Intermolecular Forces (Edexcel A Level Chemistry): Revision Note

Intermolecular Forces

Intramolecular forces

• Intramolecular forces are forces within a molecule and are usually covalent bonds

• Covalent bonds are formed when the outer electrons of two atoms are shared

• Single, double, triple and co-ordinate bonds are all types of intramolecular forces

Intermolecular forces

• Molecules also contain weaker intermolecular forces which are forces between the molecules

• There are three types of intermolecular forces:

  • Induced dipole – dipole forces also called van der Waals or London dispersion forces

  • Permanent dipole – dipole forces are the attractive forces between two neighbouring molecules with a permanent dipole

  • Hydrogen Bonding are a special type of permanent dipole - permanent dipole forces

  • Intramolecular forces are stronger than intermolecular forces

    • For example, a hydrogen bond is about one tenth the strength of a covalent bond

  • The strengths of the types of bond or force are as follows:

The varying strengths of different types of bonds

Induced dipole-dipole forces:

• Induced dipole - dipole forces exist between all atoms or molecules

  • They are also known as London dispersion forces

    

• The electron charge cloud in non-polar molecules or atoms are constantly moving

• During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other

• This causes a temporary dipole to arise

• This temporary dipole can induce a dipole on neighbouring molecules

• When this happens, the δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other

• Because the electron clouds are moving constantly, the dipoles are only temporary

Relative strength

• For small molecules with the same number of electrons, permanent dipoles are stronger than induced dipoles

  • Butane and propanone have a similar number of electrons (34 and 32) and the same relative molecular mass (58)

  • Butane is a nonpolar molecule and will have induced dipole forces

  • Propanone is a polar molecule and will have permanent dipole forces

  • Therefore, more energy is required to break the intermolecular forces between propanone molecules than between butane molecules

  • So, propanone has a higher boiling point than butane

Pd-pd forces are stronger than id-id forces in smaller molecules with an equal number of electrons

Permanent dipole - dipole forces:

• Polar molecules have permanent dipoles

• The molecule will always have a negatively and positively charged end

• Forces between two molecules that have permanent dipoles are called permanent dipole - dipole forces 

• The δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other

Hydrogen bonding

• Hydrogen bonding is the strongest form of intermolecular bonding

  • Intermolecular bonds are bonds between molecules

  • Hydrogen bonding is a type of permanent dipole – permanent dipole bonding

• For hydrogen bonding to take place the following is needed:

  • A species which has an O, N or F (very electronegative) atom bonded to a hydrogen

• When hydrogen is covalently bonded to an O, N or F, the bond becomes highly polarised

• The H becomes so δ⁺ charged that it can form a bond with the lone pair of an O, N or F atom in another molecule

• For example, in water

  • Each oxygen atom has two lone pairs, so it can accept two hydrogen bonds

  • Each water molecule also has two δ+ hydrogen atoms that can form hydrogen bonds

  • So, a water molecule can take part in up to four hydrogen bonds (an average of two per molecule, since each bond is shared by two molecules)