Van der Waals' Forces (Cambridge (CIE) AS Chemistry): Revision Note

Exam code: 9701

Richard Boole

Last updated

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

Intermolecular and intramolecular forces in water

Diagram of water molecules showing hydrogen bonds as intermolecular forces and polar covalent bonds as intramolecular forces with labels.
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

Instantaneous dipoles

Diagram showing formation of instantaneous dipoles: non-polar molecule becomes a temporary dipole, inducing a dipole in another molecule, illustrating ID-ID forces.
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

 

Two graphs showing noble gases. Graph A: enthalpy of vaporisation versus number of electrons. Graph B: boiling point versus number of electrons.
Going down the Group, the id-id forces increase due to the increased number of electrons in the atoms
Chemical structures of pentane and 2,2-dimethylpropane with boiling points; pentane has 36°C, 2,2-dimethylpropane has 10°C, with contact points noted.
The increased number of contact points in pentane 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

Diagram of two polar molecules with permanent dipoles, indicating dipole-dipole forces between them. Each molecule shows partial positive and negative charges.
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 structures: butane with boiling point 0°C and propanone with boiling point 56°C, showing molecular polarity with delta symbols.
Pd-pd forces are stronger than id-id forces in smaller molecules with an equal number of electrons

Examiner Tips and Tricks

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

Hydrogen bonds in water molecules

Diagram illustrating hydrogen bonding in water, showing polar O-H bonds, permanent dipole, and intermolecular forces with delta symbols.
Hydrogen bonding in water occurs between the oxygen lone pair of one water molecule and the δ+ hydrogen atoms of another water molecule
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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.