Edexcel A-Level Chemistry Paper 1, June 2019: Question 3

7 marks · Medium difficulty · Synoptic Questions

Calculate the enthalpy change of neutralisation for propanoic acid and explain the difference in enthalpy of neutralisation between ethanoic acid and hydrochloric acid using experimental data and calorimetry principles.

Practise this question

Question

A multi-part question about standard molar enthalpy change of neutralisation. Part (a)(i) asks to identify the piece of equipment with the smallest measurement uncertainty from a table. Part (a)(ii) gives experimental data for the reaction between propanoic acid and sodium hydroxide and asks to calculate the enthalpy change of neutralisation. Part (b)(i) shows four temperature-time graphs A, B, C, and D for an extrapolation determination and asks which one is correct. Part (b)(ii) asks to explain why the enthalpy of neutralisation of ethanoic acid differs from that of hydrochloric acid.
Question text

3 The standard molar enthalpy change of neutralisation is the enthalpy change when

an acid and an alkali react under standard conditions to form one mole of water.

(a) An experiment was carried out to determine the enthalpy change of

neutralisation for the reaction between propanoic acid and sodium hydroxide.

The equation for this reaction is

CH CH2COOH(aq) + NaOH(aq) o CH CH COO–Na+(aq) + H O(l)

33 2 2

50.0 cm3 of sodium hydroxide solution, of concentration 1.00 mol dm–3, was placed

in a polystyrene cup. The initial temperature was measured.

(i) Which piece of equipment has the smallest measurement uncertainty for the

measurement of 50.0 cm3 of sodium hydroxide solution?

(1)

Equipment Measurement uncertainty for each reading

A burette ±0.05 cm3

B 50 cm3 measuring cylinder ±1 cm3

C 25 cm3 pipette ±0.06 cm3

D 50 cm3 pipette ±0.08 cm3

(ii) 50.0 cm3 of propanoic acid solution, of concentration 1.00 mol dm–3, was

added and thoroughly mixed with the sodium hydroxide solution in the

polystyrene cup.

The maximum temperature rise was 6.5 °C.

Calculate the enthalpy change of neutralisation for propanoic acid, in kJ mol–1,

giving your answer to the nearest whole number.

[Assume density of the mixture = 1.00 g cm–3, specific heat capacity of the

mixture = 4.18 J g–1 °C–1]

(3)

(b) Another experiment was carried out with a solution of ethanoic acid and

sodium hydroxide solution of the same concentration.

(i) Which graph shows the correct way that the maximum temperature rise

should be determined?

(1)

A B

45 45

40 40

35 35

30 ¨T 30 ¨T

256 25

20 *P58306A0628*20

15 15

0 60 120 180 240 300 360 420 480 540 600 0 60 120 180 240 300 360 420 480 540 600

NaOH NaOH

Time / s Time / s

added added

C D

45 45

40 40

35 35

30 30 ¨T

¨T

25 25

20 20

15 15

0 60 120 180 240 300 360 420 480 540 600 0 60 120 180 240 300 360 420 480 540 600

NaOH NaOH

Time / s Time / s

added added

(ii) Explain why the data book value for the standard enthalpy change of

neutralisation of ethanoic acid with sodium hydroxide is –55.2 kJ mol–1 but the

value for hydrochloric acid is –57.1 kJ mol–1.

(2)

… *P58306A0728*

(Total for Question 3 = 7 marks)

Mark scheme

Show the mark scheme The mark scheme details answers for each section. Part 3(a)(i) gives D as the correct multiple-choice answer. Part 3(a)(ii) provides the calculation steps for Q, enthalpy change, and the final value of -54 kJ mol-1. Part 3(b)(i) gives B as the correct graph. Part 3(b)(ii) awards marks for stating that ethanoic acid is weak or partially dissociated and that energy is required to fully ionise it.

How to answer it

Enthalpy Change of Neutralisation Study Guide

What this question tests

This question assesses your practical skills regarding measurement uncertainties, graphical determination of maximum temperature changes in calorimetry, enthalpy change calculations (q = mcdeltaT and deltaH), and thermodynamic explanations involving strong versus weak acids (bond breaking and ionisation energies).

