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Intermolecular Forces

Intermolecular Forces. Chapter 11 Part I. States of matter Gas. Gases are compressible with no definite shape nor volume. Particles are in constant rapid motion. Liquid. Liquids have a constant volume (at a constant temperature) but not a constant shape.

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Intermolecular Forces

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  1. Intermolecular Forces Chapter 11 Part I

  2. States of matterGas • Gases are compressible with no definite shape nor volume. • Particles are in constant rapid motion.

  3. Liquid • Liquids have a constant volume (at a constant temperature) but not a constant shape. • Molecules move in relationship to one another.

  4. Solid • Solids have a constant shape and a constant volume. • Molecules vibrate in place.

  5. Bonding versus Attractions • Molecules form stable states by sharing electron in covalent bonds. • This is called intramolecular bonding or bonding within the molecule.

  6. Forces • Molecules may be held together as a liquid or a solid by strong covalent or ionic bonds

  7. Intermolecular Forces: IMF • Attractions (not bonds) between different molecules are called intermolecular forces. • When these forces are “broken” the molecules remain intact. • For example: when water is vaporized, the water molecules remain as water, but the attractions between the molecules are interrupted.

  8. IMF’s summary

  9. Dipole-Dipole • Molecules with a dipole moment (align in a magnetic field) line up so the positive ends are in close proximity to the negative ends.

  10. Dipole-Dipole • In a condensed state such as a liquid, the molecules find the best compromise between attraction and repulsion • Dipole-dipole Forces are typically only 1% as strong as ionic or covalent bonds.

  11. Hydrogen Bonding • The strongest of the dipole-dipole attractions are seen in molecules in which hydrogen is bound to a highly electronegative element such as oxygen, nitrogen or fluorine.

  12. Hydrogen Bonding and Physical Properties • The boiling points of covalent hydrides ranging in group 4A are in the next slide. • Note that there is a steady increase in boiling point with an increase in molar mass of these non-polar tetrahedral hydrides.

  13. Boiling points of Covalent Hydrides

  14. Boiling points of Hydrides from groups 4A, 5A, 6A and 7A • Compare the boiling points of all the groups 4A to 7A and note the difference in the boiling point of the highly electronegative elements relative to their molar mass. • Why? • Hydrogen bonding.

  15. Hydrogen Bonding • These unusually strong hydrogen bonds are due to two factors. • 1. the relatively high electronegative value of the lightest elements in these groups. This leads to especially polar ends. • 2. The small size of the first element in each group allows close approach of the dipoles.

  16. London Dispersion Forces • The weakest of the IMF’s • Is relative to the “squishiness” of the molecule’s electrons. • Larger molecules have more area to move electrons about, creating larger partial charges. • Larger London!!

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