AQA A Level Chemistry

Revision Notes

1.5.3 Types of Forces Between Molecules

Intramolecular Forces vs. 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

Chemical Bonding Inter and Intramolecular Forces, downloadable AS & A Level Chemistry revision notes

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:

Chemical Bonding Strengths of different types of bonds, downloadable AS & A Level Chemistry revision notes

The varying strengths of different types of bonds

Polar Bonds

Polarity

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

Chemical Bonding Nonpolar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Polar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Permanent Dipole - Permanent Dipole, downloadable AS & A Level Chemistry revision notes

  • 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:

  • Induced dipole – dipole forces exist between all atoms or molecules
    • They are also known as van der Waals forces or London dispersion forcesChemical Bonding Intermolecular Forces (2), downloadable AS & A Level Chemistry revision notes
  • 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

Chemical Bonding Pd - Pd vs Id - Id, downloadable AS & A Level Chemistry revision notes

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

Exam Tip

Remember this difference: intramolecular forces are forces within a molecule, whereas intermolecular forces are forces between a molecule.

Hydrogen Bonds

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

 

Chemical Bonding Water H-Bonds, downloadable AS & A Level Chemistry revision notes

Hydrogen bonding in water 

Exam Tip

Make sure to use a dashed, straight line when drawing your intermolecular forces!

Hydrogen bonds should start at the lone pair and go right up to the delta positive atom – it must be really clear where your H bond starts and ends.

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