Edexcel A-Level Chemistry Paper 3, June 2023: Question 4

10 marks · Medium difficulty · Open Response

Devise an experimental procedure to determine the enthalpy change of solution for hydrated magnesium sulfate, calculate the enthalpy change of hydration using Hess's law, and explain the difference in enthalpy of hydration between magnesium and calcium ions.

Practise this question

Question

Three-part chemistry exam question about hydrated magnesium sulfate. Part 4(a) asks to devise an experimental procedure to determine the enthalpy change of solution for hydrated magnesium sulfate, worth 6 marks with lined answer space. Part 4(b) provides two thermochemical equations for solution and asks to calculate the enthalpy change for hydration using Hess's law, worth 2 marks. Part 4(c) asks to explain how the enthalpy change of hydration of magnesium ions differs from calcium ions, worth 2 marks with lined space.
Question text

4 This question is about hydrated magnesium sulfate, MgSO4·7H2O.

*(a) Devise an experimental procedure to determine the enthalpy change of solution

for hydrated magnesium sulfate.

MgSO4·7H2O(s) + aq → MgSO4(aq)

Details of the method of calculation are not required.

(6)

… *P71914A0832*

*P71914A0932*

(b) The enthalpy changes of solution for anhydrous and hydrated magnesium sulfate

were found by experiment to be

MgSO (s) + aq → MgSO (aq) Δ H = –63.2 kJ mol–1

44 r

MgSO ·7H O(s) + aq → MgSO (aq) Δ H = +15.7 kJ mol–1

42 4 r

Calculate, using Hess’s law, the enthalpy change for the hydration of

anhydrous magnesium sulfate.

Include a sign and units in your answer.

MgSO4(s) + 7H2O(l) → MgSO4·7H2O(s)

(2)

(c) Explain how the enthalpy change of hydration of magnesium ions in

magnesium sulfate is different from the enthalpy change of hydration of

calcium ions in calcium sulfate.

(2)

(Total for Question 4 = 10 marks)

Mark scheme

Show the mark scheme Mark scheme showing detailed marking points for parts 4a, 4b, and 4c. Part 4a details 6 indicative points covering weighing, measuring water, calorimeter use, temperature recording, and calculation details along with structure marks. Part 4b shows a Hess's law energy cycle and calculation leading to -78.9 kJ mol^-1. Part 4c awards marks for stating magnesium hydration is more exothermic because the magnesium ion is smaller and has greater charge density.

Question

Answer Additional Guidance Mark

Number

This question assesses the student’s ability to show a coherent and Guidance on how the mark scheme should be (6)

*4 (a) logically structured answer with linkages and fully sustained reasoning. applied:

The mark for indicative content should be added to

Marks are awarded for indicative content and for how the answer is the mark for lines of reasoning. For example, a

structured and shows lines of reasoning. response with four indicative marking points that is

partially structured with some linkages and lines of

The following table shows how the marks should be awarded for reasoning scores 4 marks (3 marks for indicative

indicative content. content and 1 mark for partial structure and some

Number of indicative marking Number of marks awarded for linkages and lines of reasoning).

points seen in answer indicative marking points If there were no linkages between the points, then

64 the same indicative marking points would yield

5-4 3 and overall score of 3 marks (3 marks for

3-2 2 indicative content and zero marks for linkages).

The following table shows how the marks should be awarded for structure

and lines of reasoning

Number of marks awarded for

structure of answer and sustained

lines of reasoning

Answer shows a coherent logical 2

structure with linkages and fully

sustained lines of reasoning

demonstrated throughout

Answer is partially structured with some 1

linkages and lines of reasoning

Answer has no linkages between points 0

and is unstructured

More than one indicative marking point may be

made within the same comment or explanation

Indicative content

• IP1 – (accurately) weigh/ use a known mass of Allow amount for mass

(hydrated) magnesium sulfate Allow any reasonable stated mass between 1−10g

Allow any reasonable volume between 20−250cm3

• IP2 – (accurately weigh) known mass of

(distilled / deionised) water

• IP3 – use of polystyrene cup (and lid) Allow insulated beaker/calorimeter/styro-foam cup

Do not allow if heating is described

• IP4 – record initial temperature of the water (before adding solid) Do not award recording initial temperature of solid

Addition of water to the solid loses IP4 only

• IP5 – add magnesium sulfate (to the water in the cup) and stir Allow mix/stir the water and the solid

• IP6 - record temperature (at suitable time intervals) / Allow record the lowest or highest temperature

record final temperature reached

Allow use of graphical extrapolation

Do not award if the solution is being heated

Question

Answer Additional Guidance Mark

Number

Example of calculation and Hess cycle (2)

4(b)

MgSO4(s) + 7H2O(l) MgSO4∙7H2O(s)

+aq +aq

∆ H = −63.2 kJ mol−1 ∆ H = +15.7 kJ mol−1

r r

MgSO4(aq)

∆H = ∆H − ∆H = −63.2 − +15.7 = −78.9 (kJ mol−1)

12 3

• rearrangement using Hess’s law (1) ∆rH = (−63.2 − +15.7)

• evaluation of answer with sign (and units) (1) ∆ H = − 78.9 (kJ mol−1)

r

Correct answer with no working scores (2)

If value converted to J mol−1 then both marks can be awarded for

−78 900 J mol−1 but just −78 900 scores M1 only

Allow (1) for (+)78.9 (kJ mol−1)

Allow (1) for – 47.5 (kJ mol−1)

