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A-LevelChemistryChemical energetics9701/32 · May/June 2022 · Q216 Marks

In Question 1 you carried out a neutralisation reaction involving sodium hydroxide. H+(aq) + OH-(aq) → H₂O(I) In Question 2 you are to determine the enthalpy of neutralisation, ∆H_neut, as shown by the equation above. You will use a solution of sodium hydroxide and the diprotic acid, sulfuric acid. FB 4 is approximately 2 mol dm-³ sodium hydroxide, NaOH. FB 5 is 1.00 mol dm-³ sulfuric acid, H2SO4.

  1. (a)Method • Place the cup in the 250 cm³ beaker. • Use the 25.0 cm³ measuring cylinder to transfer 25.0 cm³ of FB 4 into the cup. • Place the thermometer in the solution. Record the temperature. • Fill the clean burette with FB 5. • Run 5.00 cm³ of FB 5 into the same cup. • Stir the mixture and record the highest temperature observed. • Repeat adding 5.00 cm³ volumes of FB 5 into the same cup until 45.00 cm³ has been added. Record the highest temperature after each addition. Results Table 2.1 volume of FB 5/cm³ 0.00 5.00 10.00 15.00 20.00 temperature/°C volume of FB 5/cm³ 25.00 30.00 35.00 40.00 45.00 temperature/°C[3]
  2. (b)(i)Plot a graph of temperature (y-axis) against volume of acid added (x-axis) on the grid provided. Select a scale on the y-axis to include a temperature 2.0°C above the highest temperature you recorded. Label any points you consider to be anomalous. Draw two lines of best fit, one for the rise in temperature and one for the temperature change after the maximum temperature has been reached. Extrapolate the two lines so they intersect.[4]
  3. (b)(ii)Use your graph to: • determine the volume of sulfuric acid, FB 5, required to neutralise 25.0 cm³ of sodium hydroxide, FB 4 • determine the maximum change in temperature, ΔT. volume of H2SO4 = .............................................. cm³ maximum ΔT = .............................................. °C[1]
  4. (c)(i)Use your answer to (b)(ii) to calculate the amount, in mol, of sulfuric acid, FB 5, neutralised in your reaction. amount of H2SO4 = .............................................. mol[1]
  5. (c)(ii)Calculate the heat energy evolved in the neutralisation reaction in (a). (Assume that 4.18J are required to change the temperature of 1.0 cm³ of solution by 1.0°C.) heat energy evolved = .............................................. J[1]
  6. (c)(iii)Use your answers to (c)(i) and (c)(ii) to calculate the enthalpy of neutralisation, ∆H_neut, for the reaction given in (a). H+(aq) + OH-(aq) → H2O(I) ΔH_neut = .............................................. sign value unit[1]
  7. (d)(i)The value for ∆H_neut quoted in a textbook is –57.6kJmol-1. Calculate the percentage error in your answer to (c)(iii) compared with the theoretical value. (If you were unable to answer (c)(iii) then assume the value was –49.2kJ mol-1.) percentage error = .............................................. %[1]
  8. (d)(ii)Without changing the apparatus, suggest one improvement that could be made to the method in (a). Explain your answer. improvement explanation[1]
  9. (e)(i)Use your graph and the information on page 4 to calculate the concentration, in moldm⁻³, of sodium hydroxide in FB 4. concentration of NaOH = .............................................. mol dm-³[1]
  10. (e)(ii)A student repeats Question 1 with a new solution of FB 1. The student decides to dilute FB 4 by a factor of 20 to make the new FB 1 solution. The student incorrectly assumes the concentration of the new FB 1 solution is 0.100 mol dm-³. Calculate the actual concentration of NaOH in the new FB 1 solution. (If you were unable to answer (e)(i) then assume the concentration of sodium hydroxide in FB 4 was 1.93 mol dm-³. This is not the correct value.) concentration of NaOH in the new FB 1 solution = .............................................. mol dm-³ Predict the effect that using the new FB 1 solution has on the value for the percentage purity of the sodium hydrogen sulfate you calculated in 1(c)(iv). The percentage purity of NaHSO4 would be larger than calculated. The percentage purity of NaHSO4 would be the same as calculated. The percentage purity of NaHSO4 would be smaller than calculated. Tick the appropriate box and explain your answer.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

  1. I Ten thermometer readings recorded, all shown to .0 or .5 °C AND at least one reading at .0 and one at .5
  2. For assessment of accuracy marks, round all thermometer readings to the nearest 0.5 °C. Calculate the supervisor's greatest ∆T (= Tmax - Tinitial) and write it ringed on each candidate script. Note volume (V) at which the Tmax occurs. Calculate the candidate's ∆T at the same volume (= Tat V - Tinitial) then calculate the difference, δ, from the supervisor value. Award of accuracy marks For supervisor ∆T 10.0 – 20.0: II Award if δ < 2.0 °C III Award if δ ≤ 1.0 °C
  3. I Axes unambiguously labelled temperature or °C on y-axis AND volume / FB 5 / H2SO4/acid or cm³ on the x-axis AND some numbers for scales
  4. II Suitable scales chosen
  5. III All points recorded in the table are accurately plotted
  6. IV Lines of best fit drawn (straight or smoothly curved lines) AND extrapolated to intersect
  7. [1] Correct volume from suitable intersect to 1 or 2 dp AND correct ∆T to 1 dp
  8. [1] Correctly calculates n(H2SO4) = 1.00 × volume (b)(ii) / 1000 AND answer to 2–4 significant figures
  9. [1] Correctly calculates Q = (25 + V(b)(ii)) × 4.18 × ∆T(b)(ii) and answer to 2–4 sf
  10. [1] Correctly uses ∆H = (c)(ii) / (2 × (c)(i)) / (1000) AND answer given to minimum 2 sf AND units kJ mol⁻¹ or J mol⁻¹ to match use or absence of ÷ 1000 AND negative sign
  11. [1] Correct expression [(-57.6 – (c)(iii)) / 57.6] × 100 OR [((c)(iii) – (- 57.6)) / 57.6] × 100
  12. [1] take more readings (near the end point) AND to improve position of intersection / owtte
  13. [1] Correctly uses n(NaOH) = (c)(i) × 2 × 1000 / 25
  14. [2] I Correctly uses (e)(i) / 20 AND answer to 2 or more sf II Explanation matches answer If conc NaOH > 0.1 mol dm⁻³ then 1st box ticked and (titre remains unchanged so actual) mol NaOH↑ so mol / amount / mass NaHSO4↑ OR If conc NaOH approx. 0.1 mol dm⁻³ then 2nd box ticked and titre not / (only) slightly changed so within experimental error OR If conc NaOH < 0.1 mol dm⁻³ then 3rd box ticked and (titre remains unchanged so actual) mol NaOH↓ so mol / amount / mass NaHSO4↓ OR If conc NaOH < 0.1 mol dm⁻³ then 1st box ticked and larger titre so moles / amount / mass NaHSO4 calculated increases OR If conc NaOH > 0.1 mol dm⁻³ then 3rd box ticked and smaller titre so mol / amount / mass NaHSO4 calculated decreases

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About this question

Topic
Chemical energetics, A-Level Chemistry (syllabus 9701). Structured question, 16 marks.
Source
A-Level Chemistry, May/June 2022 Paper 3 Variant 2, question 2.

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