Cambridge A Level Chemistry 9701 — 2019 Oct/Nov Paper 3 · Variant 3

1 In this experiment you will determine the concentration of a solution of hydrochloric acid by titration with an alkali. FA 1 is a solution containing 6.00 g dm–3 of sodium hydroxide, NaOH. FA 2 is hydrochloric acid, HCl. (This solution is also used in Questions 2 and 3.) methyl orange indicator (a) Method Dilution of FA 2 ● Pipette 10.0 cm3 of FA 2 into the 250 cm3 volumetric flask. ● Make the solution up to the mark using distilled water. ● Shake the solution in the volumetric flask thoroughly. ● This solution of hydrochloric acid is FA 3. Label the volumetric flask FA 3. Titration ● Fill the burette with FA 1. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add several drops of methyl orange indicator. ● Perform a rough titration and record your burette readings in the space below. The rough titre is .............................. cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. I ● Make sure any recorded results show the precision of your practical work. II ● Record in a suitable form below all of your burette readings and the volume of FA 1 added in each accurate titration. III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required .............................. cm3 of FA 1. [1] (c) Calculations (i) Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of sodium hydroxide, NaOH, in the volume of FA 1 calculated in (b). moles of NaOH = .............................. mol [1] (iii) Write the equation for the neutralisation of hydrochloric acid with sodium hydroxide. Include state symbols. ............................................................................................................................................. Deduce the number of moles of hydrochloric acid that reacted with the sodium hydroxide in (ii). moles of hydrochloric acid =...................... mol [1] (iv) Calculate the concentration, in mol dm–3, of hydrochloric acid in FA 2. concentration of HCl in FA 2 = .............................. mol dm–3 [2] [Total: 13]

Mark scheme: 1(a) I All the following data recorded • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations 1 II Titre values shown, for accurate titrations, and appropriate headings and units in the accurate titration table • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FB 4 and used / added • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded 1 III All accurate burette readings are recorded to the nearest .05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V if δ ⩽ 0.80 (cm3) (Where δ is difference to the supervisor’s value) 1 Award VI if δ ⩽ 0.50 (cm3) 1 Award VII if δ ⩽ 0.30 (cm3) 1 Question Answer Marks 1(b) Correctly calculates mean titre from two (or more) accurate titres where the total spread is ⩽ 0.20 cm3. AND Answer is given to 2 d.p. AND Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1 1(c)(i) Significant figures All answers in (ii)–(iv) are expressed to 3 or 4 sig. fig. 1 1(c)(ii) Correctly calculates no of moles of NaOH used. No. of moles NaOH = 6.00/40 × answer (b)/1000 1 1(c)(iii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) or H+(aq) + OH-(aq) → H2O(l) AND No. of moles of HCl = same as in moles of NaOH in (ii) 1 Question Answer Marks 1(c)(iv) Correct expression shown (for the first step) (Two steps) Concentration HCl in FA 3 = ans (c)(iii) × 1000 / 25 Allow ans (c)(iii) × 40 (Concentration of FA 2 = Concentration of FA 3 × 25) OR moles of FA 3 in 250 cm3 = ans (c)(iii) × 250 / 25 Allow ans (c)(iii) × 10 (moles of FA 2) in 1 dm3 = ans above × 1000 / 10) 1 Answer = (c)(iii) × 1000 1

