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Intermolecular Forces

Intermolecular Forces. In previous lessons you have already learned the basics of intermolecular forces. Compounds made of charged atoms have higher intermolecular forces. Ionic > Polar Covalent > Nonpolar Covalent. Intermolecular forces affect certain properties.

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Intermolecular Forces

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  1. Intermolecular Forces

  2. In previous lessons you have already learned the basics of intermolecular forces. Compounds made of charged atoms have higher intermolecular forces Ionic > Polar Covalent > Nonpolar Covalent Intermolecular forces affect certain properties Higher intermolecular attraction produces higher melting and boiling points and tend to be more brittle. You also know how to identify polar bonds based on electronegativity differences. The bonds have a dipole with one side partially positive and the other partially negative. These are drawn like the figure below.

  3. Molecules containing polar bonds may be nonpolar! You have to look at the molecular geometry to find out. To see why, we will look at two compounds with polar bonds, CF4 and CH3F. F H C C F H F H F F Every bond in these would be considered polar, C-H = .4 C-F = 1.5. However, they are not EQUALLY polar, C-F is much more polar.

  4. F H In the case of CF4, the dipoles are distributed symmetrically around the central atom and with cancel each other out. This molecule is NONpolar. C C F H F H In CH3F, because the dipoles are not symmetric (the C-F dipole is much stronger), the molecule is polar. F F

  5. N H H H In NH3, the N-H bonds are all equally polar. They are not distributed symmetrically because of the pyramidal geometry (they all point down) so the molecule is polar. So in other words, to decide if an entire molecule is polar, you must find the geometry and see if the dipoles are symmetric.

  6. Intermolecular forces and Molecule Polarity Only if an entire molecule is polar will it have the higher intermolecular forces that lead to high boiling and melting points, etc. If molecules are polar, they are said to have dipole-dipole forces.

  7. Hydrogen Bonding Hydrogen Bonding is an especially strong type of dipole-dipole force that occurs in molecules with H-F, H-O, and H-N bonds. These bonds have large electronegativity differences and therefore have strong dipoles. Molecules with hydrogen bonding have significantly higher intermolecular forces than would be predicted otherwise.

  8. Induced Dipole If a polar and a nonpolar molecule are mixed together, the polar molecule can cause a weak dipole in the nonpolar molecule. This is called an induced dipole. These are not as strong as dipole-dipole forces.

  9. London Dispersion Forces Even if there are only nonpolar molecules, there are still some intermolecular forces. These are caused when the electrons end up on one side of an atom randomly. This produces a temporary dipole and leads to weak intermolecular forces. The more electrons are in the atoms, the stronger the London Dispersion forces. This is because there is a greater chance to have random dipoles.

  10. Questions to identify Intermolecular Forces Does it have H-F, H-O, or H-N bonds? Yes – Hydrogen Bonding No – Are there polar bonds the molecule? No – London Dispersion Yes – Are the symmetric? No – Dipole-Dipole Yes – London Dispersion

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