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Intermolecular Forces and Physical Properties of Molecular Substances

This reference discusses the origins of different intermolecular forces and their effects on the physical properties of molecular substances. It covers dispersion forces, dipole forces, and hydrogen bonding, as well as their impact on boiling points and solubility.

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Intermolecular Forces and Physical Properties of Molecular Substances

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  1. Chemistry 1011 TOPIC Physical Properties of Matter TEXT REFERENCE Masterton and Hurley Chapter 9 Chemistry 1011 Slot 5

  2. 9.1 Molecular Substances; Intermolecular Forces YOU ARE EXPECTED TO BE ABLE TO: • Identify and describe the origins of different types of intermolecular forces. • Identify the intermolecular forces in different molecular substances and evaluate their effect on the physical properties of the material. Chemistry 1011 Slot 5

  3. General Properties of Molecular Substances • Nonconductors of electricity • Most are insoluble in water (but see later – polar molecules will dissolve in water) • Low melting and boiling points • Molecules are relatively easy to separate from each other because the forces between the molecules are weak Chemistry 1011 Slot 5

  4. Types of Forces BETWEEN Molecules (Intermolecular Forces) • Dispersion (London) Forces • Dipole Forces • Hydrogen “Bonding” – a special case of dipole forces Chemistry 1011 Slot 5

  5. Dispersion (London) Forces • All molecules have dispersion forces • These result from temporary dipoles formed in adjacent molecules • Their strength depends upon how readily electrons are dispersed • Forces increase with: molecular size, molar mass • This results in increasing boiling points of molecular substances as molar mass increases ( F2 < Cl2 < Br2 < I2 ) Chemistry 1011 Slot 5

  6. Effect of Molar Mass on the Boiling Points of Molecular Substances As molar mass increases, dispersion forces become stronger, and boiling points increase Chemistry 1011 Slot 5

  7. + - + - dipole forces Dipole Forces • Result from the interaction between polar molecules • They are electrical attractive forces between + end of one polar molecule and - end of adjacent molecule Chemistry 1011 Slot 5

  8. Effects of Dipole Forces • Dipole forces are quite weak, but are in addition to dispersion forces • Dipole forces will produce an increase in the melting and boiling points • Compare the boiling points of: • NO (molar mass 30)…b.pt. –151oC, and • O2 (molar mass 32)…b.pt. –183oC, and • N2 (molar mass 28)…b.pt. –196oC Chemistry 1011 Slot 5

  9. Dipole Forces in ICl Chemistry 1011 Slot 5

  10. Hydrogen “Bonding” • A unique kind of dipole force that occurs when hydrogen is bonded to a small highly electronegative element E.g. HF, H2O, NH3 • This is a strong force that results from • A large difference in electronegativity between H and F, O, and N • The small size of the hydrogen atom Chemistry 1011 Slot 5

  11. Effect of Hydrogen Bonding on Boiling Point Chemistry 1011 Slot 5

  12. Hydrogen Bonding in Water • Water has many unusual properties in addition to its high boiling point • Solvent for other polar substances and for ionic compounds • High surface tension • Expansion on freezing – ice has an open structure and so the density of ice is less that for water Chemistry 1011 Slot 5

  13. The Structure of Ice Chemistry 1011 Slot 5

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