Cambridge IGCSE Chemistry (9-1) 0971 — 2024 Oct/Nov Paper 5 · Variant 2

1 You are going to investigate the rate of the reaction between aqueous iron(III) nitrate and aqueous sodium thiosulfate. Read all of the instructions carefully before starting the experiments. Instructions You are going to do five experiments using the apparatus shown in Fig. 1.1. eye aqueous sodium thiosulfate 100 cm3 beaker 50 cm3 of aqueous iron(III) nitrate printed insert printed text Fig. 1.1 (a) Experiment 1 ● Use the 50 cm3 measuring cylinder to pour 50 cm3 of aqueous iron(III) nitrate into the 100 cm3 beaker. ● Stand the beaker on the printed text of the insert as shown in Fig. 1.1. ● Use the 25 cm3 measuring cylinder to pour 15.0 cm3 of aqueous sodium thiosulfate into the beaker. At the same time start the stop-watch. ● Stir the contents of the beaker. ● Look down from above the beaker. When the printed text on the insert becomes visible, stop the stop-watch and record the time in seconds to the nearest whole number in Table 1.1. ● Rinse the beaker with distilled water. Experiment 2 ● Use the 50 cm3 measuring cylinder to pour 50 cm3 of aqueous iron(III) nitrate into the 100 cm3 beaker. ● Stand the beaker on the printed text of the insert as shown in Fig. 1.1. ● Use the 10 cm3 measuring cylinder to pour 10.0 cm3 of aqueous sodium thiosulfate into the beaker. At the same time start the stop-watch. ● Stir the contents of the beaker. ● Look down from above the beaker. When the printed text on the insert becomes visible, stop the stop-watch and record the time in seconds to the nearest whole number in Table 1.1. ● Rinse the beaker with distilled water. Experiment 3 ● Repeat Experiment 2, using 7.0 cm3 of aqueous sodium thiosulfate instead of 10.0 cm3. Experiment 4 ● Repeat Experiment 2, using 6.0 cm3 of aqueous sodium thiosulfate instead of 10.0 cm3. Experiment 5 ● Repeat Experiment 2, using 5.0 cm3 of aqueous sodium thiosulfate instead of 10.0 cm3. Table 1.1 experiment 1 2 3 4 5 volume of aqueous sodium thiosulfate / cm3 time taken for text to become visible / s [4] (b) Write a suitable scale on the y-axis and plot your results from Experiments 1 to 5 on Fig. 1.2. Draw a smooth curve of best fit. time taken for text to become visible / s 4.0 6.0 8.0 10.0 12.0 14.0 16.0 volume of aqueous sodium thiosulfate / cm3 Fig. 1.2 [4] (c) State why the contents of the beaker are stirred after adding the aqueous sodium thiosulfate to the aqueous iron(III) nitrate. .................................................................................................................................................... .............................................................................................................................................. [1] (d) Deduce which experiment has the highest rate of reaction. .............................................................................................................................................. [1] (e) Use your graph in Fig. 1.2 to predict the time taken for the text to become visible if the volume of aqueous sodium thiosulfate is 12.5 cm3. Show your working on Fig. 1.2. time = .............................. [3] (f) (i) Explain why it would be an improvement to measure the volumes of aqueous iron(III) nitrate in a burette rather than in a measuring cylinder. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Explain why it is not possible to use a volumetric pipette to measure the volumes of the aqueous sodium thiosulfate used in the experiments. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Describe how the reliability of the results of this investigation can be checked. ............................................................................................................................................. ....................................................................................................................................... [1] (g) Describe how the results of the experiments would change if the experiments are repeated using a narrower and taller beaker. Explain your answer. change in results ....................................................................................................................... explanation ................................................................................................................................ .................................................................................................................................................... [2] (h) Describe additional measurements that must be taken to determine whether the reaction in this investigation is exothermic or endothermic. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 19]

