Intermolecular Forces

Van der waals forces

1. Type of force of attraction that’s caused by instantaneous dipoles and acts between all atoms and molecules

2. E.g if helium electrons happen to be on one side, a temporary dipole is created

3. This can induce a dipole in neighbouring atom, causing the 2 atoms to be temporarily attracted to each other (until the electrons move again)

Van der waal force diagram

Strength of van der forces increases down a group

1. Because atomic mass increases

2. So there is more electrons

3. So strength of temporary dipoles increase

4. Longer chain molecules have more interaction and more potential to interact with other molecules

5. Straight chains have stronger van der waals forces, branched have weaker, this is because long chains can pack closely together, and have a large surface area of contact with neighbouring molecules

Permanent Dipole-Dipole forces

1. Stronger than van der waal forces and only occur between polar molecules

2. Occurs when 2 atoms in a covalent bond have different Electronegativity

3. The more electronegative atom pulls the shared electrons towards itself

4. This creates a permanent charge separation, more electronegative atom is delta negative, and less electronegative atom is delta positive

5. If molecule is asymmetrical, the polarity doesn’t cancel out and results in a permanent dipole across the molecule

6. The permanent delta positive side is attracted to permanent delta negative side of a neighbouring polar molecule

Permanent Dipole-Dipole example

Hydrogen bonding

1. Occurs in compounds that have hydrogen atom attached to one of the 3 most electronegative atoms

2. Which is N,O,F, and must have a available lone pair of electrons

3. Hydrogen bonding is stronger than other 2 intermolecular forces

Anomalous boiling pints

1. The anomalously high boiling points of H2O and NH3 are caused by hydrogen bonding

2. General increase in boiling points from H2S to H2Te if caused by increasing van der waal forces, due to increasing number of electrons

Ice

Ice has lower density than water, becuase the molecules are held further apart.