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Polarity and Intermolecular Forces

Polarity and Intermolecular Forces. Review. We know how to draw Lewis structures for simple molecules and polyatomic ions. We also know how to predict the 3-D geometry of these molecules and ions, if we apply the VSEPR Theory.

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Polarity and Intermolecular Forces

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  1. Polarity and Intermolecular Forces

  2. Review • We know how to draw Lewis structures for simple molecules and polyatomic ions. • We also know how to predict the 3-D geometry of these molecules and ions, if we apply the VSEPR Theory. • Electronegativity (EN) is an atom's tendency to attract electrons in chemical bonds. • EN increases to the right and up on the periodic table, excluding the noble gases.

  3. Bond Polarity • When two nonmetal atoms bond, they share electrons. • They may or may not share electrons evenly. • Consider the following molecules: • F2 • HF

  4. Bond Polarity • H vs. F • The EN of H is 2.2. • The EN of F is 4.0. • In F2, both atoms pull with equal strength on the bonding e-. • The e- are shared evenly between them. • In HF, the F atom pulls harder than the H atom. • The e- are drawn more toward the F atom. H EN = 2.2 F EN = 4.0 F F F H

  5. F F F H Bond Polarity • EN is same for both atoms. • e- density is spread evenly around molecule. • Highest e- density occurs between atoms. • Bond is nonpolar. • EN is much higher for F than for H. • e- density is drawn toward F side. • F atom acquires partial negative charge. • H atom acquires partial positive charge. • Bond is polar.

  6. Bond Polarity • In general, a covalent bond is: • polar if it occurs between two different atoms. • nonpolar if it occurs between two identical atoms.

  7. Arrow points toward more EN atom. H F Dipole Moments • Dipole Moment - a measure of the polarity of a bond. • Is often represented by a special arrow.

  8. Polarity of Diatomic Molecules • Diatomic Molecules - molecules made of only two atoms. • If atoms are the same, molecule is nonpolar. • If atoms are diff., molecule is polar. • NOTE: Polar does not mean charged. • Is Cl2 polar or nonpolar? • Is CO polar or nonpolar?

  9. Molecules With 3 or More Atoms • A molecule with 3 or more atoms is: • Polar if its central atom has lone pairs OR • If the outer atoms are not all the same. • Nonpolar if its central atom has no lone pairs AND • All the outer atoms are identical.

  10. CO2 vs. H2O • Consider the Lewis structure of CO2: This molecule is nonpolar.

  11. CO2 vs. H2O • Consider the Lewis structure of H2O: This molecule is polar.

  12. CH4 vs. CH3Cl • Neither CH4 nor CH3Cl has any lone pairs on the central carbon atom. • Is CH4 polar or nonpolar? • Is CH3Cl polar or nonpolar? H H H C H H C Cl H H

  13. “Like Dissolves Like” • Polar molecules mix with each other. • Nonpolar molecules mix with each other. • Polar and nonpolar molecules do not easily mix.

  14. Amphipathic Molecules • Amphipathic - has a hydrophobic region and a hydrophilic region. • Hydrophobic - “water-fearing” • Nonpolar. • Hydrophilic - “water-loving” • Polar or charged. • Dish detergents contain amphipathic molecules. • Why?

  15. Grease and Water Don't Mix!

  16. Amphipathic Molecules Hydrophilic head Hydrophobic tails

  17. Amphipathic Molecules

  18. Intermolecular Forces • Intermolecular force - a force between two molecules that does not result from chemical bonding. • Dipole-dipole interaction. • Hydrogen bonding. • London force.

  19. Dipole-Dipole Interactions • Dipole - polar molecule. • Like magnets, except poles are + and  - instead of N and S. • Polar molecules generally have higher melting and boiling points than similar nonpolar molecules. • EXAMPLE: O2 (nonpolar) boils at -183ºC. • EXAMPLE: NO (polar) boils at -152ºC. • NO has a higher boiling point due to its polarity. • Still far below the boiling point of any ionic cmpd.

  20. Dipole-Dipole Interactions

  21. Hydrogen Bonding • Hydrogen bond - a stronger form of dipole-dipole interaction. • Occurs in molecules that have H atoms bonded to O, N, or F atoms. • The small size of the H atom allows these molecules to get closer together. • Closer together = stronger forces. • EXAMPLE: H2O has a boiling point of 100ºC. • EXAMPLE: H2S has a boiling point of -60ºC. • The b.p. of H2O is higher b/c of hydrogen bonding.

  22. Hydrogen Bonding

  23. London Force (Dispersion) • London force - attraction between temporary dipoles. • e- move randomly around molecules. • Nonpolar molecules become temporarily polar. • Allows for very weak attractions between nonpolar molecules. • Named for Fritz London.

  24. London Forces - +

  25. London Forces • The more e- a molecule has, the greater its London forces are. • Large molecules tend to have higher melting/boiling points than small molecules. • London forces apply to all molecules.

  26. London Forces

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