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A-LevelChemistryChemical energetics9701/31 · Oct/Nov 2018 · Q125 Marks

In this experiment you will determine the percentage purity of a sample of impure anhydrous sodium carbonate. You will use two different methods to measure the enthalpy change of reaction when a sample of impure anhydrous sodium carbonate reacts with excess dilute hydrochloric acid. FA 1 is a sample of the impure anhydrous sodium carbonate. FA 2 is 2.00 mol dm⁻³ hydrochloric acid, HCl. FA 3 is a second sample of the impure anhydrous sodium carbonate used in FA 1.

  1. (a)Method 1 • Weigh the container with FA 1. Record this mass. mass of container with FA 1 = .............................................. g • Support one of the plastic cups in the 250 cm³ beaker. • Use the measuring cylinder to place 25 cm³ of FA 2 into the cup. • Measure the temperature of the FA 2 in the cup. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. Record this temperature at time t = 0. • Start the stopclock and leave it running for the whole experiment. • Measure and record the temperature of FA 2 in the cup every half minute for 2 minutes. • At t = 2½ minutes tip all the FA 1 into the cup. Stir the contents of the cup. • Measure and record the temperature of the contents of the cup at t = 3 minutes and then every half minute up to t = 9 minutes. • Weigh the container with any residual FA 1. Record this mass. mass of container with residual FA 1 = .............................................. g[5]
  2. (b)(i)On the grid on page 3, plot a graph of temperature (y-axis) against time (x-axis). You should choose a scale that allows you to plot 2°C above the maximum temperature reached. On your graph, draw two straight lines of best fit. One line is for the temperature before adding FA 1 and the other line for the cooling of the solution once reaction is complete. Extrapolate these two lines to t = 2½ minutes.[4]
  3. (b)(ii)From your graph, find the theoretical temperature rise at t = 2½ minutes. theoretical temperature rise = ..................................................... °C[1]
  4. (c)(i)Calculate the energy released in the reaction. (Assume 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0°C.) energy released = .............................................. J[1]
  5. (c)(ii)The equation for the reaction between anhydrous sodium carbonate and hydrochloric acid is shown. Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l) The literature value for the enthalpy change of this reaction is –27.0 kJ mol⁻¹. Use this figure, and the value that you found in (i), to find the mass of anhydrous sodium carbonate you used in (a). You should assume that no energy was lost to the surroundings in your experiment. mass Na₂CO₃ = .............................................. g[2]
  6. (c)(iii)Calculate the percentage of anhydrous sodium carbonate present in FA 1. percentage Na₂CO₃ in FA 1 = .............................................. %[1]
  7. (d)In your calculation in (c), what assumption have you made about the impurity present in FA 1?[1]
  8. (e)Method 2 • Weigh a clean, dry plastic cup and record the mass. • Add between 1.70g and 1.90 g of FA 3 to the plastic cup and record the mass. • Support the plastic cup in the 250 cm³ beaker. • Pour 25 cm³ of FA 2 into the measuring cylinder. • Measure and record the initial temperature of FA 2 in the measuring cylinder. • Pour the 25 cm³ of FA 2 into the plastic cup. • Stir the contents of the cup and record the maximum temperature. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. • Calculate and record the mass of FA 3 used and the change in temperature.[2]
  9. (f)Use the temperature rise in (e), and the fact that the enthalpy change for the reaction between anhydrous sodium carbonate and hydrochloric acid is –27.0 kJ mol⁻¹, to calculate the percentage of anhydrous sodium carbonate in FA 3. percentage Na₂CO₃ in FA 3 = .............................................. %[2]
  10. (g)FA 1 and FA 3 are both samples of the same impure anhydrous sodium carbonate and so the percentage of anhydrous sodium carbonate found using Method 1 and Method 2 should be the same. In practice the percentages are sometimes different from each other. In both methods, percentage errors occur due to measuring the mass of solid and the temperature rise. Ignoring these errors, which method is more accurate? Tick the correct box and explain your answer. Method 1 more accurate [ ] Method 2 more accurate [ ] Method 1 and Method 2 equally accurate [ ][1]
  11. (h)A student decided to confirm by experiment the literature value for the enthalpy change of the reaction between anhydrous sodium carbonate and hydrochloric acid. By mistake the student weighed a sample of hydrated sodium carbonate, Na₂CO₃.10H₂O, instead of anhydrous sodium carbonate, Na₂CO₃. State what effect this would have on the calculated value of the enthalpy change for the reaction. Explain your answer.[2]
  12. (i)A student used 3.00g of anhydrous sodium carbonate that was 80.0% pure by mass. Calculate the minimum volume of 2.00 mol dm⁻³ hydrochloric acid that would be needed to react completely with this sample of impure anhydrous sodium carbonate. volume of HCl = .............................................. cm³[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

