Cambridge A Level Chemistry 9701 — 2021 May/June Paper 5 · Variant 1

1 Hydrogen peroxide decomposes slowly at room temperature to give water and oxygen. 2H2O2(aq) → 2H2O(l) + O2(g) The initial rate of this reaction can be increased by the addition of a metal oxide catalyst. A student is asked to investigate which metal oxide catalyst is best at increasing the initial rate of this reaction by using a method which involves the collection of oxygen. The student is provided with the following metal oxides: copper(II) oxide, iron(III) oxide, manganese(IV) oxide, nickel(II) oxide and titanium(IV) oxide. The student is also provided with an excess volume, of a known concentration, of aqueous hydrogen peroxide and any laboratory equipment needed. (a) (i) State the independent variable. ....................................................................................................................................... [1] (ii) State the dependent variable. ....................................................................................................................................... [1] (b) State two variables that would need to be controlled. 1 ................................................................................................................................................. 2 ................................................................................................................................................. [2] (c) Draw a labelled diagram of the assembled apparatus that could be used to carry out these experiments. The apparatus should allow the accurate recording of the oxygen produced. [3] (d) (i) What measurements need to be recorded during the course of each experiment to allow the initial rate to be determined? ....................................................................................................................................... [1] (ii) How is the initial rate determined using these measurements? ............................................................................................................................................. ....................................................................................................................................... [1] (e) How can the student ensure that the results are reliable? .............................................................................................................................................. [1] (f) Suggest an alternative method to investigate these reactions which does not include the collection of gas. .................................................................................................................................................... .............................................................................................................................................. [1] (g) Once the reaction has finished, how can the student demonstrate that the metal oxide has not been affected by the reaction? .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] (h) When aqueous hydrogen peroxide is stored there is a small hole in the lid of the bottle. Suggest why this is necessary. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 14]

Mark scheme: 1(a)(i) the metal oxide 1 1(a)(ii) count bubbles in a set time OR measure volume (of oxygen) in a certain time OR measure time to produce a certain volume 1 1(b) Any two from: the volume of the hydrogen peroxide / solution the concentration of the hydrogen peroxide temperature particle size 2 1(c) correctly drawn diagram showing: M1: contents of flask M2: sealed apparatus M3: collection of the gas in a measuring cylinder (graduations assumed) (via a delivery tube into water) / gas syringe 3 1(d)(i) volume of gas / oxygen AND time 1 1(d)(ii) plot a (volume / time) graph AND measure gradient / produce a tangent at t = 0 or at the start 1 1(e) repeat (the experiments / trials) (until consistent results) 1 Question Answer Marks 1(f) measuring the decrease / change in (total) mass of the solution / mixture / reaction vessel (and contents) (and time) OR measuring the mass / weight of the mixture/reaction vessel (and contents) AND time 1 1(g) M1: (filter/remove) dry the residue/metal oxide M2: weigh the residue / metal oxide to see if the mass is unchanged OR M1: re-use with new hydrogen peroxide / do the same reaction again with the same catalyst M2: and check if the gas volumes identical / rate of reaction the same 2 1(h) to allow the oxygen / gas (that is formed) to be released / escape / diffuse out 1

