4.3.3Unit 4. Rates, Equilibria and Further Organic Chemistry
Polarisation — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 25 of 56 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 4Distorting an Ion
A small, highly charged cation pulls on the electron cloud of a neighbouring anion and pulls it out of shape.
Polarising power
- Polarising power A cation with a high charge and a small radius has a high charge density,
- so it strongly attracts the outer electrons of nearby anions.
Polarisability
- Polarisability A large anion with a diffuse electron cloud is easily distorted because its outer electrons are far from the nucleus and weakly held.
Resulting distortion
- Resulting distortion The electron cloud of the anion is drawn toward the cation, so electron density builds up in the region between the two nuclei.
Covalent character
- Covalent character Shared electron density between the ions means the bond is no longer purely ionic but has partial covalent character.
| Factor | Increases polarisation | Example |
|---|---|---|
| Cation charge | Higher charge | Al³⁺ polarises more than Na⁺ |
| Cation radius | Smaller radius | Li⁺ polarises more than K⁺ |
| Anion size | Larger anion | I⁻ is distorted more than F⁻ |
| Anion charge | Higher charge | S²⁻ is distorted more than Cl⁻ |
Part 2 of 4Evidence from Lattice Energies
The size of the effect is found by comparing two values for the same lattice energy.
1
Calculate a theoretical lattice energy by assuming the compound is made of perfect, spherical ions held only by electrostatic attraction.
2
Find the experimental lattice energy from a Born–Haber cycle, which uses measured enthalpy changes and so reflects the real bonding.
3
Compare the two values: when they agree closely the perfect ionic model is a good description of the compound.
4
When the experimental value is more exothermic than the theoretical one, extra attraction from covalent character is strengthening the lattice.
5
The larger the gap between the two values, the greater the degree of covalent character in the compound.
Part 3 of 4Trends Across Compounds
Trends Across Compounds
Sodium chloride
- Sodium chloride Agreement between the two values is very close because Na⁺ is large for its charge and Cl⁻ is fairly hard to distort.
Silver halides
- Silver halides AgI shows a clear difference because the iodide ion is large and easily distorted by the silver ion.
Down a halide series
- Down a halide series For a given metal, covalent character grows from fluoride to iodide as anion size and polarisability increase.
Across period 3 chlorides
- Across period 3 chlorides NaCl is ionic but AlCl₃ behaves covalently because Al³⁺ has a very high charge density.
Part 4 of 4Exam Checklist & Tips
4.3.3 – Key Points for the Exam
- Polarisation Distortion of the electron cloud of an anion by a neighbouring cation.
- Polarising power The ability of a cation to distort an anion, increased by high charge and small radius.
- Polarisability How easily the electron cloud of an ion is distorted.
- Theoretical lattice energy Value calculated for a perfectly ionic model with spherical ions.
Common Mistakes
- Saying a large cation is strongly polarising; polarising power rises as the cation gets smaller.
- Forgetting that the anion must be large or highly charged to be easily polarised.
- Describing the bond as fully covalent rather than ionic with some covalent character.
- Mixing up the direction of the difference: experimental values are more exothermic than theoretical ones when covalent character exists.
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