AQA A-Level Chemistry Paper 3, June 2018: Question 33

Lattice Enthalpies: Born–Haber Cycles vs Perfect Ionic Model

📌 What this question tests

This question assesses your ability to relate theoretical calculations of lattice enthalpy to experimental values, specifically understanding:

The assumptions behind the perfect ionic model (purely spherical ions with point charges and non-directional electrostatic forces).
Why experimental lattice enthalpies from Born–Haber cycles deviate from calculated theoretical values (presence of covalent character).
Factors that maximise polarization: high charge density cation (small ionic radius) and easily polarisable anion (large ionic radius).

Question 33

Comparing Experimental vs Theoretical Lattice Enthalpy

Multiple Choice (1 Mark)

✅ Correct Answer

D — LiI

Mark Scheme Breakdown:
1 mark for selecting option D.

The compound with the greatest difference between the Born–Haber experimental value and the theoretical calculation is the one with the greatest degree of covalent character (i.e. greatest distortion of the electron cloud).

💡 Key Knowledge

Theoretical Model: Assumes a 100% ionic lattice of perfectly spherical, non-distorted ions.
Born–Haber Value: Experimental value that reflects real bonding, including any extra bonding energy from covalent character.
Polarisation of Anions: Occurs when a cation distorts the electron cloud of an anion toward itself:
- Cation needs high charge density (small ionic radius, high positive charge).
- Anion needs high polarisability (large ionic radius, outer electrons held less tightly).

📐 Step-by-Step Comparative Analysis

Compound	Cation Size & Polarising Power	Anion Size & Polarisability	Resulting Bonding Character
A: CsF	Cs⁺ is very large → very low polarising power	F⁻ is very small → very difficult to distort	Closest to the perfect ionic model (least difference)
B: CsI	Cs⁺ is very large → very low polarising power	I⁻ is large → polarisable	Low covalent character due to weak cation power
C: LiF	Li⁺ is very small → strong polarising power	F⁻ is very small → tightly held electrons, resistant to distortion	Slight polarization, mostly purely ionic
D: LiI	Li⁺ is very small → highest polarising power	I⁻ is very large → most polarisable electron cloud	Highest covalent character → Greatest discrepancy

🧠 Exam Technique: Two-Step Selection Rule

Step 1: Identify what the question asks: "Greatest percentage difference" means maximum covalent character (most departure from the ionic model).
Step 2: Find the extreme cation and anion:
- Group 1 cations: Li⁺ (period 2) is much smaller than Cs⁺ (period 6) → choose Li⁺.
- Group 7 anions: I⁻ (period 5) is much larger than F⁻ (period 2) → choose I⁻.
Conclusion: Pairing the smallest cation with the largest anion produces the most distorted, polarised ionic bond: LiI.

❌ Common Errors & Pitfalls

Confusing greatest difference with closest agreement: CsF has the closest agreement (~0% difference) because it is virtually 100% ionic. Students picking CsF misread the question as asking for the best match to the model.
Confusing lattice enthalpy magnitude with percentage difference: LiF has the most exothermic (highest numerical) lattice enthalpy because both ions are small, but its theoretical and experimental values match very closely because neither ion is readily polarised.
Mixing up polarisation criteria: Forgetting that cations must be small and anions must be large to cause distortion.

AQA A-Level Chemistry Paper 3, June 2018: Question 33

Question

Mark scheme

How to answer it