Cambridge A Level Chemistry 9701 — 2022 May/June Paper 3 · Variant 4

1 Calcium carbonate reacts with hydrochloric acid to release carbon dioxide. CaCO3(s) + 2HCl (aq) → CaCl 2(aq) + CO2(g) + H2O(l) The concentration of the hydrochloric acid can be determined by reacting it with calcium carbonate and measuring the volume of carbon dioxide formed. FB 1 is hydrochloric acid, HCl. FB 2 is calcium carbonate, CaCO3. (a) Method ● Fill the tub with tap water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub. ● Using the 50 cm3 measuring cylinder, transfer 50 cm3 of FB 1 into the flask. Check that the bung fits tightly into the neck of the flask, clamp the flask and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of the flask. Tip all the FB 2, from the container, into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. ● Replace the flask in the clamp. Leave for several minutes, swirling the flask occasionally. You may wish to start Question 3 while the gas is being produced. ● When the reaction stops producing gas, record the final volume of gas in the measuring cylinder. Keep the flask and contents for use in 1(c)(i). volume of gas = .............................. cm3 [2] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder. (Assume that 1 mol of gas occupies 24.0 dm3 under these conditions.) amount of CO2 = .............................. mol [1] (ii) Use your answer to (b)(i) and the equation on page 2 to calculate the concentration, in mol dm–3, of hydrochloric acid in FB 1. Show your working. concentration of HCl = .............................. mol dm–3 [2] (c) (i) A student thinks that the mass of FB 2 should be measured. Explain, by observing the contents of the flask, why this is not necessary. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) In this experiment some carbon dioxide is lost before the bung is replaced. Suggest a change that could be made to minimise the loss of gas. Explain how this change minimises the loss of gas. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 8]

Mark scheme: 1(a) Accuracy marks Write and ring the supervisor’s volume of gas evolved on each candidate script. M1 volume of gas recorded within 25% of supervisor value M2 volume of gas recorded within 10% of supervisor value AND rounded to nearest integer 1(b)(i) Correctly calculates amount of CO2 = volume of gas 24000 mol AND answer to 2–4 significant figures 1 1(b)(ii) M1 correctly uses amount of HCl = (b)(i)  2 in 50 cm3 mol M2 concentration of HCl = answer above  20 mol dm−3 AND answer to 2–4 significant figures 2 1(c)(i) M1 solid / calcium carbonate / CaCO3 / FB 2 / residue still present / when reaction finished / at the end M2 solid / calcium carbonate / CaCO3 / FB 2 is in excess OR the acid is the limiting reagent (ora) 2 1(c)(ii) One of the following:  specified method to keep solid and acid apart until bung fitted (e.g. divided flask / suspend solid in ignition tube / weighing boat / small beaker and may be shown as a labelled diagram)  use lumps of solid to reduce rate of reaction  use more dilute acid to reduce rate of reaction  cool (reaction) flask / acid to reduce rate of reaction 1

