Edexcel A-Level Chemistry Paper 1, November 2020: Question 4

✅ Correct Answer

C (magnesium sulfate)

💡 Key Knowledge

• The solubility of Group 2 sulfates decreases down the group (from magnesium down to barium).
• Magnesium sulfate (MgSO₄) is the most soluble among the options provided (barium, calcium, magnesium, and strontium sulfates).

✅ Correct Answer

B (-157 kJ mol⁻¹)

📐 Step-by-Step Calculation

1. Write the Hess's Law route: ΔH_sol = ΣΔH_hydration - ΔH_lattice
2. Identify values from the diagram:
   Lattice energy of MgCl₂(s) = -2526 kJ mol⁻¹
   Hydration enthalpy of Mg²⁺(aq) = -1921 kJ mol⁻¹
   Hydration enthalpy of Cl⁻(aq) = -381 kJ mol⁻¹
3. Account for stoichiometry: There are 2 moles of chloride ions, so multiply the Cl⁻ hydration enthalpy by 2:
   2 × (-381) = -762 kJ mol⁻¹
4. Calculate total hydration enthalpy: (-1921) + (-762) = -2683 kJ mol⁻¹
5. Apply the cycle equation: ΔH_sol = (-2683) - (-2526) = -157 kJ mol⁻¹

❌ Common Calculation Traps

• Forgetting stoichiometry: Forgetting to multiply the Cl⁻ hydration enthalpy by 2 leads to distractor C (+224).
• Sign errors / wrong direction: Using the cycle backwards results in positive values like +157 (Distractor A) or +224.

💡 Key Knowledge (Indicative Content)

• 2-methylpentane (non-polar): Cannot form hydrogen bonds with water because it lacks sufficiently electronegative atoms attached to hydrogen.
• 2-methylpentane in hexane: Soluble because both are non-polar molecules sharing similar London (dispersion) forces of comparable strength/size. Resulting forces in the mixture match those in pure liquids.
• Potassium bromide (ionic): Soluble in water because ion-dipole bonds form and ions are effectively hydrated.
• Enthalpy compensation: The energy released during hydration of K⁺ and Br⁻ ions is sufficient to compensate for the energy needed to break the ionic lattice.
• Potassium bromide in hexane: Insoluble because any London forces forming between the ionic lattice and non-polar hexane are vastly too weak to overcome strong electrostatic forces holding the ionic lattice together.

🧠 Exam Technique & Mark Structure (6 Marks Total)

This is a Levels of Response question evaluated across two criteria:

• Indicative Content (IC - up to 4 marks): Awarded based on how many correct chemical points (bullet points above) you successfully make.
• Lines of Reasoning (RoR - up to 2 marks): Awarded for a coherent, logical structure linking the properties of both compounds in both solvents with clear terminology (e.g. explicitly mentioning intermolecular forces, polarity, and hydration).