Edexcel A-Level Chemistry Paper 1, June 2022: Question 5
9 marks · Medium difficulty · Calculations
Calculate enthalpy changes including hydration enthalpy and construct Born-Haber/enthalpy cycles, define standard enthalpy of formation, and explain factors affecting enthalpy of hydration.
Practise this questionQuestion
Question text
5 This question is about enthalpy changes and energy changes.
(a) Magnesium carbonate reacts with dilute hydrochloric acid at room temperature.
MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l)
When the reaction is carried out in a sealed container with a constant volume, the
heat energy change is not the same as the enthalpy change for this reaction.
Give a reason why this is so.
(1)
(b) State what is meant by the standard enthalpy change of formation of
aluminium oxide, Al2O3(s). Include standard conditions.
(3)
(c) Use the data in the table to answer the questions.
Enthalpy change Value / kJ mol–1
Enthalpy change of hydration of K+ –322
Enthalpy change of hydration of Ca2+ –1650
Enthalpy change of solution of KCl +17.2
Lattice energy of KCl –711
14 (i) Name the two properties of ions that affect the value of their
enthalpy change of hydration.*P67093RA01432*
(2)
(ii) Calculate the enthalpy change of hydration for chloride ions by completing
the energy cycle, including labels, and using the data in the table.
(3)
ΔsolnH + –
KCl(s) + aq K (aq) + Cl (aq)
enthalpy change of hydration for Cl– ions kJ mol–1
(Total for Question 5 = 9 marks)
Mark scheme
Show the mark scheme
5(a) An answer that makes reference to the following point: (1)
• (the system / it) is not at constant pressure Allow a gas / carbon dioxide is produced and
or this increases the pressure
enthalpy change is the heat change at a constant
Allow the pressure is increased / increases
pressure
Ignore reference to temperature
Question
Answer Additional Guidance Mark
Number
5(b) An answer that makes reference to (3)
the following points: Allow
• (the enthalpy/energy change when) 1 2Al(s) + 1½O2(g) → Al2O3(s) for M1 and M2
mol of aluminium oxide (1) If state symbols are missing or incorrect only M1 can be
awarded
• is formed from its elements in their Allow M2 for multiples in equation provided state symbols
standard states (1) for the elements are correct
• at 100 kPa and a ‘specified’ / ‘stated’ Allow 1 atm / 1 x 105 Pa / 101 kPa / 1.01 x 105 Pa for pressure
temperature (1) Allow a value for the temperature of 298K / 250C
Ignore 273K
Ignore other standard conditions e.g. 1 mol dm-3
Do not allow oK
Question
Answer Additional Guidance Mark
Number
5(c)(i) An answer that makes reference to the following (2)
points:
• (ionic) radius (1) Allow size (of ions)
Do not award atomic radius/size of atoms
• (ionic) charge (1) Do not award atomic charge/charge of atoms
Allow charge density for 1 mark if no other
mark awarded
Question
Answer Additional Guidance Mark
Number
5(c)(ii) Example of cycle (3)
(∆solnH)
(KCl(s) + aq) (K+(aq) + Cl−(aq))
∆ H K+
Lattice energy hyd
+
∆ H Cl−
hyd
K+(g) + Cl−(g) + aq
• correct species with state symbols in Ignore missing aq
bottom box (1)
• arrows in correct directions Allow any clear labels for arrows, including values for
lattice energy and ∆ H K+, e,g, LE, HE
and hyd
labelled (1) Allow arrow on left reversed if labelled – lattice
energy/+711
Allow two separate arrows on the RHS
• calculation of enthalpy change of hydration Standalone mark
− ∆ H Cl− = −711 +17.2 – (−322)
of Cl ions (1) hyd
= −371.8 (kJ mol−1)
No TE on incorrect arrows
Ignore SF apart from 1SF
(Total for Question 5 = 9 marks)
How to answer it
Enthalpy Changes and Energy Changes Study Guide
This question assesses your understanding of thermodynamic definitions, enthalpy cycles (Born-Haber/Hess's Law), factors affecting ion hydration, and the fundamental distinction between heat energy changes at constant pressure versus constant volume.
Enthalpy Change vs. Heat Energy Change in Sealed Containers
✅ Correct Answer
The system is not at constant pressure (because CO₂ gas is produced, increasing the pressure inside the sealed container).
💡 Key Knowledge
By definition, enthalpy change ( ΔH ) is the heat energy change measured under a constant pressure. If a reaction generates gas in a sealed, constant-volume container, pressure builds up, meaning the heat released/absorbed does not equal the enthalpy change.
❌ Common Errors
Students frequently try to explain the difference by referencing changes in temperature or reaction rate. The examiner explicitly instructs to ignore references to temperature.
Standard Enthalpy Change of Formation
✅ Correct Answer
The enthalpy change when 1 mole of aluminium oxide ( Al₂O₃(s) ) is formed from its elements in their standard states, under standard conditions (100 kPa and a stated temperature, usually 298 K).
🧠 Exam Technique
Always memorise definitions using strict 3-part templates: (1) Amount formed (1 mole of the substance), (2) Source components (elements in standard states), (3) Conditions (100 kPa and a stated temperature).
❌ Common Errors
Forgetting to specify 1 mol of the product, or failing to state standard conditions (temperature and pressure). Writing 273 K instead of room conditions or omitting state symbols in supporting equations.
Factors Affecting Enthalpy Change of Hydration
✅ Correct Answer
- Ionic radius (or ionic size)
- Ionic charge
💡 Key Knowledge
Hydration enthalpy depends on the attraction between polar water molecules and gaseous ions. Smaller ions with higher charges have higher charge density, leading to stronger electrostatic attractions with water and therefore more exothermic hydration enthalpies.
❌ Common Errors
Writing "atomic radius" or "atomic charge" instead of ionic radius/charge will lose you the marks. Precision in chemical terminology is heavily penalised.
Enthalpy Cycle and Hydration Calculation
📐 Step-by-Step Calculation
- Complete the bottom box: K⁺(g) + Cl⁻(g) + aq (or K⁺(g) + Cl⁻(g) ).
- Set up cycle arrows:
• From KCl(s) + aq down to gaseous ions = Lattice energy ( -711 kJ mol⁻¹ ).
• From gaseous ions up to K⁺(aq) + Cl⁻(aq) = Sum of hydration enthalpies ( Δ_hyd H(K⁺) + Δ_hyd H(Cl⁻) ). - Apply Hess's Law:
Δ_soln H = - (Lattice energy) + Δ_hyd H(K⁺) + Δ_hyd H(Cl⁻)
+17.2 = -(-711) + (-322) + Δ_hyd H(Cl⁻)
+17.2 = +711 - 322 + Δ_hyd H(Cl⁻)
+17.2 = +389 + Δ_hyd H(Cl⁻)
Δ_hyd H(Cl⁻) = +17.2 - 389 = -371.8 kJ mol⁻¹
❌ Common Calculation Traps
Sign errors are the most common pitfall. Lattice energy values given in tables are negative, but going against an arrow direction or substituting values into Hess loops requires careful tracking of signs. Double-check your algebra before final submission!
Topics
Physical Chemistry · Topic 8: Energetics I · Topic 13: Energetics II
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.