2.2.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Physical Properties — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 19 of 53 Unit quiz
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 4London Forces and Boiling Point
Boiling requires enough energy to overcome the intermolecular forces, so stronger forces mean a higher boiling point.
More electrons
- More electrons Molecules with more electrons form larger temporary dipoles more often, so London forces and boiling points increase.
Noble gases
- Noble gases Boiling points rise from helium at about -269 °C to xenon at about -108 °C because atoms contain more electrons.
Chain length
- Chain length Longer alkane chains have more electrons and more surface contact with neighbours, so their boiling points rise.
Branching
- Branching Branched isomers are more compact with less surface contact, so London forces are weaker and boiling points are lower.
Same electrons
- Same electrons Isomers share the same number of electrons, so differences in boiling point come from surface area and shape.
Section 2 of 4Hydrogen Bonding Raises Boiling Points
Hydrogen Bonding Raises Boiling Points
Alcohols versus alkanes
- Alcohols versus alkanes An O-H group allows hydrogen bonding, which is much stronger than the London forces between alkane molecules of similar size.
Example
- Example Propane boils at about -42 °C but propan-1-ol, with a similar electron number, boils at about 97 °C because of hydrogen bonds.
Hydrogen fluoride
- Hydrogen fluoride HF boils far higher than HCl because its very polar H-F bond and small fluorine lone pairs allow hydrogen bonds.
Other hydrogen halides
- Other hydrogen halides HCl, HBr and HI have rising boiling points because the larger molecules have more electrons and stronger London forces.
| Compound | Main intermolecular force | Approximate boiling point |
|---|---|---|
| Propane, C₃H₈ | London forces | -42 °C |
| Propan-1-ol, C₃H₇OH | Hydrogen bonds plus London forces | 97 °C |
| HCl | Permanent dipole-dipole and London | -85 °C |
| HF | Hydrogen bonds | 20 °C |
Section 3 of 4Reasoning Through a Comparison
1
Identify the type of each substance: simple molecular, ionic or giant covalent, because only simple molecules rely on intermolecular forces.
2
List every intermolecular force present in each substance, starting with London forces which are always there.
3
Check for N-H, O-H or H-F bonds that allow hydrogen bonding, then for polar molecules with permanent dipoles.
4
If the forces are the same type, compare the number of electrons and the surface area available for contact.
5
State that more energy is needed to overcome the stronger forces, so that substance has the higher boiling point.
Section 4 of 4Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Boiling point The temperature at which a liquid's particles gain enough energy to escape as a gas throughout the liquid.
- London forces Induced dipole-dipole forces whose strength increases with the number of electrons in a molecule.
- Isomers Compounds with the same molecular formula but different structures.
- Surface area The extent of contact between neighbouring molecules, reduced by branching.
Common Mistakes
- Saying a branched isomer has fewer electrons;
- isomers have the same number, but a smaller surface area for contact.
- Referring to breaking covalent bonds when explaining why boiling points differ between molecular substances.
- Forgetting that alcohols also have London forces, which still increase with chain length.
- Assuming hydrogen bonding in every hydrogen-containing compound; it needs H bonded directly to N, O or F.
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