Intermolecular Forces

3 types of forces:

• permanent dipole-dipole forces

• induced dipole-dipole forces (van der Waals, dispersion, London)

• hydrogen bonding

Van der Waals

-weakest

-acts as induced dipole between molecules

Van der Waals Strength

• dependant on Mr of molecule + its shape

• larger Mr = stronger intermolecular forces

straight chain molecules experience stronger van der waals forces than -

branched chain molecules

• as they line up and pack closer together

• reduces distance over which the force acts, so is stronger

permanent dipole

acts between molecules with a polar bond

• + and - regions attract each other

• holds molecules in lattice like structure

Hydrogen Bonding

form between hydrogen and three most electronegative atoms (nitrogen, fluorine and oxygen)

-strongest

Drawing hydrogen bonds:

lone pair on atoms forms a bond with a hydrogen atom and another molecule (shown by dotted line)

Hydrogen bonds:

-higher melting points and boiling points