2.2.7Additional lessons
Molecular Polarity — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 28 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6From Bond Dipoles to Molecular Dipoles
Molecular polarity depends on both polar bonds and the three-dimensional shape.
Vector sum
- Vector sum Each polar bond has a dipole with size and direction, and the molecular dipole is their vector sum.
Cancellation
- Cancellation If bond dipoles of equal size point in opposite directions, they cancel and the molecule is non-polar.
No cancellation
- No cancellation If the dipoles do not cancel, one side of the molecule is slightly negative and the other slightly positive, so it is polar.
Shape matters
- Shape matters A molecule with polar bonds can still be non-polar if its shape is symmetrical.
Section 2 of 6Testing Common Molecules
Testing Common Molecules
Non-polar with polar bonds
- Non-polar with polar bonds CO₂ (linear), BF₃ (trigonal planar), CH₄ and CCl₄ (tetrahedral) have symmetrical shapes so their dipoles cancel.
Polar molecules
- Polar molecules H₂O (bent), NH₃ (pyramidal), HCl (diatomic) and CH₃Cl (tetrahedral but unsymmetrical) have dipoles that do not cancel.
Diatomic elements
- Diatomic elements Molecules such as O₂ and Cl₂ have identical atoms and no bond polarity, so they are non-polar.
Lone pairs
- Lone pairs Lone pairs on the central atom usually break the symmetry, as seen in water and ammonia.
| Molecule | Shape | Bond dipoles cancel? | Polarity |
|---|---|---|---|
| CO₂ | linear | yes | non-polar |
| CCl₄ | tetrahedral | yes | non-polar |
| CH₃Cl | tetrahedral | no | polar |
| H₂O | bent | no | polar |
| NH₃ | pyramidal | no | polar |
Section 3 of 6Deciding Polarity
1
Draw the Lewis formula and work out the shape using VSEPR.
2
Use electronegativity values to decide whether each bond is polar and which way each dipole points.
3
Draw the dipoles as arrows on the three-dimensional shape.
4
Check whether the arrows cancel by symmetry: identical polar bonds in a symmetrical shape cancel.
5
State clearly that the molecule is polar or non-polar and justify with shape and dipoles.
Section 4 of 6Evidence and Effects
Electric field
- Electric field A stream of a polar liquid is deflected towards a charged rod, whereas a non-polar liquid is not.
Solubility
- Solubility Polar molecules dissolve well in polar solvents such as water;
- non-polar substances dissolve in non-polar solvents like hexane.
Boiling point
- Boiling point Polar molecules attract each other by dipole-dipole forces, which usually raises boiling points.
Mixing
- Mixing Oil is non-polar and water is polar, so they separate into two layers.
Section 5 of 6Worked Example
Question Explain why CCl₄ is non-polar although each C–Cl bond is polar.
- Chlorine (3.2) is more electronegative than carbon (2.6), so each C–Cl bond has a dipole pointing towards Cl.
- CCl₄ has four identical bonds and no lone pairs, so the shape is tetrahedral and symmetrical.
- The four equal bond dipoles point in directions that cancel, so the molecule has no overall dipole.
Answer CCl₄ is non-polar because the symmetrical tetrahedral shape makes the four bond dipoles cancel.
Section 6 of 6Exam Checklist & Tips
2.2.7 – Key Points for the Exam
- Molecular dipole The overall separation of charge in a molecule found by adding all bond dipoles.
- Polar molecule A molecule with an overall dipole because bond dipoles do not cancel.
- Symmetry An arrangement in which identical parts are placed so that effects cancel.
- Non-polar molecule A molecule with no overall dipole.
Common Mistakes
- Assuming every molecule with polar bonds is polar.
- Forgetting to check the shape before deciding on polarity.
- Thinking lone pairs have no effect on polarity.
- Describing CH₄ as polar because C and H differ in electronegativity.
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