Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.14 Experimental vs theoretical lattice energy-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.14 Experimental vs theoretical lattice energy- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.14 Experimental vs theoretical lattice energy- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
12.14 Lattice Energy and Evidence for Covalent Character
The lattice energy of an ionic compound can be determined experimentally using a Born–Haber cycle, or calculated theoretically using electrostatic models that assume purely ionic bonding. Comparing these two values provides insight into whether the bonding is purely ionic or has some covalent character.
Key Idea
A difference between experimental and theoretical lattice energy indicates the presence of covalent bonding.
Theoretical Lattice Energy
- Calculated using electrostatic theory.
- Assumes ions are spherical and interact via purely ionic (electrostatic) forces.
- Does not account for covalent interactions.
Experimental Lattice Energy
- Determined using a Born–Haber cycle.
- Based on real measured enthalpy changes.
- Includes all interactions present in the compound.
Comparison and Interpretation
- If experimental ≈ theoretical → bonding is mainly ionic.
- If experimental is more exothermic than theoretical → covalent character is present.
Reason for Difference
- Small, highly charged cations can polarise large anions.
- This distorts the electron cloud of the anion.
- Electron density becomes shared between ions, introducing covalent character.
- This extra attraction makes the lattice energy more exothermic than predicted.
Fajans’ Rules
- Small cation → high polarising power.
- Large anion → easily polarised.
- High charge → stronger polarisation.
- Greater polarisation → greater covalent character.
Key Features
- Theoretical values assume purely ionic bonding.
- Experimental values include real bonding effects.
- A large difference indicates significant covalent character.
- Polarisation of ions explains the deviation.
Example 1:
Explain why \( \mathrm{AgCl} \) shows a large difference between experimental and theoretical lattice energy values.
▶️ Answer/Explanation
The \( \mathrm{Ag^+} \) ion is relatively small and has a high charge density, giving it strong polarising power.
The \( \mathrm{Cl^-} \) ion is large and easily polarised.
This leads to distortion of the electron cloud and partial sharing of electrons.
Therefore, covalent character is introduced, making the experimental lattice energy more exothermic than the theoretical value.
Example 2:
Explain why \( \mathrm{NaCl} \) shows little difference between experimental and theoretical lattice energy values.
▶️ Answer/Explanation
The \( \mathrm{Na^+} \) ion has relatively low polarising power due to its larger size and lower charge density.
The \( \mathrm{Cl^-} \) ion is less strongly polarised.
Therefore, the bonding remains predominantly ionic.
As a result, the experimental and theoretical lattice energy values are very similar.