Edexcel A-Level Chemistry Paper 1, June 2025: Question 7

✅ Correct Answer

C: the energy change when 1 mol of an ionic substance is formed from its gaseous ions

💡 Key Knowledge

• Lattice Formation Energy: Always exothermic (negative ΔH). Written as: Na⁺(g) + Cl⁻(g) → NaCl(s) .
• "Energy Change": Using the neutral term "energy change" accommodates both formation (exothermic) and dissociation (endothermic) conventions, whereas "energy absorbed" strictly means endothermic.
• Starting species must be gaseous ions, not standard elements.

❌ Common Distractors & Errors

• Confusing with Δ_fH (Options B & D): "Formed from its elements in standard states" defines standard enthalpy of formation, not lattice energy.
• Assuming lattice energy must be positive (Option A): Lattice energy of formation releases energy; it is not "energy absorbed".

🧠 Exam Technique

Read definitions by checking three criteria:

1. Amount: Exactly 1 mol of product solid.
2. Starting state: Gaseous ions ( M⁺(g) and X⁻(g) ).
3. Process type: Energy change / released upon formation.

✅ Correct Answer

A: NaCl

📐 Step-by-Step Comparison

1. Check charges: All four salts (NaCl, NaI, KCl, KI) consist of 1+ cations and 1- anions. Charge magnitude is identical.
2. Compare cation radii: Na⁺ has fewer electron shells than K⁺, so radius of Na⁺ < K⁺ .
3. Compare anion radii: Cl⁻ has fewer electron shells than I⁻, so radius of Cl⁻ < I⁻ .
4. Interionic distance: NaCl has the shortest sum of ionic radii ( r⁺ + r⁻ ).
5. Conclusion: Smallest ionic radii result in the strongest electrostatic attraction, giving the most exothermic (most negative) lattice energy.

💡 Key Knowledge

Lattice energy is governed by Coulomb's Law:

Lattice Energy ∝ (q₁ × q₂) / (r⁺ + r⁻)

Smaller ionic separation produces stronger attraction and a more exothermic value.

❌ Common Errors

Picking KI (Option D) because it is the "heaviest" or has the highest relative formula mass. Remember: lattice energy depends on charge density and ionic radius, not molar mass.

✅ Correct Answer

B: NaI

💡 Key Knowledge: Polarisation & Fajans' Rules

• Theoretical lattice energy: Calculated using the purely ionic electrostatic model (assuming perfectly spherical ions).
• Experimental lattice energy: Determined via the Born-Haber cycle.
• Discrepancy: A large difference occurs when the bond possesses significant covalent character due to polarisation of the electron cloud.
• Conditions for maximum polarisation:
  • Small, highly charged cation = high charge density / high polarising power ( Na⁺ > K⁺ ).
  • Large, highly charged anion = easily distorted / highly polarisable ( I⁻ > Cl⁻ ).

🧠 Exam Strategy: Two-Way Matrix

Set up a quick mental grid to find maximum polarisation:

• Cation polarising power: Na⁺ (radius = 102 pm) > K⁺ (radius = 138 pm). Na⁺ polarises more strongly.
• Anion polarisability: I⁻ (radius = 220 pm) > Cl⁻ (radius = 181 pm). I⁻ is more easily polarised.
• Combining strongest polarising cation (Na⁺) with most polarisable anion (I⁻) gives NaI.

❌ Examiner Pitfalls

• Selecting NaCl (Option A): NaCl has the strongest purely ionic bonding, meaning theoretical and experimental values agree closely (< 1% difference).
• Selecting KI (Option D): While I⁻ is easily polarised, K⁺ has lower charge density than Na⁺, leading to less cation-induced distortion.