CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.3.14 Van der Waals' Forces

van der Waals' Forces & Dipoles

  • Covalent bonds are strong intramolecular forces
  • Molecules also contain weaker intermolecular forces which are forces between molecules
  • These intermolecular forces are called van der Waals’ forces
  • There are two types of van der Waals’ forces:
    • Instantaneous (temporary) dipole – induced dipole forces also called London dispersion forces
    • Permanent dipole – permanent dipole forces

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

The polar covalent bonds between O and H atoms are intramolecular forces and the permanent dipole – permanent dipole forces between the molecules are intermolecular forces as they are a type of van der Waals’ force

Instantaneous dipole – induced dipole (id – id)

  • Instantaneous dipole – induced dipole forces or London dispersion forces exist between all atoms or molecules
  • 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

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

Id-id (London dispersion) forces between two non-polar molecules

  • Id – id forces increase with:
    • Increasing number of electrons (and atomic number) in the molecule
    • Increasing the places where the molecules come close together

 

Chemical Bonding Enthalpy and Boiling Point, downloadable AS & A Level Chemistry revision notes

Going down the Group, the id-id forces increase due to the increased number of electrons in the atoms

Chemical Bonding Contact Points, downloadable AS & A Level Chemistry revision notes

The increased number of contact points in petane means that it has more id-id forces and therefore a higher boiling point

Permanent dipole – permanent dipole (pd – pd)

  • 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 – permanent 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

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

The delta negative end of one polar molecule will be attracted onwards the delta positive end of a neighbouring polar molecule

  • For small molecules with the same number of electrons, pd – pd forces are stronger than id – id
    • Butane and propanone have the same number of electrons
    • Butane is a nonpolar molecule and will have id – id forces
    • Propanone is a polar molecule and will have pd – pd 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 Bonding as a Permanent Dipole

  • Hydrogen bonding is an intermolecular force between molecules with an -OH/-NH group and molecules with an N/O atom
  • Hydrogen bonding is a special case of a permanent dipole – dipole force between molecules
    • Hydrogen bonds are stronger forces than pd – pd forces
  • The hydrogen is bonded to an O/N atom which is so electronegative, that almost all the electron density from the covalent bond is drawn towards the O/N atom
  • This leaves the H with a large delta positive and the O/N with a large delta negative charging resulting in the formation of a permanent dipole in the molecule
  • A delta positive H in one molecule is electrostatically attracted to the delta negative O/N in a neighbouring molecule

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

Hydrogen bonds in water molecules

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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