• 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

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

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

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

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

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

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

• 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

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