Cambridge IGCSE Chemistry (9-1) 0971 — 2025 May/June Paper 6 · Variant 1

1 A student prepares some crystals of pure potassium sulfate by reacting aqueous potassium carbonate with dilute sulfuric acid. The equation for the reaction is shown. K2CO3(aq) + H2SO4(aq) K2SO4(aq) + H2O(l) + CO2(g) The student: • places 25.0 cm3 of aqueous potassium carbonate in a conical flask • adds a few drops of a suitable indicator to the conical flask • uses a burette to add dilute sulfuric acid to the conical flask until the end-point is reached and the indicator changes colour. Table 1.1 shows the burette readings the student obtains. Table 1.1 final burette reading / cm3 18.8 initial burette reading / cm3 4.2 volume of dilute sulfuric acid added / cm3 (a) Complete Table 1.1 by calculating the volume of dilute sulfuric acid added to the conical flask. [1] (b) State which solution, aqueous potassium carbonate or dilute sulfuric acid, is the least concentrated. Give a reason for your answer. least concentrated solution ....................................................................................................... reason ....................................................................................................................................... [1] (c) Name a piece of apparatus suitable for measuring the 25.0 cm3 of aqueous potassium carbonate. ............................................................................................................................................. [1] (d) Name a suitable indicator and give the colour change of this indicator at the end-point. indicator .................................................................................................................................... colour change at end-point ................................................. to .................................................. [2] (e) As the student added the dilute sulfuric acid to the aqueous potassium carbonate they looked for a colour change. State what the student should do as they add the dilute sulfuric acid to the aqueous potassium carbonate in the conical flask. Do not include observations in your answer. ................................................................................................................................................... ............................................................................................................................................. [1] (f) Describe what the student should now do to obtain crystals of pure potassium sulfate. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] [Total: 9]

Mark scheme: Question Answer Marks 1(a) 14.6 1 1(b) (least concentrated solution) potassium carbonate 1 and (because) a larger volume of potassium carbonate was used 1(c) (volumetric) pipette / burette 1 1(d) M1 methyl orange 1 M2 yellow to orange 1 1(e) swirl / mix (the contents of the flask) 1 1(f) M1 mix / add / combine together 25.0 cm3 aqueous potassium carbonate and 14.6 cm3 dilute sulfuric acid (without indicator) 1 M2 (place in an evaporating basin) and warm until crystals start to form / warm until crystalising point / warm until 1 saturated / half / most water gone M3 cool (solution) (so that crystals form) / leave solution from M2 so that crystals form 1

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2 A student investigates how the rate of reaction of magnesium ribbon with dilute acid changes as the concentration of the acid is changed. The student uses five solutions of the same acid, A, B, C, D, and E. Each solution has a different concentration. The acid is in excess in all experiments. The student does five experiments. Experiment 1 • Use a 50 cm3 measuring cylinder to pour 30 cm3 of acid A into a 100 cm3 conical flask. • Add a coil of magnesium ribbon to the acid in the conical flask and immediately start a stop- watch. • Continually swirl the mixture in the conical flask until the magnesium ribbon disappears completely. Immediately stop the stop-watch and record the time in seconds to the nearest second. • Empty and rinse the conical flask with distilled water. Experiment 2 • Repeat Experiment 1 using 30 cm3 of acid B instead of acid A. Experiment 3 • Repeat Experiment 1 using 30 cm3 of acid C instead of acid A. Experiment 4 • Repeat Experiment 1 using 30 cm3 of acid D instead of acid A. Experiment 5 • Repeat Experiment 1 using 30 cm3 of acid E instead of acid A. (a) Use the stop-watch diagrams to complete Table 2.1 Table 2.1 time for concentration of experiment acid stop-watch diagram magnesium to acid in mol / dm3 disappear in s 0 seconds 0 1 A 2.0 45 15 5 15 10 minutes 30 0 seconds 0 2 B 1.5 45 15 5 15 10 minutes 30 0 seconds 0 3 C 1.0 45 15 5 15 10 minutes 30 0 seconds 0 4 D 0.8 45 15 5 15 10 minutes 30 0 seconds 0 5 E 0.5 45 15 5 15 10 minutes 30 [2] (b) Write a suitable scale on the y-axis and plot the results from Experiments 1 to 5 on Fig. 2.1. Draw a line of best fit. time for magnesium to disappear in s 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 concentration of acid in mol / dm3 Fig. 2.1 [4] (c) From your graph in Fig. 2.1, deduce the time for the magnesium to disappear when the concentration of the acid is 1.3 mol / dm3. Show clearly on Fig. 2.1 how you worked out your answer. time for magnesium to disappear = .............................. s [2] (d) The mean rate of reaction is calculated using the equation shown. length of magnesium ribbon in cm mean rate of reaction = time for magnesium to disappear in s The length of each coil of magnesium ribbon used in all five experiments was 5 cm. (i) Calculate the mean rate of reaction in Experiment 1. Give units for the rate you have calculated. mean rate of reaction = .............................. units .............................. [2] (ii) Deduce in which Experiment, 1, 2, 3, 4 or 5, the mean rate of reaction is the slowest. ..................................................................................................................................... [1] (e) Explain why repeating each experiment is an improvement. ................................................................................................................................................... ............................................................................................................................................. [1] (f) The student does another experiment to find the temperature change when magnesium reacts with acid A. • Use the measuring cylinder to pour 30 cm3 of acid A into the 100 cm3 conical flask. • Measure the initial temperature of the acid in the conical flask. • Add a coil of magnesium ribbon to the acid in the conical flask. • Continually swirl the conical flask until the magnesium ribbon disappears completely. • Measure the final temperature of the acid in the conical flask. (i) Use the thermometer diagrams to complete Table 2.2. Table 2.2 thermometer thermometer initial final diagram for diagram for temperature temperature temperature initial final change / °C / °C / °C temperature temperature 30 50 20 40 [2] (ii) Explain why controlling the temperature of the acid so that it remains constant is an improvement. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Explain why using a polystyrene cup instead of the 100 cm3 conical flask does not control the temperature of the acid. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Describe how the temperature of the acid can be controlled and kept constant. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 17]