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2 In this experiment you will determine the enthalpy change, ΔH, for the reaction shown. MgO(s) + CO2(g) MgCO3(s) To do this, you will determine the enthalpy changes for the reactions of magnesium oxide and magnesium carbonate with hydrochloric acid. Excess hydrochloric acid will be used in each reaction. You will then use Hess’ Law to calculate the enthalpy change for the reaction. FA 2 is hydrochloric acid, HCl. FA 4 is magnesium oxide, MgO. FA 5 is magnesium carbonate, MgCO3. (a) Determination of the enthalpy change for the reaction of magnesium oxide, FA 4, with hydrochloric acid, FA 2 Method ● Support a plastic cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 40 cm3 of FA 2 into the plastic cup. ● Measure and record the initial temperature of the solution. ● Weigh the container with FA 4. Record the mass. ● Add all the FA 4 from the container to the FA 2 in the plastic cup. ● Stir constantly until the maximum temperature is reached. ● Measure and record the maximum temperature. ● Weigh the container with any FA 4 remaining. Record the mass. ● Calculate and record the mass of FA 4 used. ● Calculate and record the temperature rise. I II III IV [4] (b) Calculations (i) Calculate the energy produced during this reaction. (Assume that 4.2 J are needed to raise the temperature of 1.0 cm3 of solution by 1.0 °C.) energy produced = .............................. J [1] (ii) Calculate the number of moles of MgO used. moles of MgO = .............................. mol [1] (iii) Calculate the enthalpy change, in kJ mol–1, for the reaction below. MgO(s) + 2HCl (aq) MgCl 2(aq) + H2O(l) enthalpy change = ......... ................... kJ mol–1 [1] sign value (c) By means of a calculation, use your answer to 1(c)(iv) to show that the hydrochloric acid, FA 2, was in excess for the reaction in 2(a). (If you were unable to carry out the calculation in 1(c)(iv), you should assume that the concentration of HCl in FA 2 is 3.75 mol dm–3. This may not be the correct value.) [1] (d) Determination of the enthalpy change for the reaction of magnesium carbonate, FA 5, with hydrochloric acid, FA 2 (i) Method ● Support the second plastic cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 40 cm3 of FA 2 into the plastic cup. ● Measure and record the initial temperature of the solution. ● Weigh the container with FA 5. Record the mass. ● Add approximately half of the FA 5 from the container to the FA 2 in the plastic cup. ● Stir constantly for approximately 30 seconds. ● Then add the remainder of the FA 5. ● Stir constantly until the maximum temperature is reached. ● Measure and record the maximum temperature. ● Weigh the container with any FA 5 remaining. Record the mass. ● Calculate and record the mass of FA 5 used. ● Calculate and record the temperature rise. [2] (ii) Apart from the change in temperature, what observations did you make during the reaction? ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Calculation Calculate the enthalpy change for this reaction. MgCO3(s) + 2HCl (aq) MgCl 2(aq) + CO2(g) + H2O(l) enthalpy change = ......... ................... kJ mol–1 [1] sign value (e) Use your values for the enthalpy changes calculated in (b)(iii) and (d)(iii) to calculate the enthalpy change for the reaction below. (If you were unable to calculate the enthalpy changes, assume that the magnitude of the enthalpy change in (b)(iii) is 110.3 kJ mol–1 and the magnitude of the enthalpy change in (d)(iii) is 65.9 kJ mol–1. Note: these may not be the correct magnitudes and the signs have been deliberately omitted.) MgO(s) + CO2(g) MgCO3(s) enthalpy change = ......... ................... kJ mol–1 [1] sign value (f) Outline one improvement to the method that would lead to more accurate values for the enthalpy changes. Do not include a change to the apparatus used, such as the use of a lid. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] (g) Give the error in a single balance reading. error = ± .............................. g Which is greater, the percentage error in the mass of magnesium oxide used or the percentage error in the mass of magnesium carbonate used? Percentage error is greater in ..................................................................... . Calculate the greater percentage error. greater % error = .............................. % [2] [Total: 16] 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: 2(a) I Suitable tables/lists with values entered in the space provided. • Three clearly labelled masses (do not allow ‘weight’) with correct units • Three unambiguous labelled temperatures, with units 1 II Readings and subtraction in 2(a) and 2(d) • All four measured temperatures recorded to .0 or .5 °C • Both temperature changes correctly subtracted. • All masses in (a) and (d) recorded to one or more d.p. (balance readings must be consistent d.p. within each experiment) • Both masses correctly subtracted. 1 Accuracy marks Round all thermometer readings to 0.5 °C. Check the candidate’s and supervisor’s subtractions and compare the candidate’s [corrected] temperature rise with the supervisor’s. The difference between candidate and supervisor is δ. See table below for marks available. 2 Sup ∆Tmax > 40.0 °C 30.5–40.0 °C 20.5–30.0 °C 10.5–20.0 °C 5.5–10.0 °C < 5.5 °C 1 mark δ ⩽ 5.0 °C δ ⩽ 4.0 °C δ ⩽ 3.0 °C δ ⩽ 2.0 °C δ ⩽ 1.0 °C δ ⩽ 0.5 °C 2 marks δ ⩽ 2.5 °C δ ⩽ 2.0 °C δ ⩽ 1.5 °C δ ⩽ 1.0 °C δ ⩽ 0.5 °C not available Question Answer Marks 2(b)(i) Correctly calculates heat change Heat change = 40 × 4.2 × temp rise AND Answer is correct to 2–4 s.f. 1 2(b)(ii) Correctly calculates number of moles of MgO No. of moles of MgO used = mass used/40.3 AND Answer is correct to 2–4 s.f. 1 2(b)(iii) Correctly uses (i)/(ii) and negative sign shown Enthalpy change = − (b)(i)/(b)(ii) × 1/1000 AND Answer is given to 2–4 s.f. 1 2(c) Correct expression(s) shown in ‘excess’ calculation Candidate must compare the number of moles of MgO and HCl used and correctly use the mole ratio of 1:2. Example of working No. of moles of HCl used = 0.04 × ans 1(c)(iv) (or n = 0.04 × 3.75 = 0.15) Maximum no. moles MgO that can be used = 0.5 × n Note – there are other valid ways of doing this calculation. 1 Question Answer Marks 2(d)(i) Readings written in space provided • Two thermometer readings are recorded both above 10 °C • Two masses are recorded giving mass of MgCO3 between 2–5 g. 1 Accuracy mark Check temperature subtractions of candidate and of supervisor. Compare the [corrected] candidate’s temperature rise with the supervisor’s. The difference is δ. • If δ is less than or equal to 2.0 °C, award the mark 1 2(d)(ii) Observations (two required) Fizzing / bubbles / effervescence AND either solid dissolves / disappears / colourless solution formed or reaction is brisk / rapid / vigorous / violent 1 2(d)(iii) Correct expression(s) for enthalpy change, with negative sign Enthalpy change = −40 × 4.2 × temp rise × 84.3/ mass MgCO3 ×1/1000 1 2(e) Enthalpy change correctly calculated with correct sign Answer = (b)(iii) – (d)(iii) and to minimum 2 s.f. unless answer is an integer. If default values are used, the answer must be −44.4 kJ mol−1. 1 Question Answer Marks 2(f) Plot a cooling curve (after the maximum temperature reached) OR plot a graph to get better value of ∆T OR use increased masses of FA 4 and / or FA 5 / mass of solid (added) OR heat the acid before adding solid / FA 4 and / or FA 5 Ignore answers related to the apparatus used. 1 2(g) Correct single reading error (see below) and MgO has the greater % error (Allow MgCO3 if this was the smaller mass.) If candidate used a 1 d.p. balance for this solid, error is 0.05 or 0.1 g If candidate used a 2 d.p. balance for this solid, error is 0.005 or 0.01 g If candidate used a 3 d.p. balance for this solid, error is 0.0005 or 0.001 g 1 Correct calculation of % error for MgO % error = 2 × (single error/mass of MgO used) × 100 1