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2 In this experiment you will use a thermometric method to determine the concentration of a sample of alkali. You will mix varying volumes of acid with a fixed volume of the alkali and measure the temperature rises that occur. You will use your experimental data to calculate the enthalpy change for the neutralisation of the acid with alkali. FB 3 is aqueous sodium hydroxide, NaOH. FB 4 is 2.20 mol dm–3 hydrochloric acid, HCl. (a) Method ● Support the cup in the 250 cm3 beaker. ● Use the thermometer to measure the initial temperature of FB 3. initial temperature of FB 3 = .............................. °C ● Fill a burette with distilled water. ● Fill the other burette with FB 4. Label this burette FB 4. ● For Experiment 1, use the 10 cm3 pipette to transfer 10.0 cm3 of FB 3 into the cup. ● Add 9.00 cm3 of distilled water from the burette into the same cup. ● Add 1.00 cm3 of FB 4 from the other burette into the same cup. ● Stir the mixture and use the thermometer to measure the maximum temperature obtained. ● Record the maximum temperature in the table. ● Empty and shake dry the cup ready for use in Experiment 2. ● Repeat the method using 10.0 cm3 of FB 3 for each experiment and the volumes of water and FB 4 shown in the table. In each case, measure and record the maximum temperature. Table 2.1 volume water volume FB 4 maximum experiment / cm3 / cm3 temperature / °C 1 9.00 1.00 2 7.00 3.00 3 5.00 5.00 4 3.00 7.00 I 5 1.00 9.00 II III 6 IV 7 V Carry out two further experiments which will enable you to determine more precisely the volume of FB 4 that gives the highest maximum temperature. Record your measurements for these two experiments in the table. [5] (c) (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of hydrochloric acid in the volume of FB 4 in (b). If you were unable to determine an answer to (b) use 4.10 cm3 as the volume of FB 4. amount of HCl = .............................. mol [1] (iii) Use your answer to (c)(ii) and the information on page 4 to calculate the concentration, in mol dm–3, of sodium hydroxide in FB 3. concentration of NaOH = .............................. mol dm–3 [1] (iv) Calculate the energy released when the volume of FB 4 in (b) is neutralised by sodium hydroxide. Show your working. (Assume that 4.18 J of energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.) energy released = .............................. J [2] (v) Use your answers to (c)(ii) and (c)(iv) to calculate the enthalpy change of neutralisation, in kJ mol–1, for 1.0 mol of hydrochloric acid. enthalpy change = ...... ............................. kJ mol–1 sign value [1] (d) (i) The theoretical value of the enthalpy change of neutralisation is –57.6 kJ mol–1. Calculate the percentage error in your value of the enthalpy change from (c)(v). Show your working. (Assume that the conditions under which you carried out your experiment in (a) are identical to the conditions used to determine the theoretical value.) percentage error = .............................. % [1] (ii) Suggest one modification to the procedure used in (a) that would give a more accurate value for the enthalpy change of neutralisation. Do not suggest any modifications to apparatus in your answer. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 17] 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 Seven thermometer readings recorded II All thermometer readings recorded to nearest 0.5 °C with at least one at .0 and one at .5 AND all burette readings to .#0 or .#5 cm3. III Two appropriate extra volumes chosen AND volume of water used + volume of FB 4 = 10(.00) cm3 The volumes selected must be at least 0.50 cm3 from any other. One value must be the volume between the first highest temperature and one of the adjacent temperatures. The second must not be between the same two set experiments as value one. If there is no maximum temperature for experiments 1–5 then the one value must be between 7.00 and 9.00 cm3 and the other 9.50 or 10.00 cm3. Accuracy marks Calculate the supervisor’s T at 5.00 cm3 of added FB 4 (initial thermometer reading). This value must be written and ringed on each candidate’s script. Calculate and record candidate’s T for the same pair of readings. Calculate and record the difference, , from supervisor. IV Award if ⩽1.0 ºC V Award if ⩽0.5 ºC Question Answer Marks 2(b) I Axes correctly labelled with temperature AND / ºC or (ºC) on the y-axis volume / FB 4 AND / cm3 or (cm3) on the x-axis AND suitable scales selected to occupy more than half of the available space on both axes and including 2 ºC above the highest recorded temperature on the y-axis 4 II All points recorded in the table are accurately plotted. Those on the line must be exactly on the line; those not on the line must be in the correct half of the small square. III Lines of best fit Two straight lines of best fit drawn with a ruler. The two lines must give a sharp intersection at (or above) the highest temperature. There must be at least 3 points considered for the line of best fit where the maximum temperature is increasing. Any points labelled anomalous are disregarded. IV Correct volume of FB 4 at intersection AND given to 1 (or 2) d.p. 2(c)(i) All answers to (c)(ii), (iii), (iv) and (v) are given to 2–4 significant figures 1 2(c)(ii) Correctly calculates amount of HCl = 2.2 volume in 1000  (b) mol 1 2(c)(iii) Correctly uses concentration of NaOH = (c)(ii)  100 mol dm−3 1 2(c)(iv) M1 correctly calculates Tmax (from table or graph) OR correct figures for calculation shown M2 correctly calculates energy released = 20(.00)  4.18  Tmax AND answer correct 2 2(c)(v) Correctly uses enthalpy change = [ 1000]  (c)(iv) (c)(ii) kJ mol−1 AND answer with negative sign given 1 Question Answer Marks 2(d)(i) Correct expression % error = ( )57.6 ( ) ( )57.6          (c)(v)  100 1 2(d)(ii) Any one of:  Measure initial temperatures of water AND FB 4  Take more readings near the largest maximum temperature  Record initial temperature for each experiment 1

