Intermolecular Forces and Physical Properties

• intermolecular forces - forces between molecules

• determine physical properties

Ion-dipole forces

• ionic substance (charged ion) dissolves in polar liquid

• positive and negative charges hold

Ion-induced dipole

• an ion attracts a non polar molecule causing a shift in its electrons inducing it to become temporarily polar

Van der waal forces

a) dipole-dipole forces

• electrostatic attraction between the positive and negative poles of molecules

  • therefore occur in moleculers with permanent dipoles (hence dipole-dipole)

  • hydrogen bonding is a type of dipole-dipole

b) dipole-induced dipole forces

• polar and non polar substance mixed

  • polar induces polarity in the non-polar which then causes them electrostatic attraction

c) London forces

• two non polar molecules

• electrons are mobile and therefore can be an uneven distrubition at any point

  • this causes a temporary dipole which can induce a temporary dipole in the neighbouring molecule

• strength

  • more valence electrons means higher probability of temporary dipoles meaning stronger forces

  • larger cloud is easier to be polarised

  • increased surface area increases strength → greater contact in linear than branched (can get closer)

Hydrogen bonding

• hydrogen bonded to highly electronegative nitrogen, oxygen or flourine

• lone pair on N, O, F

• not a bond → actually a intermolecular force

• stronger than expected bonds

• highly positive hydrogen attracted to neighbouring double bond

• strong and directional bond

• explains water and ice props

Viscosity

• resistance of liquid to flow

• based on temp and strength of inermolecular bonds → stronger, more viscious

Evaporation

• change of liquid into vapour below its boiling point

• stronger bonds → lower rate of evaporation

• volatile → evaporates readily

Melting and Boiling points

• bonds need to be weakened to allow liquid to form

• bonds must be weakened enough to completely seperate to boil

• stronger intermolecular forces requires more energy to break

Surface Tension

• intermolecular forces near surface lead to sort of surface film

• stronger intermolecular bonds → higher surfae tension

Capillarity

• liquid inside capillary tube or absorbent material to rise/fall because of surface tension

• greater surface tension → greater rise or fall of liquid

• addhesive - forces between liquid and the contianer

• cohesive - intermolecular forces between molecules in the substance

• adhesive>cohesive = rise /. vice versa