Cambridge A Level Chemistry 9701 — 2023 May/June Paper 3 · Variant 4

1 You will investigate the enthalpy change of neutralisation, ∆Hneut, between aqueous sodium hydroxide of known concentration and a dilute organic acid. You will use your results to suggest the identity of the organic acid. The acid is a halogenocarboxylic acid containing one halogen atom, X, per molecule. NaOH(aq) + CH3CHXCOOH(aq) CH3CHXCOONa(aq) + H2O(l) FB 1 is 1.90 mol dm−3 sodium hydroxide, NaOH. FB 2 is a solution containing 312.5 g dm−3 of the organic acid CH3CHXCOOH. (a) Method ● Support the cup in the 250 cm3 beaker. ● Pipette 25.0 cm3 of FB 1 into the cup. ● Place the thermometer into FB 1. Record the temperature of FB 1 in Table 1.1. This is the temperature when the volume of FB 2 is 0.00 cm3. ● Fill the burette with FB 2. ● Run 5.00 cm3 of FB 2 into the cup containing FB 1. I ● Stir the mixture. Record the highest temperature observed. ● Run further 5.00 cm3 portions of FB 2 into the same cup. II ● On each addition of FB 2 stir the contents of the cup. Record the highest III temperature after each addition. Results Table 1.1 total volume of 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 FB 2 added / cm3 temperature / ºC [3] (b) (i) Plot a graph of temperature (y-axis) against volume of FB 2 added (x-axis) on the grid. Select a scale on the y-axis to include a temperature of 2 ºC above your maximum thermometer reading. Label any points you consider to be anomalous. Draw two lines of best fit, the first for the increase in temperature and the second for after the maximum temperature has been reached. Extrapolate the two lines so they intersect. This intersection corresponds to the volume of FB 2 required to form a neutral solution. (c) Calculations (i) Calculate the energy change, in J, when the volume of FB 2 recorded in (b)(ii) neutralises 25.0 cm3 of FB 1. energy change = .................................... J [1] (ii) Calculate the amount, in mol, of sodium hydroxide, FB 1, pipetted into the cup. amount of NaOH = ................................ mol [1] (iii) Calculate the enthalpy change of neutralisation, ∆Hneut, in kJ mol−1, for 1.00 mol of sodium hydroxide reacting with FB 2. ∆Hneut = ......... ................. kJ mol−1 [1] sign value (iv) Use your answers to (b)(ii) and (c)(ii) and the information given on page 2 to calculate the relative formula mass, Mr, of the organic acid CH3CHXCOOH. Show your working. Mr of CH3CHXCOOH = ....................................... [1] (v) The acid is known to be one of the following: CH3CHFCOOH, CH3CHCl COOH, CH3CHBrCOOH or CH3CHICOOH. Use your answer to (c)(iv) to identify the acid used to make solution FB 2. The acid in FB 2 is ....................................................... . [1] (vi) Calculate the percentage error in the relative formula mass, Mr, you calculated in (c)(iv). percentage error in Mr = ................................... % [1] [Total: 14]

Mark scheme: 1(a) I All thermometer readings to .0 or .5 °C AND at least one at .0 °C and one at .5 °C 3 Accuracy marks Examiner calculates Tmax for supervisor. Examiner calculates Tmax for candidate. Calculate and record the difference between supervisor and candidate value, . II and III Award accuracy marks as shown: Supervisor Tmax ⩽ 6.5 °C 7.0–15.0 °C >15.0 °C 1 mark  ⩽ 1.0 °C  ⩽ 1.5 °C  ⩽ 2.0 °C 2 marks  ⩽ 0.5 °C  ⩽ 1.0 °C  ⩽ 1.5 °C 1(b)(i) M1 Temperature on y-axis and volume of FB 2 added on the x-axis AND with unambiguous names or units AND some numbers for scales M2 Linear scales which include 2 °C above highest T in table AND plotted points/data and 2 °C above highest T in table, would occupy at least 5  5 big squares M3 All recorded points plotted correctly M4 Two lines of best fit drawn (straight line or smooth curve) covering all plotted points ignoring points labelled anomalous AND extrapolated to intersect 4 1(b)(ii) Reads and records volume of FB 2 at intersection to 1 or 2 decimal places 1 1(c)(i) Correctly uses results to calculate energy released Q = (25 + volume in (b)(ii))  4.18  temp rise AND answer to 2 or more significant figures 1 Question Answer Marks 1(c)(ii) Correctly calculates amount of NaOH (= 1.9  25 / 1000) = 4.75  10−2 AND answer given to 2–4 significant figures 1 1(c)(iii) Correct use of (c)(i) and (c)(ii) to calculate enthalpy change = − (c)(i) / [(c)(ii)  1000] AND answer given to 2–4 significant figures with negative sign shown 1 1(c)(iv) Correct expression evaluated Mr CH3CHXCOOH = [312.5  (b)(ii)] / [(c)(ii)  1000] 1 1(c)(v) Correct use of (c)(iv) to identify acid Ar = (c)(iv) – 73 AND correct choice of acid CH3CHFCOOH ⩽ 100.25 100.25 < CH3CHCl COOH ⩽ 130.7 130.7 < CH3CHBrCOOH ⩽ 176.4 176.4 < CH3CHICOOH 1 1(c)(vi) Correct expression evaluated [actual Mr – (c)(iv)] x 100 / actual Mr CH3CHFCOOH = 92.0 CH3CHClCOOH = 108.5 CH3CHBrCOOH = 152.9 CH3CHICOOH = 199.9 1