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3 Half fill the 250 cm3 beaker with water, heat until the water is nearly boiling and then turn off the Bunsen burner. This is your hot water bath for use in (a)(i). (a) You are provided with aqueous solutions FB 5, FB 6, FB 7 and FB 8. The solutions are known to be hydrochloric acid, hydrogen peroxide, methanoic acid and sulfuric acid. All the solutions have the same concentration. Note: the order of FB 5 to FB 8 does not correspond to the order of identities given above. (i) Carry out the following tests using a 2 cm depth of each reagent in a test-tube. Record your observations in Table 3.1. Table 3.1 8 FB 7 FB observations 6 FB 5 FB water aqueous hot the acidified in test of magnesium. of dropsmanganate(VII). test-tube piece few the 1a 2a TestAdd TestAddpotassiumPlacebath. [5] (ii) Use your observations to complete the sentences. Explain your answer. FB 7 is ........................................... . FB 8 is ........................................... . explanation .......................................................................................................................... ............................................................................................................................................. [2] (iii) Carry out one additional test that allows you to distinguish between FB 5 and FB 6. Record your test and the result you obtained. FB 5 is ........................................... . FB 6 is ........................................... . [2] (iv) Write an ionic equation for the reaction of magnesium with FB 5. Include state symbols. ....................................................................................................................................... [1] (v) Describe the observations that you would expect to see if the tests in (a)(i) were repeated using aqueous ethanoic acid. observation with magnesium ............................................................................................... ............................................................................................................................................. observation with acidified aqueous potassium manganate(VII) ......................................... ............................................................................................................................................. [2]

Mark scheme: 3(a)(i) test FB 5 FB 6 FB 7 FB 8 Test 1 Mg fizz * H2 / gas pops with lighted splint / burns with pop * fizz * H2 / gas pops with lighted splint / burns with pop slower * fizz * H2 / gas pops with lighted splint / burns with pop no change / no effect / no visible reaction / no fizzing * Test 2 KMnO4 no change / no effect / no (visible) reaction OR KMnO4 remains purple * no change / no effect / no (visible) reaction OR KMnO4 remains purple * purple to colourless / yellow / brown OR KMnO4 turns colourless / decolourises / yellow / brown * purple to colourless OR KMnO4 turns colourless / decolourises * 2 * = 1 mark The relative rate of reaction of Mg with the three acids may be awarded from ‘rapid’ (owtte) fizz with either or both FB 5 and FB 6. The gas test for H2 may be awarded in any one of the three boxes for FB 5, FB 6 or FB 7. However, an incorrect gas ‘identified’ in Test 1 negates this *. A slight change in colour (purple to brown *) may be noted with KMnO4 and FB 6 on heating. Question Answer Marks 3(a)(ii) M1 FB 7 is methanoic acid / HCOOH / HCO2H AND FB 8 is hydrogen peroxide / H2O2 M2 Correct explanation for one of FB 7 or FB 8 from observations in table: Methanoic acid / FB 7 Fizzes with Mg / H2 produced and slowest (reaction) OR oxidised by KMnO4 / decolourises KMnO4 (allow yellow / brown unless the same observations are given with FB 6) OR Hydrogen peroxide / FB 8 no fizz / no reaction with Mg OR decolourises KMnO4 immediately / without heating / oxidised by KMnO4 immediately 2 3(a)(iii) M1 EITHER add (aqueous) AgNO3 / silver nitrate AND white ppt with FB 6 OR add (aqueous) barium chloride / BaCl2 / barium nitrate / Ba(NO3)2 AND white ppt with FB 5 M2 FB 5 is sulfuric acid / H2SO4 AND FB 6 is hydrochloric acid / HCl AND from correct observation 2 3(a)(iv) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) 1 3(a)(v) M1 magnesium: effervescence / fizz / bubbling M2 potassium manganate(VII): no effect / no (visible) reaction / KMnO4 remains purple 2 3(b) 2 * = 1 mark use of (aqueous) NaOH AND NH3 * with NaOH white ppt * ppt insoluble in excess * with NH3 white ppt * ppt insoluble in excess * cation is Mg2+ * 3

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