Intermolecular Forces - Introduction (Edexcel International A Level Chemistry)

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Intermolecular Forces - Introduction

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:

1-5-3-types-of-forces-between-molecules-new

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

      Chemical 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

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

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 

Examiner 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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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.