Question

Answer Additional Guidance Mark

Number

(2)

4(c)

• the enthalpy change of hydration of magnesium/Mg (ion)

Do not award just ‘enthalpy change is greater’

will be more exothermic/more negative (1)

Do not allow calcium (ion) enthalpy change of

hydration is more positive/ endothermic

• because the magnesium (ion) is smaller than the calcium (ion) Accept reverse argument

but has the same charge Allow magnesium (ion) has a greater charge density

(so the attraction between the water molecules and the gaseous Allow Mg2+ is smaller than Ca2+

ion is stronger) (1)

Do not award atomic radius

(Total Question 4 = 10 marks)

How to answer it

Hydrated Magnesium Sulfate Study Guide

Edexcel A-Level Chemistry — Energetics

What this question tests

This question assesses your practical design skills (calorimetry experimental procedure), application of Hess's Law cycles with enthalpy changes of solution, and your understanding of ionic radii and charge density on enthalpies of hydration.

Part (a): Designing an Enthalpy of Solution Experiment

6-Mark Extended Response Question

💡 Key Knowledge Required

  • Calorimetry setup for measuring temperature change upon dissolving solids.
  • Need for mass measurements of both solid and liquid (or known volume of water).
  • Thermal insulation to minimize heat loss to the surroundings.

✅ Indicative Content (Mark Scheme Points)

  • IP1: Accurately weigh/use a known mass of hydrated magnesium sulfate (1–10 g).
  • IP2: Accurately weigh/use a known mass or volume of distilled/deionized water (20–250 cm³).
  • IP3: Use a polystyrene cup (or insulated beaker/calorimeter) with a lid.
  • IP4: Record the initial temperature of the water before adding the solid.
  • IP5: Add the magnesium sulfate to the water in the cup and stir.
  • IP6: Record temperature at suitable time intervals or record the final/maximum/minimum temperature reached.

🧠 Exam Technique & Quality of Written Communication

Out of the 6 total marks, 4 marks are awarded for identifying the correct experimental points (indicative content), and 2 marks are awarded for the coherence, logical structure, and sustained reasoning of your written response.

Organize your steps chronologically: preparation → setup → taking initial readings → reaction → final temperature tracking.

❌ Common Errors & Pitfalls

  • Recording the initial temperature of the solid instead of the water (loses IP4).
  • Adding water to the solid rather than the solid to the water (loses IP4).
  • Proposing heating the mixture with a Bunsen burner — enthalpy of solution is measured by dissolution, not combustion or reflux!
Mark Breakdown: 6 marks total (4 marks for physics/chemistry points IP1–IP6; 2 marks for logical structure and narrative flow).

Part (b): Hess's Law Calculation

Calculate Enthalpy of Hydration

📐 Step-by-Step Calculation

Given equations:

1. MgSO₄(s) + aq → MgSO₄(aq)      ΔH = -63.2 kJ mol⁻¹

2. MgSO₄•7H₂O(s) + aq → MgSO₄(aq)      ΔH = +15.7 kJ mol⁻¹

Target equation:

MgSO₄(s) + 7H₂O(l) → MgSO₄•7H₂O(s)      ΔH₁ = ?

  1. Construct Hess Cycle: Both pathways lead to the common aqueous solution MgSO₄(aq) .
  2. Apply Hess's Law: ΔH₁ = ΔH₂ - ΔH₃
  3. Substitute values: (-63.2) - (+15.7) = -78.9 kJ mol⁻¹

✅ Final Answer

-78.9 kJ mol⁻¹

Note: Correct answer with no working scores full marks (2 marks). Watch out for units and correct sign.

❌ Calculation Traps

  • Subtracting the equations in the wrong direction, giving +78.9 kJ mol⁻¹ or -47.5 kJ mol⁻¹ .
  • Forgetting to include the negative sign (a negative sign is vital here as hydration is exothermic).
  • Converting units into joules unnecessarily without clear reason (if converted to J mol⁻¹, must be written as -78900 J mol⁻¹ ).
Mark Breakdown: 2 marks (1 mark for correct algebraic rearrangement/Hess cycle logic; 1 mark for correct evaluation with sign and units).

Part (c): Enthalpy of Hydration Comparison

Magnesium Ions vs. Calcium Ions

💡 Key Knowledge: Charge Density

Enthalpy of hydration depends on the attraction between gaseous ions and polar water molecules. Higher charge density leads to a more exothermic (more negative) enthalpy of hydration.

✅ Examiner-Approved Answer Points

  • Point 1: The enthalpy change of hydration of magnesium ions is more exothermic / more negative than that of calcium ions.
  • Point 2: Because the magnesium ion ( Mg²⁺ ) is smaller than the calcium ion ( Ca²⁺ ), while having the same charge (+2). Therefore, the electrostatic attraction between the Mg²⁺ ion and water molecules is stronger.

❌ Common Misconceptions

  • Saying vaguely that "hydration enthalpy is greater" without specifying more negative/exothermic.
  • Comparing atomic radius instead of ionic radius.
  • Failing to mention that both ions have the same charge (+2), which is why ionic radius becomes the determining factor for charge density.
Mark Breakdown: 2 marks (1 mark for stating magnesium's hydration enthalpy is more exothermic/negative; 1 mark for linking this to smaller ionic radius / higher charge density / stronger attraction to water).

Topics

Physical Chemistry · Core Practicals · Core Practical 8: Determine the enthalpy change of a reaction using Hess’s law · Topic 8: Energetics I · Topic 13: Energetics II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.