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2 A student is given 250.0 cm3 of solution containing a mixture of Fe2+ and Fe3+ ions. The student is asked to find the total mass of iron ions and the percentage by mass of Fe3+ in the solution by performing titrations with aqueous potassium manganate(VII), KMnO4. The student is told that the Fe3+(aq) ions can be reduced to Fe2+(aq) ions by reaction with zinc. The student is given the following instructions. ● Calculate the mass of KMnO4 needed to make 500.0 cm3 of 0.0200 mol dm–3 KMnO4(aq). ● Record the mass of an empty plastic weighing boat (a small container used to hold solid samples). ● Add the calculated mass of KMnO4 to the weighing boat. ● Transfer the KMnO4 from the weighing boat into a 100 cm3 beaker. ● Add 50 cm3 of distilled water to the beaker. ● Transfer the mixture from the beaker into a 500.0 cm3 volumetric flask. ● Make up to the graduation mark, dropwise, with distilled water. (a) (i) Calculate the mass of KMnO4 needed to make 500.0 cm3 of 0.0200 mol dm–3 KMnO4(aq). [Ar: K, 39.1; Mn, 54.9; O, 16.0] mass of KMnO4 needed = .............................. g [1] (ii) The student used a balance accurate to two decimal places. Calculate the percentage error in weighing the mass of the KMnO4 by difference. If you were unable to calculate a value for 2(a)(i) use the mass 1.75 g. This is not the correct answer to 2(a)(i). Show your working. percentage error = .............................. % [1] (iii) The student noticed that some crystals of KMnO4 were stuck to the weighing boat after adding the KMnO4 solid to the beaker. State how the student should modify the instructions to ensure that the measured mass of KMnO4 was accurate. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iv) Give two additional instructions that should be given to the student to ensure that the solution is prepared as accurately as possible. 1 .......................................................................................................................................... ............................................................................................................................................. 2 .......................................................................................................................................... ............................................................................................................................................. [2] (b) When the KMnO4(aq) is ready for use, the student is given additional instructions. step 1 Fill a burette with 0.0200 mol dm–3 KMnO4(aq). step 2 Using a measuring cylinder, transfer 25.00 cm3 of Fe2+(aq) / Fe3+(aq) solution into a conical flask. step 3 Add 10 cm3 of 1.0 mol dm–3 sulfuric acid to the conical flask. step 4 Titrate this acidified solution of Fe2+(aq) / Fe3+(aq) with 0.0200 mol dm–3 KMnO4(aq) until the end-point. step 5 Repeat titrations until the titres are concordant. This set of results is set A. step 6 Using a measuring cylinder, add 100 cm3 of the Fe2+(aq) / Fe3+(aq) solution into a beaker then add excess zinc. Allow time for reduction to Fe2+(aq) to take place. step 7 Filter the mixture into a beaker. step 8 Transfer 25.00 cm3 of the filtrate into a conical flask and add 10 cm3 of 1.0 mol dm–3 sulfuric acid. step 9 Titrate this acidified solution of the filtrate with 0.0200 mol dm–3 KMnO4(aq) until the end-point. step 10 Repeat steps 8 and 9 twice. This set of results is set B. (i) How should the burette be prepared for use before it is filled in step 1? ............................................................................................................................................. ....................................................................................................................................... [1] (ii) What must be done to ensure as accurate an end-point as possible? ............................................................................................................................................. ....................................................................................................................................... [1] (c) (i) Identify an experimental weakness in step 2. Explain how this would affect the results. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) How could this weakness be overcome? ....................................................................................................................................... [1] (d) The results for each set of titrations are shown. set A rough titration 1 titration 2 titration 3 final volume / cm3 18.40 17.25 34.55 18.00 initial volume / cm3 0.65 0.15 17.25 0.95 titre / cm3 set B rough titration 1 titration 2 titration 3 final volume / cm3 45.05 43.60 43.70 initial volume / cm3 0.20 0.15 0.10 titre / cm3 (i) Complete both tables and calculate an appropriate average titre for each set of results. The student could not carry out titration 3 in set B. Record the average titre to one decimal place. set A average titre = .............................. cm3 set B average titre = .............................. cm3 [2]

Mark scheme: 2(a)(i) Mr KMnO4 = 158.0 500 × 0.02 / 1000 = 0.01 0.01 × 158.0 = 1.58 g 2(a)(ii) (2 × 0.005 / 1.58) × 100 = 0.63% 1 2(a)(iii) rinse the solid off the weighing boat into the beaker OR weighing the mass directly into the beaker OR (re)-weigh the weighing boat after transferring the KMnO4 into the beaker 1 2(a)(iv) Any two from: stir / agitate / mix (to ensure that the solid has dissolved) rinse the beaker and transfer the washings shaking / inverting / homogenising of volumetric solution 2 Question Answer Marks 2(b)(i) rinse / run through / wash the burette with some of the KMnO4 solution OR run some of the KMnO4 solution from the burette to fill to the tip 1 2(b)(ii) add the KMnO4 dropwise (near the end point) 1 2(c)(i) the Fe2+(aq) / Fe3+(aq) mixture measured using a measuring cylinder AND leads to increased likelihood of non-concordant titres 1 2(c)(ii) (measure the volume) using a (volumetric / 25 cm3) pipette 1 Question Answer Marks 2(d)(i) Set A rough titration 1 titration 2 titration 3 final volume / cm3 18.40 17.25 34.55 18.00 initial volume / cm3 0.65 0.15 17.25 0.95 titre / cm3 17.75 17.10 17.30 17.05 mean titre = 17.1 (cm3) Set B rough titration 1 titration 2 final volume / cm3 45.05 43.60 43.70 initial volume / cm3 0.20 0.15 0.10 titre / cm3 44.85 43.45 43.60 mean titre = 43.5 (cm3) M1: correct titres (to 2 decimal places) M2: correct means 2 2(d)(ii) M1: n MnO4– (aq) = 43.5 × 0.02 / 1000 = 8.70 × 10–4 n Fe2+ (aq) = 8.70 × 10–4 × 5 = 4.35 × 10–3 M2: mass of Fe 2+ (aq) = 4.35 × 10–3 × 55.8 × 100 25 = 0.971 g 2 Question Answer Marks 2(d)(iii) n MnO4- (aq) = 17.1 × 0.02 / 1000 = 3.42 × 10–4 n Fe2+ (aq) = 3.42 × 10–4 × 5 = 1.71 × 10–3 mass of Fe 2+ (aq) = 1.71 × 10–3 × 55.8 × 250 25 = 0.954 g 1 2(d)(iv) % Fe3+ = ((2(d)(ii) × 250 100 ) – 2(d)(iii))/(2(d)(ii) × 250 100 )) × 100% OR (2(d)(ii) × 2.5 – 2(d)(iii)) / (2(d)(ii) × 2.5) × 100% 1 2(d)(v) reduce / use (in step 6) more than 100 cm3 of solution (Fe2+ and Fe3+) 1

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