Cambridge IGCSE Chemistry (9-1) 0971 — 2021 May/June Paper 5 · Variant 1

1 You are going to investigate the temperature decrease when sodium hydrogencarbonate reacts with dilute hydrochloric acid. Read all of the instructions carefully before starting the experiments. Instructions You are going to do six experiments. (a) Experiment 1 ● Use a measuring cylinder to pour 25 cm3 of dilute hydrochloric acid into a conical flask. ● Use a thermometer to measure the initial temperature of the acid. Record the initial temperature in the table. ● Add the 1 g sample of sodium hydrogencarbonate to the conical flask. At the same time start the stop-clock. ● Continually stir the acid and sodium hydrogencarbonate mixture in the conical flask using the thermometer. ● Measure the temperature reached by the mixture after 1 minute. Record the temperature of the mixture in the table. ● Calculate and record the temperature decrease in the table. ● Rinse the conical flask with distilled water. Experiment 2 ● Repeat Experiment 1 using 2 g of sodium hydrogencarbonate instead of 1 g. Experiment 3 ● Repeat Experiment 1 using 3 g of sodium hydrogencarbonate instead of 1 g. Experiment 4 ● Repeat Experiment 1 using 5 g of sodium hydrogencarbonate instead of 1 g. Experiment 5 ● Repeat Experiment 1 using 6 g of sodium hydrogencarbonate instead of 1 g. Experiment 6 ● Repeat Experiment 1 using 7 g of sodium hydrogencarbonate instead of 1 g. mass of sodium initial temperature after temperature experiment hydrogencarbonate / g temperature / °C 1 minute / °C decrease / °C 1 1 2 2 3 3 4 5 5 6 6 7 [4] (b) Complete a suitable scale on the y-axis and plot your results from Experiments 1 to 6 on the grid. Draw two best-fit straight lines through your points. The first straight line should be for the first three points and must pass through (0,0). The second straight line should be for the last three points and must be horizontal. Extend your straight lines so that they meet each other. temperature decrease / °C 0 0 1 2 3 4 5 6 7 mass of sodium hydrogencarbonate / g [5] (c) (i) From your graph, determine the temperature decrease and mass of sodium hydrogencarbonate where your two straight lines meet. Include appropriate units in your answer. Show clearly on the grid how you worked out your answer. temperature decrease = .............................. mass of sodium hydrogencarbonate = .............................. [3] (ii) Explain why the temperature decrease becomes constant for high masses of sodium hydrogencarbonate. ............................................................................................................................................. ....................................................................................................................................... [1] (d) The investigation could be repeated with dilute hydrochloric acid of half the concentration, but the same volume. Sketch on the grid the graph you would expect to obtain. Label your line D. [2] (e) Suggest two changes that could be made to the apparatus that would improve the accuracy of the results. For each change explain why it would improve the accuracy of the results. change 1 .................................................................................................................................... explanation 1 .............................................................................................................................. .................................................................................................................................................... change 2 .................................................................................................................................... explanation 2 .............................................................................................................................. .................................................................................................................................................... [4] [Total: 19]

Mark scheme: 1(a) all temperatures completed and all temperatures and temperature changes recorded to the same precision 1 all temperatures after 1 minute are lower than the starting temperatures. 1 all temperature decreases calculated correctly 1 temperature change for 3 g is comparable to supervisor 1 1(b) suitable scale for y-axis 1 plotting 1 ruler straight line through first three points and line passes through (0,0) 1 horizontal ruler drawn straight line drawn through last three points 1 lines extended so that they meet / cross 1 1(c)(i) indication at correct point on graph 1 values read correctly from graph 1 units (°C and g) 1 1(c)(ii) (all) acid used up / sodium hydrogen carbonate in excess 1 1(d) temperature change when line becomes horizontal is half of temperature change for experimental results 1 the mass at which the line becomes horizontal is half of mass at which plotted line becomes horizontal 1 1(e) change: use a pipette 1 explanation: more accurate than a measuring cylinder 1 change: use a polystyrene / Styrofoam cup 1 explanation: insulator / reduces heat gain 1

