WJEC A-Level Chemistry AS Unit 2, June 2025: Question 8
17 marks · Medium difficulty · Structured Questions
Investigate the endothermic reaction between hydrated barium hydroxide and ammonium chloride by completing a reaction profile, calculating enthalpy change of formation, plotting experimental temperature data, and determining experimental enthalpy change.
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Question text
8. The reaction between hydrated barium hydroxide and ammonium chloride is a rare example of
an endothermic solid-state reaction.
(a) Complete the reaction profile below to show the enthalpy change involved in an
endothermic reaction. Label the enthalpy change clearly on the diagram. [1]
Energy
reactants
Reaction progress9
(b) The enthalpy change for this reaction can be calculated using standard enthalpy
changes of formation.
(i) Give the meaning of the term standard enthalpy change of formation. [2]
(ii) The standard enthalpy changes of formation, Δ H θ, of the substances involved in
f
this reaction are given below.
Substance Δ H θ / kJ mol–1
f
H2O –286
NH3 –45.9
BaCl2.2H2O –1460
Ba(OH)2.8H2O –3350
NH4Cl –314
Using a Hess cycle or otherwise, calculate the standard enthalpy change for this
reaction. [3]
Ba(OH)2.8H2O + 2NH4Cl BaCl2.2H2O + 8H2O + 2NH3
08 © WJEC CBAC Ltd. (2410U20-1)
Enthalpy change =10 kJ mol–1
(c) (i) A student performed an experiment to determine the enthalpy change for this
reaction.
He weighed 31.5g of hydrated barium hydroxide into a glass beaker and recorded
09 its temperature. He started his stopwatch and recorded the temperature again©WJEC CBAC Ltd.(2410U20-1)
after 30 seconds.
After 1 minute, he added excess ammonium chloride and thoroughly mixed the
solids using a stirring rod. He then measured the temperature every 30 seconds
for a further 4 minutes.
His results are shown below.
Time/s Temperature/°C
0 21
30 21
60 —
90 17
120 5
150 –8
180 –20
210 –19
240 –18
270 –17
300 –16
Plot the results on the grid opposite. Calculate the maximum temperature change11
and record it below your graph. [4]
10 © WJEC CBAC Ltd. (2410U20-1)
0 Time/s
60 120 180 240 300
_
_
_
_
_
Maximum temperature change =12 °C
(ii) Use the balanced equation to calculate the mass of water released during this
reaction. Give your answer to an appropriate number of significant figures. [3]
11 Ba(OH)2.8H2O + 2NH4Cl BaCl2.2H2O + 8H2O + 2NH3
Mr 315
Mass of water = g
(iii) Use the mass of water from part (ii) and the maximum temperature change from
part (i) to calculate the enthalpy change of reaction per mole of hydrated barium
hydroxide. Give your answer in kJ mol–1.
Assume that the specific heat capacity of the reaction mixture is 1.13 J g–1 K–1. [2]
Enthalpy change =13 kJ mol–1
(d) (i) Explain why the experimental value calculated in part (c)(iii) is much lower than
the theoretical value calculated in part (b)(ii). [1]
12 © WJEC CBAC Ltd. (2410U20-1)
(ii) Suggest a possible change to the method that would improve the accuracy of the
experimentally determined value. [1]
Mark scheme
Show the mark scheme
Marks available
Question Marking details
AO1 AO2 AO3 Total Maths Prac
8 (a)
energy level of products above the level of the reactants and
enthalpy change clearly labelled
(b) (i) enthalpy change for the formation of 1 mol of a substance (1)
from its constituent elements with everything in its standard 2 2
state under standard conditions (1)
(ii) ΣΔ Hθ = –3840 (1)
f products
ΣΔ Hθ = –3978 (1)
f reactants
Δ Hθ = ΣΔ Hθ – ΣΔ Hθ
r f products f reactants 3 3 3
Δ Hθ = –3840 – (–3978) = 138 (1)
r
ecf possible
Marks available
AO1 AO2 AO3 Total Maths Prac
(c) (i) all points plotted correctly (2)
any seven points plotted correctly (1) 2
line of best fit drawn back to time = 60 s (1)
43 4
temperature change = 45 C (1) 2
accept any value in the range 44-45.5 C
(ii) n(Ba(OH)2.8H2O) = 0.100 mol (1)
n(H2O) = 8 × 0.100 = 0.800 mol (1)
mass of H2O = 0.800 × 18.02 = 14.4 g (1)
33 2
must be given to 3 sig figs
ecf possible
(iii) 𝑚𝑐∆𝑇
ΔH = − 𝑛 (1)
14.4 × 1.13 × (−45) –1
ΔH = − 0.100 = 7322 J mol
ΔH = 7.32 kJ mol–1 (1) 2 2 2
ecf possible e.g. from incorrect temperature change from graph
if incorrect award (1) for q = –732 J
Marks available
AO1 AO2 AO3 Total Maths Prac
(d) (i) heat is transferred from the surroundings into the7beaker
(making the temperature decrease smaller than it should be)
11 1
do not accept ‘heat lost to the surroundings’
(ii) award (1) for any of following
use a polystyrene cup / insulated calorimeter
insulate the beaker 1 1 1
put a lid on the beaker
Question 8 total 7 10 0 17 10 6
How to answer it
Energetics: Enthalpy of Formation & Solid-State Calorimetry
What this question tests
This 17-mark question assesses fundamental and practical thermochemistry skills from Unit 2:
- Sketching and labelling an endothermic reaction profile showing products at a higher energy level than reactants.
