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1.5.4Physical Chemistry

Effects of Forces Between Molecules — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 30 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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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.
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Part 1 of 4Boiling Point Trends

Larger molecules
  • Larger molecules As the number of electrons increases, induced dipole–dipole forces get stronger, so boiling points rise down a homologous series.
Branching
  • Branching Branched isomers have a smaller surface area for contact between molecules, so their forces are weaker and boiling points are lower.
Polar molecules
  • Polar molecules Propanone boils higher than butane, which has the same number of electrons, because of additional permanent dipole–dipole forces.
Hydrogen bonding
  • Hydrogen bonding Hydrogen fluoride, water and ammonia have much higher boiling points than the other hydrides in their groups because of hydrogen bonds.
Halogen hydrides
  • Halogen hydrides HCl, HBr and HI rise in boiling point as the number of electrons increases, while HF is highest because of hydrogen bonding.

Part 2 of 4Water

Water
Ice structure
  • Ice structure In ice each water molecule forms four hydrogen bonds in an open tetrahedral lattice, so ice is less dense than liquid water.
High boiling point
  • High boiling point Many hydrogen bonds between water molecules must be broken before it can boil, so it boils at a high temperature for its size.
Surface tension
  • Surface tension Hydrogen bonding between surface molecules pulls them inwards and gives water a high surface tension.
Solvent
  • Solvent Water dissolves many ionic compounds and polar substances because its dipoles attract ions and form hydrogen bonds to solutes.

Part 3 of 4Explaining a Difference in Boiling Point

1
Identify the strongest intermolecular force present in each substance being compared.
2
If the forces are the same, compare the number of electrons or the surface area of the molecules.
3
Explain that stronger forces need more energy to overcome, so the boiling point is higher.
4
Refer to intermolecular forces being overcome, not covalent bonds.
5
Finish by stating clearly which substance has the higher boiling point.

Part 4 of 4Exam Checklist & Tips

1.5.4 – Key Points for the Exam

  • Anomalous Different from the expected trend, as with the boiling points of HF, H₂O and NH₃.
  • Surface area The extent of contact between neighbouring molecules, greater in long chains than in branched ones.
  • Hydrogen bonding A strong dipole-based attraction that gives water its unusual properties.
  • Density
    • Mass per unit volume;
      • ice is less dense than water because of its open structure.

Common Mistakes

  • Claiming that hydrogen bonds are broken when water is converted to steam rather than describing them as intermolecular and overcome.
  • Explaining a boiling point difference using bond strength inside molecules.
  • Saying branched alkanes have more electrons, when it is the surface contact that is reduced.
  • Forgetting that ice floats because its hydrogen-bonded lattice is open and less dense than the liquid.

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