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2 A gravimetric procedure can identify the metal in many metal carbonates. You will decompose a metal carbonate, MCO3, by heating to produce the metal oxide and carbon dioxide. You may assume this metal forms a stable metal oxide on heating. M is not a transition metal. MCO3(s) MO(s) + CO2(g) FB 3 is the metal carbonate MCO3. (a) Method ● Weigh the crucible with its lid. Record the mass. ● Add between 0.90 g and 1.10 g of FB 3 to the crucible. ● Weigh the crucible, lid and FB 3. Record the mass. ● Place the crucible on the pipe-clay triangle. ● Gently heat the crucible and contents for approximately 1 minute with the lid on. ● Remove the lid. Then heat the crucible and contents strongly for approximately 5 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. While the crucible is cooling, you may wish to begin work on Question 3. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of FB 3 added to the crucible, the mass of residue obtained and the mass loss. Keep the residue for use in 2(d). Results I II III IV V [5] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide lost on heating FB 3. amount of CO2 = ................................ mol [1] (ii) Use your answer to (b)(i) and your data from (a) to calculate the relative formula mass, Mr, of MCO3. Mr of MCO3 = ....................................... [1] (iii) Use your answer to (b)(ii) to suggest the identity of metal M. Show your working. M is ....................................................... . [1] (c) A student carrying out the same experiment as in (a) spills a small quantity of solid just before carrying out the final weighing. State what effect this would have on the value of the Mr that is calculated for MCO3. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (d) Normally, in this experiment, you would reheat and reweigh the crucible and contents until the mass is constant to ensure all the metal carbonate has decomposed. Suggest a chemical test to determine whether all the metal carbonate has decomposed. Record your test, observation and conclusion. test ............................................................................................................................................ observation ............................................................................................................................... conclusion ................................................................................................................................. [2] [Total: 11] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I Three weighings recorded in the space provided AND all weighings recorded to the same decimal places (2 or 3) 5 II Correct headings AND correct displayed units  (Mass of) crucible (+ lid)  (Mass of) crucible (+ lid) + FB 3 (or ‘contents before heating’)  (Mass of) crucible (+ lid) + residue / MO / metal oxide / contents (after heating)  (Mass of) FB 3  (Mass of) residue / MO / metal oxide / contents after heating  (Mass) loss / FB 3 – residue/ CO2 Units must be ‘/’ or ‘in’ or ‘()’ AND g or ‘grams’ in column headings or with every entry III Correct subtractions to give masses of FB 3, residue and mass loss AND values are correctly labelled AND mass of FB 3 is between 0.90 and 1.10 g. Accuracy marks Calculate supervisor’s mass ratio (to 2 d.p.) = mass FB 3 / mass of residue Calculate candidate’s mass ratio (to 2 dp) = mass FB 3 / mass of residue Difference () between the candidate’s mass ratio and the supervisor’s mass ratio IV if  is within 25% of the supervisor ratio V if  is within 10% of the supervisor ratio 2(b)(i) Correctly calculates amount of CO2 lost = mass loss / 44 AND answer given to 2–4 significant figures 1 2(b)(ii) Correct use of (b)(i) to calculate Mr Mr = mass of FB 3 used / (b)(i) AND answer given to 2–4 significant figures 1 2(b)(iii) Correct use of (b)(ii) to identify Ar = (b)(ii) – 60 AND selects metal that forms a 2+ ion with suitable Ar 1 Question Answer Marks 2(c) The Mr would appear lower AND it would appear that greater amount / moles of CO2 was lost 1 2(d) M1 test: add named acid M2 fizz / effervescence / bubbles AND (some) FB 3 not decomposed (owtte) OR no fizz / effervescence / bubbles / no reaction AND FB 3 completely decomposed (owtte) 2