- Recalling the exact IUPAC definition for standard enthalpy change of formation (ΔfH⦵).
- Applying Hess’s Law to calculate standard reaction enthalpy using ΔfH⦵ data with correct stoichiometry and sign management.
- Plotting experimental temperature-time cooling data, performing an extrapolation to mixing time (t = 60 s), and deducing ΔT.
- Determining reacting quantities, mole ratios, and mass of water released to appropriate significant figures.
- Calculating enthalpy change of reaction (kJ mol⁻¹) using q = mcΔT .
- Evaluating experimental errors in endothermic reactions and suggesting specific apparatus improvements.
Reaction Profile for an Endothermic Reaction
Sketching and labelling energy levels
✅ Required Diagram Details
- Horizontal line for products drawn clearly above the reactants line.
- An activation energy curve/hump rising from reactants and dropping down to the product plateau (or a direct step).
- A vertical arrow or clearly labelled distance pointing upward from the level of reactants to products labelled enthalpy change (or ΔH).
❌ Common Errors
- Drawing products lower than reactants (confusing endothermic with exothermic).
- Labelling the activation energy hump as the enthalpy change instead of the net difference between reactants and products.
- Omitting the label or drawing double-headed arrows without clear boundaries.
Definition of Standard Enthalpy of Formation
Precise chemical terminology
✅ Correct Model Answer
The enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states under standard conditions.
💡 Key Knowledge
- Mark 1: Enthalpy change when 1 mol of a compound/substance is formed.
- Mark 2: From its elements in their standard states under standard conditions (100 kPa, 298 K).
❌ Common Errors & Lost Marks
- Saying "1 mole of reactants" instead of 1 mole of product formed.
- Forgetting to specify elements (writing "from its molecules" or "from its atoms").
- Missing "standard states" or "standard conditions".
• Enthalpy change for the formation of 1 mol of a substance [1]
• From its constituent elements with everything in its standard state under standard conditions [1]
Hess's Law Calculation
Calculating ΔrH⦵ from enthalpies of formation
📐 Step-by-Step Calculation
Balanced Equation:
Ba(OH)₂·8H₂O(s) + 2NH₄Cl(s) → BaCl₂·2H₂O(s) + 8H₂O(l) + 2NH₃(g)
- Sum of ΔfH⦵ for Products:
ΣΔfH⦵(products) = [1 × (-1460)] + [8 × (-286)] + [2 × (-45.9)]
= -1460 + (-2288) + (-91.8) = -3839.8 ≈ -3840 kJ mol⁻¹ - Sum of ΔfH⦵ for Reactants:
ΣΔfH⦵(reactants) = [1 × (-3350)] + [2 × (-314)]
= -3350 + (-628) = -3978 kJ mol⁻¹ - Apply Hess’s Law:
ΔrH⦵ = ΣΔfH⦵(products) - ΣΔfH⦵(reactants)
ΔrH⦵ = -3840 - (-3978) = +138 kJ mol⁻¹ (or +138.2 kJ mol⁻¹)
🧠 Exam Technique
Always remember the stoichiometric multipliers: 8 for H₂O, 2 for NH₃, and 2 for NH₄Cl. Watch the double negative when subtracting a negative reactant sum: - (-3978) = +3978 .
❌ Common Errors
- Formula inverted: calculating reactants - products giving -138 kJ mol⁻¹.
- Forgetting to multiply NH₄Cl by 2 or H₂O by 8.
- Arithmetic slip with minus signs.
• ΣΔfH⦵ products = -3840 [1]
• ΣΔfH⦵ reactants = -3978 [1]
• ΔrH⦵ = -3840 - (-3978) = +138 kJ mol⁻¹ [1] (allow ecf)
Graphical Analysis & Maximum Temperature Change
Plotting, extrapolating cooling curve to time of mixing
🧠 Graph Construction Method
- Plotting Points: Plot all data accurately on the grid. Notice that at t = 60 s, no temperature is recorded (time of mixing). Negative temperatures must be plotted accurately below 0 °C.
