Cambridge A Level Chemistry 9701 — 2018 Feb/March Paper 3 · Variant 3

1 You will investigate how increasing temperature affects the rate of a reaction. Sodium thiosulfate reacts with acid to form a pale yellow precipitate of sulfur. The ionic equation for the reaction is given. S2O32–(aq) + 2H+(aq) S(s) + SO2(g) + H2O(l) You will measure the time it takes for the sulfur formed in the reaction to obscure the print on the Insert supplied. Record your results in a table on page 4. Your table should include the rate of reaction for each experiment. FA 1 is an 18.1 g dm–3 solution of hydrated sodium thiosulfate, Na2S2O3.5H2O. FA 2 is a 0.050 mol dm–3 solution of a strong monoprotic acid, HZ. (a) Method ● Approximately half fill the 250 cm3 beaker with tap water and place it on the tripod and gauze over the Bunsen burner. ● Heat the water in the beaker to about 55 °C and then switch off the Bunsen burner. This will be your hot water bath. ● Use the 25 cm3 measuring cylinder to transfer 10 cm3 of FA 1 into boiling tube 1. Place boiling tube 1 into your hot water bath. ● Use the 50 cm3 measuring cylinder to transfer 20 cm3 of FA 2 into boiling tube 2. Place boiling tube 2 into your hot water bath. ● Leave boiling tubes 1 and 2 in the hot water bath to heat up for use in Experiment 2. ● Start Experiment 1. Experiment 1 ● Use the 50 cm3 measuring cylinder to transfer 20 cm3 of FA 2 into the 100 cm3 beaker. ● Measure and record the temperature of FA 2. ● Use the 25 cm3 measuring cylinder to transfer 10 cm3 of FA 1 into the same beaker and start timing immediately. ● Swirl the beaker once to mix the solutions and place the beaker on the Insert. ● Look down through the beaker and contents onto the Insert. ● Stop timing as soon as the precipitate of sulfur obscures the print on the Insert. ● Record the reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in Experiment 2. Experiment 2 ● Measure and record the temperature of FA 2 in boiling tube 2. ● Carefully transfer the hot contents of boiling tube 2 into the 100 cm3 beaker. ● Carefully transfer the hot contents of boiling tube 1 into the same beaker and start timing immediately. ● Swirl the beaker once to mix the solutions and place the beaker on the Insert. ● Look down through the beaker and contents onto the Insert. ● Stop timing as soon as the precipitate of sulfur obscures the print on the Insert. ● Record the reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in Experiment 3. Experiment 3 ● Use the 25 cm3 measuring cylinder to transfer 10 cm3 of FA 1 into boiling tube 1. Place boiling tube 1 into your hot water bath. ● Use the 50 cm3 measuring cylinder to transfer 20 cm3 of FA 2 into boiling tube 2. Place boiling tube 2 into your hot water bath. ● Place the thermometer in boiling tube 2. When the temperature of FA 2 is about 8 °C lower than that for Experiment 2 record the temperature. Remove the thermometer and transfer the contents of boiling tube 2 into the 100 cm3 beaker. ● Transfer the contents of boiling tube 1 into the same beaker and start timing immediately. ● Swirl the beaker once to mix the solutions and place the beaker on the Insert. ● Look down through the beaker and contents onto the Insert. ● Stop timing as soon as the precipitate of sulfur obscures the print on the Insert. ● Record the reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in Experiments 4 and 5. Experiments 4 and 5 ● Repeat the method for Experiment 3 but at two different temperatures. ● Keep the temperature of FA 2 between room temperature and 55 °C. Do not exceed 55 °C. Record all your results in your table on page 4. Results The rate of reaction can be calculated as shown. 1000 rate = reaction time Calculate the rate of reaction for each of your five experiments. Record these rates in your table. I II III IV V VI VII VIII [8] (c) Use your graph to calculate the time to the nearest second that the reaction would have taken if you had carried it out at 17.5 °C. Show on the grid how you obtained your answer. time = .............................. s [2] (d) Explain, by referring to your graph or your table of results, how the rate of reaction is affected by increasing temperature. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] (e) Calculations (i) Calculate the concentration of hydrated sodium thiosulfate, Na2S2O3.5H2O, in FA 1 in mol dm–3. concentration of Na2S2O3.5H2O in FA 1 = .............................. mol dm–3 [1] (ii) Calculate the concentration of the strong monoprotic acid, HZ, in the solution immediately after FA 1 was added to FA 2 in the beaker. concentration of HZ = .............................. mol dm–3 [1] (iii) Use the equation on page 2 to determine which reagent, FA 1 or FA 2, was in excess. The reagent in excess was .............................. . [2] (f) (i) Calculate the maximum percentage error in measuring the reaction time you recorded for Experiment 2. Assume that the maximum error of the timer is ±0.5 s. maximum percentage error in the reaction time = .............................. % [1] (ii) A student suggested that the error in measuring the reaction time in Experiment 1 was greater than for Experiment 2. Give one reason why the student could be correct. ............................................................................................................................................. ....................................................................................................................................... [1] (g) Suggest two ways to improve the accuracy of the results of these experiments. 1 ................................................................................................................................................. .................................................................................................................................................... 2 ................................................................................................................................................. .................................................................................................................................................... [2] [Total: 24] 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.

