Section 5.5—Intermolecular Forces

1. Section 5.5—Intermolecular Forces

2. Intra- versus Inter-molecular Forces • So far this chapter has been discussing intramolecular forces • Intramolecular forces = forces within the molecule (chemical bonds-ionic and covalent) • Now let’s talk about intermolecular forces • Intermolecular forces = forces between separate molecules

3. Breaking Intramolecular forces • Breaking of intramolecular forces (within the molecule) is a chemical change • 2 H2 + O2 2 H2O • Bonds are broken within the molecules and new bonds are formed to form new molecules

4. Breaking Intermolecular forces • Breaking of intermolecular forces (between separate molecules) is a physical change • Boiling water is breaking the intermolecular forces in liquid water to allow the molecules to separate and be individual gas molecules.

5. Positively charged nucleus - - - - - - - - - - As electrons move, they “gang up” on one side. Negatively charged electron + + + Electrons are fairly evenly dispersed. + London Dispersion Forces This lop-sidedness of electrons creates a partial negative charge in one area and a partial positive charge in another. Electrons move around the nuclei. They could momentarily all “gang up” on one side All molecules have electrons.

6. + - + - London Dispersion Forces • Once the electrons have “ganged up” and created a partial separation of charges, the molecule is now temporarily polar. • The positive area of one temporarily polar molecule can be attracted to the negative area of another molecule.

7. Strength of London Dispersion Forces Electrons can gang-up and cause a non-polar molecule to be temporarily polar The electrons will move again, returning the molecule back to non-polar The polarity was temporary, therefore the molecule cannot always form LDF. London Dispersion Forces are the weakest of the intermolecular forces because molecules can’t form it all the time. They are also called induced dipoles.

8. Strength of London Dispersion Forces All molecules have electrons…all molecules can have London Dispersion Forces The more electrons that gang-up, the larger the partial negative charge. The larger the molecule, the stronger the London Dispersion Forces Larger molecules have more electrons Larger molecules have stronger London Dispersion Forces than smaller molecules.

9. + - + - Dipole Forces • Polar molecules have permanent partial separation of charge. • The positive area of one polar molecule can be attracted to the negative area of another molecule.

10. Strength of Dipole Forces Polar molecules always have a partial separation of charge. Polar molecules always have the ability to form attractions with opposite charges Dipole forces are stronger than London Dispersion Forces

11. Hydrogen Bonding – Strongest IMF Why is it the strongest? • Hydrogen is a small atom and when its one electron attracts to one of 3 HIGH electronegative elements (F, O or N) that it is bonded to, its exposed nucleus can easily attract surrounding molecules! NOTE: This ONLY happens when Hydrogen bonds with Nitrogen, Oxygen or Fluorine (Remember as FON (phone )

12. N N H H H H Hydrogen Bond Nitrogen – one of the elements required to have a Hydrogen bond Intramolecular bond within the molecule Hydrogen with “exposed” proton Hydrogen bond –intermolecular A lone pair of electrons that will attract strongly with the H on the OTHER molecule.