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

5. 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.

7. 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. + - + -

8. Recall: How to determine bond type
Find the electronegativies of the two atoms in the bond
Find the absolute value of the difference of their values
If the difference is 0.4 or less, it’s a non- polar covalent bond
If the difference is greater than 0.4 but less than 1.9, it’s a polar covalent bond
If the difference is greater than 1.9, it’s an ionic bond

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

12. 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 )

13. Hydrogen Bond N H
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. N H

15. 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. +
Positively charged nucleus -
Negatively charged electron + - + -
Electrons are fairly evenly dispersed. + -
As electrons move, they “gang up” on one side.

16. 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. + - + -

17. 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.

19. 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.