Types of Forces Between Molecules (AQA A Level Chemistry)

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

Polarity

• When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar

The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms

• When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom
• As a result of this:
  • The negative charge centre and positive charge centre do not coincide with each other
  • This means that the electron distribution is asymmetric
  • The less electronegative atom gets a partial charge of δ+ (delta positive)
  • The more electronegative atom gets a partial charge of δ- (delta negative)

• The greater the difference in electronegativity the more polar the bond becomes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive

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

Induced dipole-dipole forces:

• Induced dipole - dipole forces exist between all atoms or molecules
  • They are also known as van der Waals forces or 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 the same number of electrons
  • 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

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
  • Water can form two hydrogen bonds, because the O has two lone pairs

Hydrogen bonding in water