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Intermolecular forces

Questions. Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? The answers have to do with …. Intermolecular forces. Generall y, inter molecular forces are much weaker than intra molecular forces.

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Intermolecular forces

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  1. Questions • Why do some solids dissolve in water but others do not? • Why are some substances gases at room temperature, but others are liquid or solid? • The answers have to do with … Intermolecular forces

  2. Generally, intermolecular forces are much weaker than intramolecular forces. Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. “Measure” of intermolecular force - boiling point - melting point - Surface tension - evaporation rate

  3. Types Of Intermolecular Forces: • There are three main types of IMFs • London Dispersion Forces (IMF) • Dipole-Dipole Forces (DD) • Hydrogen Bonding Forces (HB)

  4. London Dispersion Forces • The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors. • London forces increase with the size of the molecules. Fritz London 1900-1954

  5. London Dispersion Forces • Non-polar molecules do not have dipoles like polar molecules. How, then, can non-polar compounds form solids or liquids? • London forces are attractive forces caused due to small temporary dipoles that exist in non-polar molecules • Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical • The resulting tiny dipoles cause attractions between molecules

  6. London forces Induced dipole: Instantaneous dipole: Eventually electrons shift so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

  7. London Dispersion Forces

  8. Orientation of PolarMolecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules

  9. Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond in one molecule and an electronegative O, N, or F atom.

  10. Hydrogen Bonding Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen Hydrogen bonding between ammonia and water

  11. Maximum Density 40C Ice is less dense than water Water is a Unique Substance Density of Water

  12. The hexagonal structure of ice

  13. The H-bonding abilitiy of the water molecule

  14. Forces & Properties • Bigger size = Stronger force = higher melting and boiling point because more energy is needed to separate the molecules Ex. H2 Vs. Cl2 • Heavier the molecule = stronger force • Ex. F2 Vs. Cl2

  15. Molecular shape and boiling point Longer molecule = stronger force C5H12 (straight structure) Vs. C5H8 (branched structure)

  16. Properties of Liquids Surface tensionis the amount of energy required to stretch or increase the surface of a liquid by a unit area. (polar molecules, liquid metals). Strong intermolecular forces = High surface tension (liquid beads up)

  17. Adhesion Cohesion Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules

  18. stronger cohesive forces adhesive forces Shape of water or mercury meniscus in glass Figure 12.20

  19. Like Dissolves Like • This is why oil and water will not mix! Oil is nonpolar, and water is polar. • The two will repel each other, and so you can not dissolve one in the other • Polar substances dissolves Polar substances • Non-Polar Dissolves Non-polar sbubstances

  20. Bond Polarity • “Like Dissolves Like” • Polar dissolves Polar • Nonpolar dissolves Nonpolar

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