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2 You are provided with one solid, solid E, and one solution, solution F. Do the following tests on the substances, recording all of your observations at each stage. tests on solid E (a) Place about half of solid E in a hard-glass test-tube. Heat the solid gently for about 30 seconds. Record your observations. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] Transfer the remaining solid E to a boiling tube. Add about 10 cm3 of distilled water to the boiling tube. Place a stopper in the boiling tube and shake the tube to dissolve solid E and form solution E. Divide solution E into three approximately equal portions in two test-tubes and one boiling tube. (b) To the first portion of solution E in a test-tube, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous barium nitrate. Record your observations. .............................................................................................................................................. [1] (c) To the second portion of solution E in a test-tube, add excess aqueous ammonia. Record your observations. .............................................................................................................................................. [1] (d) To the third portion of solution E in the boiling tube, add aqueous sodium hydroxide dropwise and then in excess. Keep the product for use in (e). Record your observations. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] (e) Gently warm the product from (d). Test any gas produced. Record your observations. .................................................................................................................................................... .............................................................................................................................................. [1] (f) Identify the three ions contained in solid E. .................................................................................................................................................... .............................................................................................................................................. [3] tests on solution F (g) Carry out a flame test on solution F. Record your observations. .............................................................................................................................................. [1] (h) Divide the remaining solution F into two approximately equal portions in two test-tubes. (i) To the first portion of solution F add a few drops of universal indicator solution. Record your observations. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) To the second portion of solution F add approximately 2 cm depth of aqueous copper(II) sulfate. Record your observations. ............................................................................................................................................. ....................................................................................................................................... [1] (i) Identify solution F. .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 15]

Mark scheme: 2(a) any two from: • melts / forms a liquid • steam • condensation 2 2(b) white precipitate 1 2(c) white precipitate 1 2(d) white precipitate 1 dissolves / disappears 1 2(e) (red) litmus turns blue 1 2(f) ammonium / NH4+ 1 aluminium / Al 3+ 1 sulfate / SO42– 1 2(g) yellow 1 2(h)(i) blue 1 2(h)(ii) blue ppt 1 2(i) sodium / Na+ 1 hydroxide / OH– 1

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3 Dilute hydrochloric acid reacts with calcium carbonate to make carbon dioxide gas. The apparatus shown in the diagram can be used to follow the progress of the reaction. The carbon dioxide gas leaves the flask causing the mass shown on the balance to decrease. cotton wool dilute hydrochloric acid calcium carbonate balance 260.00 Plan an investigation, using the apparatus shown in the diagram, to find out how the temperature of the dilute hydrochloric acid affects the rate of the reaction. Your plan should include how your results will show how the temperature of the dilute hydrochloric acid affects the rate of the reaction. You are provided with dilute hydrochloric acid, calcium carbonate and common laboratory apparatus. ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ..................................................................................................................................................... [6]

Mark scheme: 3 any 6 from: • stated / set / same / measured volume of acid • stated / set / same / measured mass of calcium carbonate • add / combine / put together and start timing • Repeat (with acid) at higher / lower temperature then: graphical method: • measure / record mass at known / regular / specified times • plot graph • steepest line is fastest OR mass loss in a set time • measure / record mass at a specified time • calculate / measure mass lost • largest mass loss is fastest or calculates rate by mass loss ÷ time OR time to end of reaction • react until mass stops changing / reaction stops • record time • shortest time is fastest or calculates rate by mass loss ÷ time OR time to lose a set mass • react until it reaches / loses a certain mass • record time • shortest time is fastest or calculates rate by mass loss ÷ time OR mass of calcium carbonate left after a set time • filter after a set time • find mass of calcium carbonate left • lower mass of calcium carbonate is fastest or calculates rate by mass loss ÷ time

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