Mark scheme: Question Answer Marks 1(a) M1 all five volumes of aqueous sodium thiosulfate recorded correctly (15.0, 10.0, 7.0, 6.0, 5.0) 4 M2 all five times recorded in seconds AND to whole number of seconds M3 times increase from Experiment 1 to 5 as volume of sodium thiosulfate decreases M4 time for Experiment 5 is approximately twice time for Experiment 1 1(b) M1 y-axis scale is linear AND points extend over halfway up scale 4 M2 and M3 all points plotted correctly M4 best-fit line 1(c) to mix the reactants / solutions 1 OR so that concentration(s) are uniform throughout the mixture 1(d) Experiment 1 1 1(e) M1 working shown at 12.5 cm3 on graph 3 M2 correct value from graph M3 units given as s 1(f)(i) burette is more accurate (than a measuring cylinder) 1 1(f)(ii) volume is not the same in each run / volume is not 25 cm3 1 1(f)(iii) repeat AND compare / repeat AND look for/exclude anomalies / repeat AND compare curve shape 1 1(g) M1 times longer 2 M2 greater depth of liquid 1(h) (measure) temperature before AND after (the reaction) 1

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2 You are provided with two solids: solid M and solid N. Do the following tests on solid M and solid N. Record all of your observations at each stage. Tests on solid M (a) Carry out a flame test on solid M. Record your observations. .............................................................................................................................................. [1] Transfer the remaining solid M to a boiling tube. Add about 5 cm depth of distilled water to solid M, place a stopper in the boiling tube and shake the boiling tube to dissolve solid M and form solution M. Divide solution M into four approximately equal portions in one boiling tube and three test-tubes. (b) To the first portion of solution M in the boiling tube, add aqueous sodium hydroxide dropwise until it is in excess. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [2] (c) To the second portion of solution M in a test-tube, add the piece of magnesium ribbon. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [1] (d) To the third portion of solution M in a test-tube, add about 2 cm depth of aqueous hydrogen peroxide. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [2] (e) To the fourth portion of solution M in a test-tube, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. Leave the mixture to stand for about two minutes. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [1] (f) Identify the three ions in solid M. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [3] Tests on solid N (g) Transfer approximately half of solid N to a boiling tube. Add about 5 cm depth of dilute sulfuric acid to the boiling tube. Test any gas produced. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [2] (h) Add the remaining solid N to a boiling tube and add about 2 cm depth of distilled water. Do not place a stopper in the boiling tube. Shake the boiling tube carefully to dissolve solid N and form solution N. To solution N, add about 2 cm depth of aqueous sodium hydroxide. Gently warm the mixture and test any gas given off. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [1] (i) Identify solid N. .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 15]

Mark scheme: 2(a) yellow (flame colour) 1 2(b) M1 green precipitate 2 M2 remains in excess 2(c) fizzing / bubbles / effervescence 1 2(d) M1 effervescence / fizzing / bubbles 2 M2 becomes orange/brown 2(e) cream precipitate 1 2(f) M1 sodium / Na+ 3 M2 iron(II) / Fe2+ M3 bromide / Br– 2(g) M1 effervescence / fizzing / bubbles 2 M2 limewater turns milky 2(h) (damp red) litmus (paper) turns blue 1 2(i) ammonium carbonate / (NH4)2CO3 2

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3 Limes and lemons are citrus fruits which contain aqueous citric acid in their juice. Citric acid reacts with alkalis such as aqueous sodium hydroxide. Plan an investigation to find which of lime juice and lemon juice contains the most concentrated aqueous citric acid. Assume that citric acid is the only acid present in the juices. Your plan must include: ● the method to use ● how the results are used to determine which juice contains the most concentrated aqueous citric acid. You are provided with lime juice, lemon juice, aqueous sodium hydroxide and common laboratory apparatus and chemicals. ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ..................................................................................................................................................... [6]

Mark scheme: 3 any six from: 6 MP1 known / same / specified volume of juice MP2 add a named acid-alkali indicator to the juice MP3 suitable apparatus for titration (conical flask / beaker) MP4 NaOH added from burette to juice MP5 mix / swirl MP6 indicator / juice changes colour MP7 the juice that needs most NaOH is most concentrated OR MP1 known / same / specified volume of NaOH MP2 add a named acid-alkali indicator to the NaOH MP3 suitable apparatus for titration (conical flask / beaker) MP4 juice added from burette to NaOH MP5 mix / swirl MP6 indicator / alkali changes colour MP7 the juice with the smallest volume is most concentrated

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