Question 3(a)(i) - Measurement Uncertainty

Identifying the equipment with the smallest measurement uncertainty

✅ Correct Answer

D (50 cm³ pipette with an uncertainty of ±0.08 cm³)

💡 Key Knowledge

When evaluating uncertainties for delivering a specific volume (50 cm³):

  • A burette (A) is used twice (initial and final reading), doubling its uncertainty to ±0.10 cm³.
  • A 50 cm³ measuring cylinder (B) has a massive baseline uncertainty of ±1 cm³.
  • A 25 cm³ pipette (C) must be filled twice to reach 50 cm³, doubling its uncertainty to ±0.12 cm³.
  • A single 50 cm³ pipette (D) is filled only once, so its uncertainty remains at ±0.08 cm³.
Marks available: 1 mark

Question 3(a)(ii) - Calorimetry Calculation

Calculating the enthalpy change of neutralisation for propanoic acid

📐 Step-by-Step Calculation

  1. Calculate total mass (m) of the mixture:
    50 cm³ + 50 cm³ = 100 cm³. Assuming density = 1.00 g cm⁻³, mass = 100 g .
  2. Calculate heat energy released (Q):
    Q = m × c × deltaT
    Q = 100 × 4.18 × 6.5 = 2717 J (or 2.717 kJ ).
  3. Calculate moles of water / limiting reactant:
    Moles of NaOH = (50 / 1000) × 1.00 = 0.050 mol.
    Moles of propanoic acid = 0.050 mol. Therefore, moles formed = 0.050 mol .
  4. Calculate enthalpy change per mole (deltaH):
    deltaH = -Q / n = -2.717 kJ / 0.050 mol = -54.34 kJ mol⁻¹ .
  5. Apply final formatting:
    Nearest whole number with a negative sign = -54 kJ mol⁻¹ .

❌ Common Calculation Traps

  • Volume mixing trap: Forgetting to add the volumes of acid and alkali together (using 50 instead of 100 g for mass m ).
  • Sign omission: Forgetting the negative sign ( - ) for an exothermic neutralisation reaction.
  • Significant figures: Failing to round to the requested nearest whole number.
Marks available: 3 marks (1 for Q, 1 for deltaH calculation, 1 for correct sign and rounding)

Question 3(b)(i) - Graphical Temperature Analysis

Determining maximum temperature change from cooling curves

✅ Correct Answer

B

🧠 Exam Technique & Extrapolation

To find the true maximum temperature rise (deltaT) accounting for heat loss to the surroundings:

  • The cooling trend after mixing must be extrapolated back to the exact time the alkali was added (indicated by the arrow/marker).
  • Graph B correctly back-extrapolates the cooling line to the time of mixing to find the theoretical maximum temperature reached.
  • Graph A lacks extrapolation entirely; C and D extrapolate at incorrect time coordinates.
Marks available: 1 mark

Question 3(b)(ii) - Strong vs. Weak Acid Enthalpy

Explaining differences in enthalpy of neutralisation values

💡 Key Knowledge

The standard enthalpy change of neutralisation for a strong acid (like HCl) is typically more exothermic (−57.1 kJ mol⁻¹) than for a weak acid (like ethanoic acid, −55.2 kJ mol⁻¹).

✅ Marking Points

  • Point 1: Ethanoic acid is a weak acid / only partially ionised or dissociated in solution.
  • Point 2: Energy is absorbed / used to fully ionise the ethanoic acid molecules during the neutralisation process.

❌ Common Errors & Misconceptions

Do not state that "more NaOH reacts so more energy is given off." The enthalpy change is defined per mole of water formed; the difference is entirely due to the endothermic requirement of pulling apart covalent bonds during weak acid dissociation before neutralisation can finish.

Marks available: 2 marks

Topics

Physical Chemistry · Core Practicals · Topic 8: Energetics I · Topic 12: Acid-base Equilibria · Core Practical 8: Determine the enthalpy change of a reaction using Hess’s law

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.