  1. [1] I Table drawn with suitable headings and units; Temperature / T and time / t / °C, (s) or / minutes or mins or min
  2. [1] II Table completed with all temperature readings from 0 to 9 minutes at half minute intervals and 2 balance readings shown to the same number of decimal places
  3. [1] III All recorded thermometer readings to .5 °C with at least one ending in .0 °C and at least one ending in .5 °C.
  4. [1] IV ΔT within 1 °C of supervisor.
  5. [1] V ΔT within 0.5 °C of supervisor.
  6. [1] Axes unambiguously labelled. Scale chosen so that plotted points (and 2 °C extra on y-axis) occupy more than half the available space in both directions.
  7. [1] All recorded points plotted. Points plotted to within half a small square. Points that should be on lines must be on the line and points that should not be on lines must not be on lines. Do not award if crosses / blobs are > half square thick.
  8. [1] Two straight lines of best fit drawn (with a ruler) – one up 2 minutes and the other after the maximum temperature.
  9. [1] Both lines extrapolated to 2½ minutes.
  10. [1] Candidate ΔT to at least 1 decimal place is within half a small square or 1 decimal place (whichever is more appropriate) of examiner's calculated ΔT.
  11. [1] Correctly calculates energy evolved = 25 × 4.2 × ΔT ΔT from (b)(ii) or table and answer to 2–4 sf
  12. [1] Correct use of moles Na₂CO₃ = (c)(i) / 27000
  13. [1] Correct conversion of moles to mass (moles × 106).
  14. [1] Correct use of % Na₂CO₃ = (c)(ii) / mass FA1 used and answer to 2–4 sf.
  15. [1] Assumption: the impurity does not react with hydrochloric acid or FA 2 or impurities are not alkaline
  16. [1] Data with unambiguous headings (not weight). and Correct subtraction of mass and ΔΤ.
  17. [1] Examiner calculates expression: mass FA 1 × ΔT in (e) / mass FA 3 × ΔT in (a) to 2 decimal place. Award mark if this value lies in range 0.80 to 1.25
  18. [1] Calculation of energy released and moles of Na2CO3 in FA 3, using temperature rise from (e).
  19. [1] Calculation of mass of Na2CO3 and percentage in FA 3.
  20. [1] First box ticked (If there is a temperature fall after the max in Method 1 then this is more accurate) because as heat lost is compensated for / cooling curve plotted OR Third box ticked (special case if no temp fall in (a)) If there is no temperature fall after the maximum in Method 2 then both seem to be equally accurate because the temperature rise is potentially the same for both methods.
  21. [1] (The student used) fewer moles / less amount of carbonate
  22. [1] (The temperature increase is less and hence calculated) enthalpy change would be less exothermic / ΔH is less negative.
  23. [1] Correctly calculates Moles Na₂CO₃ = (3 / 106) × 0.8 = 0.0226 / 0.02264
  24. [1] Ratio 1: 2 moles HCl =0.0452 / 0.0453
  25. [1] Vol HCl = 0.0452 / 0.002 = 22.6(0) / 22.7(0) cm³ and answer to 3 or 4 significant figures (or nearest 0.05 cm³)

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About this question

Topic
Chemical energetics, A-Level Chemistry (syllabus 9701). Structured question, 25 marks.
Source
A-Level Chemistry, Oct/Nov 2018 Paper 3 Variant 1, question 1.

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