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3 Half fill the 250 cm3 beaker with water and place it on a gauze on the tripod. Heat the water until boiling then switch off your Bunsen burner. This will be your hot water bath. (a) FB 4, FB 5 and FB 6 are acids with the same concentration. Only one is an organic acid. (i) Carry out the following tests and record your observations in Table 3.1. Use a 1 cm depth of FB 4, FB 5 or FB 6 in a test-tube for each test. Table 3.1 observations test FB 4 FB 5 FB 6 Test 1 Add 1 or 2 drops of acidified aqueous potassium manganate(VII), then place the test-tube in the hot water bath. Test 2 Add a few copper turnings and place the test-tube in the hot water bath for a few minutes. Test 3 Add a 1 cm strip of magnesium ribbon, then leave the test-tube for 3 minutes and then shake the test-tube gently. At the end of the experiments rinse the test-tubes thoroughly. [7] (ii) Each of FB 4, FB 5 and FB 6 is one of the following acids: methanoic acid, nitric acid or sulfuric acid. Use your observations to suggest the identity of each acid. Explain your answers. FB 4 is ............................................... . explanation ........................................................................................................................ ........................................................................................................................................... FB 5 is ............................................... . explanation ........................................................................................................................ ........................................................................................................................................... FB 6 is ............................................... . explanation ........................................................................................................................ ........................................................................................................................................... [3] (b) The halogenocarboxylic acid in FB 2 was hydrolysed by heating with excess aqueous sodium hydroxide. The resulting solution is FB 7. (i) Carry out a test to check the identity of the halogen atom present in FB 2. Use a 1 cm depth of FB 7 in a test-tube for your test. State your reagents and record your observations at each stage of your test. From this test only, give the identity of the halogen present in FB 2. The halogen is .................................................................. . [3] (ii) Give the equation for the hydrolysis reaction of the halogenocarboxylic acid, FB 2, with excess hot aqueous sodium hydroxide. (If you were unable to identify the halogen in (b)(i), then use the formula CH3CHXCOOH.) ..................................................................................................................................... [2] [Total: 15] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+

Mark scheme: 3(a)(i) test observations FB 4 FB 5 FB 6 Test 1 + KMnO4 (and warm) no change / (KMnO4) stays purple / no (visible) reaction * (solution) turns colourless / KMnO4 decolourised * look to box below ignore no change / (solution stays) purple / no (visible) reaction * Test 2 + Cu turnings effervescence / fizzing / bubbling (on warming) * solution (turns) (pale) blue / green-blue / blue-green OR (pale) brown gas / bubbles * no (visible) reaction / no change * no (visible) reaction / no change * Test 3 + Mg ribbon (slow) effervescence / fizzing / bubbling * (slow) effervescence / fizzing / bubbling * fast / rapid / vigorous gas made / fizz / effervescence / bubbling * gentle shake after leaving Mg remaining OR effervescence * Mg remaining OR effervescence * becomes hot owtte / exothermic * no Mg remaining / colourless solution only * anywhere in Test 3: (gas / H2) burns with a ‘pop’ / pops with a lighted splint * * 2 * = 1 mark (round down) Question Answer Marks 3(a)(ii) M1 FB 4 is nitric acid / HNO3 AND slower effervescence than FB 6 OR forms brown gas / NO2 (with Cu) OR blue solution (with Cu) OR reacts with Cu M2 FB 5 is methanoic acid / HCOOH AND (only acid to) decolourise KMnO4 (owtte) OR (only) methanoic acid can be oxidised M3 FB 6 is sulfuric acid / H2SO4 AND faster / fastest bubbling with Mg OR Mg disappears 3 3(b)(i) M1 acidifies FB 7 with nitric acid (before testing with AgNO3) M2 adds (aqueous) silver nitrate / AgNO3 AND off-white / cream ppt M3 adds (aqueous) ammonia / NH3 AND ppt partially soluble / insoluble AND bromine 3 3(b)(ii) Two marks for fully correct equation (with excess NaOH): 2NaOH + CH3CHXCOOH  CH3CH(OH)COONa + NaX + H2O where X is the halogen identified in (b)(i) or ‘X’ One mark for: unbalanced equation with correct formulae OR NaOH + CH3CHXCOOH  CH3CH(OH)COOH + NaX OR NaOH + CH3CHXCOOH  CH3CH(OH)COONa + HX 2

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