Mark scheme: 1(a) I Single table to show temperature of FA 2/reactant(s), time and rate for 5 experiments. (not all experiments need have been done – minimum 2) 1 II Headings unambiguous and units correct – displays: (°C), / s, in s–1 (ignore factor of 1000 in rate unit) 1 III All temperatures recorded to .0 or .5, all times as integers. (minimum 4 experiments carried out) 1 IV Selects temperatures in experiments 4 and 5 that are ⩾ 4 °C apart from all others and none above 60 °C. (Paper states 55 °C but T for Expt 2 may be slightly higher.) 1 V Rates correctly calculated to 2‒4 sf (minimum 3 results) 1 Award VI if candidate for expt 1 is within 10% of supervisor (If expts have been renumbered by candidate then compare time for the expt carried out at the lowest temperature.) 1 Award VII if all times decrease with increasing temperature. 1 Award VIII if all results give an increasing gradient graph (Allow if 4 out of the 5 points show an increasing gradient line.) (Do not award if no graph drawn or fewer than 5 points plotted.) 1 1(b) I Axes labelled (name or unit) and linear scales chosen so graph occupies more than half the available length for both axes including 15 °C on x-axis and 0 on y-axis. 1 II All points recorded (minimum 4 recorded) accurately plotted Any point which should be on a line must be on that line. Any point not on a line must be in the correct part of the small square. If blobs shown then they must be correctly centred and be less than ½ a small square across. 1 III Line of best fit drawn (smooth curve expected but allow suitable straight line) Ignore any obviously anomalous points. 1 IV Anomalous points indicated and line extrapolated to 15 °C If no points anomalous then smooth line very close to all points 1 Question Answer Marks 1(c) Both construction lines at 17.5 °C shown Allow other clear indication linking 17.5 °C with rate 1 Correctly calculates time from rate reading (ignore sf) Rate must be correctly read from the graph (to within 0.5 s–1 of examiner value) If no construction lines are drawn examiner infers rate and checks rate and time given by candidate. If construction lines / ‘point’ drawn in wrong place then allow as ecf (i.e. wrong temp selected) 1 1(d) Rate of reaction increases with / is proportional to increase in temperature because it / graph line curves upwards / has a positive gradient or figures from table. Directly proportional is CON 1 Rate of rate of reaction increases because gradient increases with temperature / rate of reaction increases more / at a greater rate than increase in temperature as gradient increases (or from relevant figures from graph or results table) 1 1(e)(i) Correctly calculates initial concentration of thio to 2–4 sf. (Penalise incorrect sf only once in this section.) 18.1/248.2 = 0.073 / 0.0729 / 0.07293 mol dm–3 1 1(e)(ii) Correctly calculates concentration of acid in the mixture to 2–4 sf 0.05 × 2/3 = 0.033(3) mol dm–3 1 Question Answer Marks 1(e)(iii) Shows working to compare concentration of thio in mixture with (ii) or moles of thio and of acid in mixture [conc of thio in mixture = 0.073 × 1/3 = 0.024(31) mol dm–3] or [moles of thio (in 10 cm3) = (7.3 / 7.29 / 7.293) × 10‒4 mol and moles of acid (in 20 cm3) = 1(.000) × 10‒3 mol] 1 Comparison using equation moles thio : acid = 1 : 2 (thio/acid = 0.5) and thio / FA 1 in excess [concentration of thio/acid in mixture = 0.024/0.033 > 0.5 This may be shown as thio : acid = 0.0243 > 0.0167 or 0.0486 > 0.033] or [moles of thio/acid in mixture = 0.00073/0.001 > 0.5 This may be shown as thio : acid = 0.00146 > 0.001] Allow ecf from (ii) (7.29 × 10–2 × 2/3 > 3.33 × 10–2 and FA 1 / thio in excess gains both marks) 1 1(f)(i) Correct working shown or correct answer to minimum 2 sf (0.5/ time for expt 2) × 100 If ‘ × 100’ not in working allow if answer shows its use. (t2 must match the time recorded for the expt labelled 2 in the results table) 1 1(f)(ii) It is more difficult to distinguish exactly when the printing disappears at the lower temperature (in expt 1) ora 1 Question Answer Marks 1(g) One of Take the temperature on initial mixing and the temperature as soon as the printed sheet is obscured (and calculate a mean T). Take the temperature of FA 1 / both solutions (and calculate (weighted) mean) Use a thermostatically controlled water bath (to prevent temperature fluctuations) 1 One of Use (graduated) pipette / burette / measuring cylinders calibrated to greater precision / smaller percentage error to measure volumes. Use (graduated) pipette / burette to measure FA 1 / thio and FA 2 / acid / (volumes of) solutions / reactants (instead of the measuring cylinders) Use light sensor/colorimeter (to avoid subjective judgement of turbidity) (Do not allow use a more accurate thermometer) 1

