggest a reason for this difference. . …… . …… [1] [Total:16] 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) (i) FB 6, FB 7 and FB 8 are all aqueous solutions. Each contains one anion and one cation. Carry out the following tests and record your observations. observations test FB 6 FB 7 FB 8 To a 1 cm depth in a test‑tube add a 1 cm depth of dilute sulfuric acid and then add a few drops of FB 4, KMnO4(aq). To a 1 cm depth in a boiling tube add aqueous sodium hydroxide, then warm gently. To a 1 cm depth in a test‑tube add a 1 cm depth of FB 1, H2O2(aq), and then add aqueous sodium hydroxide. To a 1 cm depth in a test‑tube add aqueous barium chloride or aqueous barium nitrate. To a 1 cm depth in a boiling tube add a 1 cm depth of aqueous sodium hydroxide and a piece of aluminium foil and then warm gently. [8] (ii) Identify, with a reason, the cation present in FB 6. . …… . …… [1] (iii) Identify, with a reason, two anions that could be present in FB 6. . …… . …… [1] (iv) Identify, with a reason, a cation that could not be present in FB 7. . …… . …… [1] (v) Identify, with a reason, an anion that could be present in FB 8. . …… . …… [1] (b) A student is given an unlabelled bottle containing a liquid that is either propan‑1‑ol, CH3CH2CH2OH, or ethanoic acid, CH3COOH. Describe tests that would allow the student to confirm the identity of the liquid. Record in a suitable table the tests and the expected positive result for each of your tests. [3] [Total: 15] 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. 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 CO3 2– 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) SO42–(aq) sulfite, gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) SO3 (aq)2– 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. 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