Mark scheme: 2(a) M1 times are correct for all five experiments (20, 25, 42, 56, 95) 1 M2 All times recorded in whole numbers of seconds only 1 2(b) M1 y-axis scale is linear and points take up over half of space available 1 M2 and M3 All points plotted correctly 2 M4 best fit line 1 2(c) M1 correct working shown at a concentration of 1.3 on graph 1 M2 correct reading from their working shown on graph 1 2(d)(i) M1 0.25 1 M2 cm / s 1 2(d)(ii) Experiment 5 1 2(e) to check / verify / compare results / to get concordant results / 1 2(f)(i) M1 both temperatures are correct (24.0, 46.5) 1 M2 temperature change calculated correctly (22.5) and all temperatures and temperature change given to one dp 1 2(f)(ii) a change of temperature will change the rate of reaction 1 2(f)(iii) the polystyrene would prevent heat escaping 1 2(f)(iv) use a water bath (at room temperature / at starting temperature) 1

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3 A student tests two solids: solid F and solid G. Tests on solid F Solid F is calcium carbonate. (a) The student adds about 15 cm3 of dilute hydrochloric acid to the sample of solid F in a boiling tube and tests any gas produced. observations ............................................................................................................................. ................................................................................................................................................... ............................................................................................................................................. [2] (b) The student filters the product from (a) to obtain solution H as the filtrate. The student divides solution H into three approximately equal portions in three test-tubes. (i) To the first portion of solution H, the student adds about 1 cm depth of dilute nitric acid followed by a few drops of aqueous barium nitrate. observations ...................................................................................................................... ..................................................................................................................................... [1] (ii) To the second portion of solution H, the student adds aqueous sodium hydroxide dropwise and then in excess. observations when added dropwise .................................................................................. observations in excess ...................................................................................................... [2] (iii) To the third portion of solution H, the student adds about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. observations ...................................................................................................................... ..................................................................................................................................... [1] Tests on solid G Table 3.1 shows the tests and the student’s observations for solid G. Table 3.1 tests observations test 1 Do a flame test on solid G. lilac coloured flame test 2 Dissolve the remaining solid G in water to form solution G. Divide solution G into three portions. To the first portion of solution G, add about 1 cm3 orange solution forms of aqueous chlorine. test 3 To the second portion of solution G, add about remains colourless 1 cm3 of aqueous sodium hydroxide. test 4 To the third portion of solution G, add about cream precipitate forms 1 cm3 of dilute nitric acid followed by a few drops of aqueous silver nitrate. (c) Identify solid G. ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 8]

Mark scheme: 3(a) M1 limewater becomes milky 1 M2 effervescence / fizzing / bubbling 1 3(b)(i) no change 1 3(b)(ii) dropwise: white precipitate 1 excess: no (further) change / precipitate remains / white precipitate / insoluble 1 3(b)(iii) white precipitate 1 3(c) M1 potassium / K+ 1 M2 bromide / Br– KBr = 2 1

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4 Solid sodium hydrogencarbonate, NaHCO3, reacts with dilute hydrochloric acid and with dilute ethanoic acid. NaHCO3(s) + HCl (aq) NaCl (aq) + CO2(g) + H2O(l) NaHCO3(s) + CH3COOH(aq) CH3COONa(aq) + CO2(g) + H2O(l) These two reactions are endothermic. Plan an investigation to find which of the two reactions is the most endothermic. Your plan must include how you can use the results to find which reaction is the most endothermic. You are provided with solid sodium hydrogencarbonate, dilute hydrochloric acid, dilute ethanoic acid and common laboratory apparatus. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [6]

Mark scheme: 4 any 6 from 6 MP1 known / specified mass of sodium hydrogencarbonate MP2 known / specified volume (and concentration) of hydrochloric or ethanoic acid MP3 use of a named suitable container for the reaction and named suitable apparatus to measure volume of acid MP4 measure temperature of acid before mixing MP5 add reagents together and mix / stir MP6 measure temperature of mixture / acid after mixing / reaction MP7 biggest temperature decrease is most endothermic max 6

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