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3 (a) You will investigate FA 6. Pour a 1 cm depth of hydrochloric acid, FA 2, into a test‑tube. Add a small spatula measure of FA 6 to the acid. Record your observations. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... What can you deduce from your observations? Explain your answer. .................................................................................................................................................... .................................................................................................................................................... [3] (b) (i) FA 7 is a sodium compound containing an anion listed in the Qualitative Analysis Notes. Heat a small spatula measure of FA 7 in a hard-glass test-tube. Heat strongly until no further change occurs, then leave the test‑tube and contents to cool. Record all your observations below. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Dissolve the remaining FA 7 in a 5 cm depth of distilled water in a boiling tube. Label this solution FA 8. FA 9 is a solution of a different sodium compound. The anion is listed in the Qualitative Analysis Notes. Carry out the following tests on FA 8 and FA 9 and record your observations in the table. test observations with FA 8 observations with FA 9 To a 1 cm depth in a test-tube, add a few drops of aqueous acidified potassium manganate(VII). To a 1 cm depth in a test-tube, add a few drops of aqueous barium chloride or aqueous barium nitrate. To a 1 cm depth in a boiling tube, add an equal volume of aqueous sodium hydroxide. Warm carefully, then add aluminium foil. [4] (iii) From your observations, suggest the anions present in FA 8 and FA 9. anion in FA 8 ................................................ anion in FA 9 ................................................ [1] (iv) Give the ionic equation for any reaction observed in (b)(ii). Include state symbols. ....................................................................................................................................... [1] [Total: 11] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a) Observation Bubbling / fizzing / effervescence (Ignore ‘gas formed’) 1 Test for hydrogen during reaction (Gas) pops with a lighted splint / burns with a pop (allow explodes) 1 Deduction FA 6 is a metal and gives hydrogen (with an acid) 1 3(b)(i) Heating FA 7 – observations • FA 7) melts / (partially) dissolves / becomes liquid / becomes a solution • (liquid is) yellow or pale brown • Fizzing / bubbling / effervescence occurs • (Gas / oxygen) re-lights a glowing splint • Gas turns (blue) litmus red • After standing / cooling white / off-white / cream / paler solid (formed). Award 1 mark for two correct observations from the list, award 2 marks for four or more correct observations. MAX. 2 marks 2 Question Answer Marks 3(b)(ii) Observations in the table (see below) Expected observations are in the table below. Award 1 mark for every two correct observations (*) Reject no observation the first time seen. Reject a dash every time. test observation with FA 8 observation with FA 9 To a 1 cm depth in a test-tube, add a few drops of acidified potassium manganate(VII). No (visible) reaction / no change or solution (in test tube) becomes pink / purple / solution / KMnO4 / MnO4– stays purple * from purple to colourless or KMnO4 / MnO4– decolourised To a 1 cm depth in a test-tube, add a few drops of aqueous barium chloride or aqueous barium nitrate. no reaction / no change / no precipitate / solution remains colourless * white precipitate (formed) * To a 1 cm depth in a boiling tube, add an equal volume of aqueous sodium hydroxide. Warm carefully, then no reaction / no change / no precipitate * no reaction / no change / no precipitate * add aluminium foil. gas / ammonia turns (moist red) litmus blue * fizzing / bubbling / effervescence or gas / H2 pops with a lighted splint * 4 3(b)(iii) Both anions correct • FA 8 = nitrate / NO3− • FA 9 = sulphite / SO32− 1 Question Answer Marks 3(b)(iv) One relevant ionic equation with state symbols • Ba2+(aq) + SO32−(aq) → BaSO3(s) • Ba2+(aq) + SO42−(aq) → BaSO4(s) • 2MnO4–(aq) + 6H+(aq) + 5SO32–(aq) → 5SO42–(aq) + 2Mn2+(aq) + 3H2O(l) • 3NO3–(aq) + 27H+(aq) + 3Al(s) → 3NH3(g) + 9H2O(l) + 3Al 3+(aq) 1

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