- Extrapolation: The temperature drops rapidly until t = 180 s (-20 °C) and then warms up steadily from t = 210 s to 300 s (-19 °C to -16 °C).
- Draw a straight line of best fit through the warming points (210 s to 300 s) and extrapolate it back to t = 60 s.
- Read the extrapolated minimum temperature at t = 60 s (approx. -24 °C).
- Calculate ΔT: Initial temperature = 21 °C. Maximum ΔT = 21 - (-24) = 45 °C (accept 44 °C to 45.5 °C).
❌ Common Errors
- Simply taking the lowest measured value (-20 °C) giving ΔT = 41 °C without extrapolating.
- Extrapolating back to t = 0 s instead of the mixing time at t = 60 s.
- Incorrect scale reading on the negative y-axis.
• All points plotted correctly [2] (any 7 points plotted correctly = 1 mark)
• Line of best fit drawn back to time = 60 s [1]
• Temperature change = 45 °C (accept 44 - 45.5 °C) [1]
Mass of Water Released
Mole calculations and significant figures
📐 Step-by-Step Calculation
- Calculate moles of Ba(OH)₂·8H₂O:
n = mass / Mr = 31.5 g / 315 g mol⁻¹ = 0.100 mol - Determine moles of water released (8:1 ratio from equation):
n(H₂O) = 8 × 0.100 mol = 0.800 mol - Calculate mass of water:
Mr(H₂O) = 2(1.01) + 16.00 = 18.02 g mol⁻¹ (or 18.0 g mol⁻¹)
mass = n × Mr = 0.800 mol × 18.02 g mol⁻¹ = 14.416 g → 14.4 g
🧠 Significant Figures Requirement
The question specifies an appropriate number of significant figures. The input data gives 31.5 g (3 sig figs) and Mr = 315 (3 sig figs), so the final mass must be quoted to 3 significant figures: 14.4 g.
• n(Ba(OH)₂·8H₂O) = 0.100 mol [1]
• n(H₂O) = 8 × 0.100 = 0.800 mol [1]
• mass of H₂O = 0.800 × 18.02 = 14.4 g [1] (must be given to 3 sig figs; ecf possible)
Calculating Enthalpy Change of Reaction (ΔH)
Applying q = mcΔT and finding ΔH in kJ mol⁻¹
📐 Step-by-Step Calculation
- Identify quantities:
• Mass being heated/cooled m = 14.4 g (from part c(ii))
• Specific heat capacity c = 1.13 J g⁻¹ K⁻¹
• Temperature change ΔT = -45 °C (or 45 K) - Calculate heat energy change (q):
q = m × c × ΔT = 14.4 × 1.13 × (-45) = -732.24 J - Calculate ΔH per mole of Ba(OH)₂·8H₂O:
ΔH = -q / n = -(-732.24 J) / 0.100 mol = +7322.4 J mol⁻¹
Convert to kJ mol⁻¹:
+7322.4 / 1000 = +7.32 kJ mol⁻¹
✅ Final Value
ΔH = +7.32 kJ mol⁻¹ (Accept values based on student's ΔT from part (i), e.g. if ΔT = 44 °C, ΔH = +7.16 kJ mol⁻¹).
❌ Common Errors
- Forgetting to convert J to kJ (giving 7322 kJ mol⁻¹).
- Using the total mass of solids instead of the mass of water as instructed by the question.
- Wrong sign: endothermic reactions must have a positive ΔH value.
• Working: ΔH = -(m × c × ΔT) / n = -(14.4 × 1.13 × (-45)) / 0.100 = 7322 J mol⁻¹ [1]
• Value with unit: ΔH = +7.32 kJ mol⁻¹ [1] (ecf possible from parts (i) and (ii))
Evaluation & Experimental Improvements
Understanding heat transfer in endothermic reactions
(d)(i) Why is the experimental value much lower?
Because the reaction mixture drops well below room temperature, heat is transferred from the surroundings into the reaction beaker. This makes the measured temperature drop smaller than it should be.
(d)(ii) Method Improvements
Any one of the following scores 1 mark:
- Use an expanded polystyrene cup / insulated calorimeter instead of a glass beaker.
- Insulate the glass beaker (e.g., wrap in cotton wool/mineral wool).
- Place a lid on the beaker.
• (d)(i): Heat is transferred from the surroundings into the beaker (making the temperature decrease smaller than it should be) [1]. Do not accept 'heat lost to the surroundings'.
• (d)(ii): Use a polystyrene cup / insulated calorimeter OR insulate the beaker OR put a lid on the beaker [1].
Topics
Physical Chemistry · Practical · 2.1 Thermochemistry · 1.3 Chemical calculations · AS Unit 2 practical work
Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.