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2 (a) FA 3 is a more concentrated solution of the strong monoprotic acid, HZ, used for Question 1. Select two sets of reagents and suitable apparatus to use in two separate tests, Test 1 and Test 2, to investigate the identity of the anion, Z–, present in FA 3. The anion is one of those listed in the Qualitative Analysis Notes. Complete the ‘test ’ boxes in the table before starting any practical work by circling whether you would use a test-tube or a boiling tube, and stating which reagents you would use. Carry out your tests and record your observations. You must carry out both Test 1 and Test 2. test observations Test 1 To a 1 cm depth of FA 3 in a test-tube / boiling tube add .................................................................. ....................................................................... . (reagent(s)) Test 2 To a 1 cm depth of FA 3 in a test-tube / boiling tube add .................................................................. ....................................................................... . (reagent(s)) [4] (b) Identify the anion present in HZ from your observations in (a). Z– is .............................. . [1] (c) FA 4 and FA 5 both contain one cation and one anion. The ions present in FA 4 are different from the ions present in FA 5. All four ions are listed in the Qualitative Analysis Notes. You are to identify the four different ions. Carry out the following tests and record your observations. test observations To a small spatula measure of FA 4 in a boiling tube, add a 4 cm depth of FA 3 and shake the tube well. Leave the tube to stand for at least five minutes. Label the solution formed FA 6. To a 1 cm depth of FA 5 in a test-tube, add aqueous sodium carbonate. To a 1 cm depth of FA 5 in a test-tube, add aqueous sodium hydroxide. To a 1 cm depth of FA 5 in a test-tube, add aqueous ammonia. To a 1 cm depth of FA 5 in a test-tube, add a few drops of aqueous silver nitrate. To a 1 cm depth of FA 5 in a test-tube, add a few drops of aqueous barium chloride or aqueous barium nitrate, then add a 1 cm depth of a suitable acid. To a 1 cm depth of FA 6 in a test-tube, add aqueous sodium hydroxide. To a 1 cm depth of FA 6 in a test-tube, add aqueous ammonia. To a 1 cm depth of FA 6 in a test-tube, add dilute sulfuric acid. To a 1 cm depth of FA 6 in a test-tube, add a 1 cm depth of FA 5. [8] (d) Give the formula of the acid you added to the mixture of FA 5 and aqueous barium chloride or aqueous barium nitrate in (c). The acid added was .............................. . [1] (e) Identify the ions present in FA 4 and FA 5 from your observations in (c). cation anion FA 4 FA 5 [2] [Total: 16]

Mark scheme: 2(a) Test: selects AgNO3 and NH3 and test-tube or boiling tube used 1 Observation: no reaction / no change / no ppt / (solution) remains colourless This mark may be awarded without NH3 being specified in test. 1 Test: selects NaOH + Al (and warm) and boiling tube used Penalise lack of test-tube/boiling tube only once 1 Observation: effervescence / gas / NH3 turns (damp) red litmus blue 1 If the Devarda’s test has been carried out first then allow Test: selects (aqueous acidified) KMnO4 and test-tube or boiling tube used (1) Observation: stays purple / does not decolourise (1) Allow the observation: no reaction / no ppt / no change if BaCl2 or Ba(NO3)2 selected as a test. 2(b) NO3 – / nitrate (ion) from evidence of ammonia (Allow NO2 – / nitrite) 1 Question Answer Marks 2(c) Gas / CO2 / effervescence turns limewater milky / cloudy white / (forms) white ppt 1 2* = 1 mark Do not penalise ‘no observation’, ‘transparent’, ‘clear’, ‘–‘ for ‘no visible reaction’ more than once. FA 4 + FA 3 effervescence / fizzing / bubbling * colourless solution formed * FA 5 + Na2CO3 white ppt * (soluble in excess is CON) FA 5 + NaOH white ppt * insoluble in excess * FA 5 + NH3 white ppt * insoluble in excess * FA 5 + Ag+ no reaction / no ppt / no change * FA 5 + Ba2+ white ppt * insoluble in acid * (addition of H2SO4 shown in observation table negates second point) FA 6 + NaOH no change / no visible reaction / no ppt / faint / slight white ppt * FA 6 + NH3 no change / no visible reaction / no ppt / faint / slight white ppt * FA 6 + H2SO4 white ppt * (soluble in excess is CON) FA 6 + FA 5 white ppt * (soluble in excess is CON) 7 Question Answer Marks 2(d) Selects either HCl or HNO3 1 2(e) FA 4: Ba2+ / barium and CO3 2– / carbonate (allow Ca2+ if white ppt insoluble in excess formed with NaOH – see 8th observation box) 1 FA 5: Mg2+ / magnesium and SO4 2– / sulfate (allow Ca2+ if no ppt with NH3 – see 4th observation box) (Do not allow Ca2+ for both. Anions are stand alone marks.) 4 ions = 2 marks; 2 or 3 ions = 1 mark 1

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