Intermolecular Forces

HideShow resource information
View mindmap
  • Intermolecular Forces
    • van der Waals forces
      • 'Attractive forces between molecules that arise when random electron movements in one molecule cause an instantaneous dipole which in turn causes an induced dipole in the other molecule'
      • Electrons do not have fixed positions within atoms and molecules but instead move in a somewhat random way wtihin their orbitals
        • As a result, at any one moment in time the electrons will be more on one side or end than the other. The molecule will then have an instantaneous dipole
          • If another molecule is close to the first, the electrons in this molecule will be influenced by the nearby instantaneous dipole, causing an induiced diple in this second molecule. As a result the two molecules are attracted to each other.
      • The effectiveness of van der Waals forces in attracting molecules depends on;
        • The number of electrons in the molecule- the more electrons, the stronger the van der Waals forces
        • The surface area of the molecule - a larger surface area means more contacts with other molecules can be made so more van der Waals forces have to be broken to seperate the molecules
        • The boiling point of a substance increases as both the number of electrons per molecule and the surface area increase
    • Permanent dipole- permanent dipole forces
      • When molecules contain atoms of different electronegativity, th eelectrons will not be shared equally and the molecules may have a dipole moment all the time. This is called a permanent dipole
      • When a simple molecular substance has a permenant dipole, it can form additional intermolecular forces called permanent dipole - permanent dipole forces
      • Because the dipole is there all the time, permanent dipole-permanent dipole forces are generally stronger than van der Waals, so polar substances are likely to have higher boiling points than non-polar molecules of a similar size
      • Diatomic molecules will always have a permanent diople unless that are made of the same atom
      • The bigger the electronegativity difference, the bigger the dipole moment
      • In highly symmetrical molecules, the individual dipoles may cancel out, so even if there are polar bonds, the whole molecule is non polar
    • Hydrogen Bonds
      • A special case of permanent dipole- permanent dipole forces which occur between a hydrogen atom and a small, very electronegative atom
        • A lone pair on this atom points directly towards the hydrogen
      • Much stronger than other intermolecular forces
      • 'The intermolecular attrraction between hydrogen covalently bonded to N, F or O on one molecule and the lone pair on N, O or F in another molecule'
    • When a material with a giant structure turns to vapour, the strong bonds that exist throughout the giant structure have to be broken
      • This takes a lot of energy, so these substances generally have high boiling points
    • When a simple molecular substance is vaporised, the covalent bonds within the molecules are not broken.
      • Instead only the intermolecular forces between the molecules have to be broken
        • The intermolecular forces are relatively weak, so simple molecular substances generally have relatively low boiling points
    • 'The volatility of a liquid decreases as the number and strength of the intermolecular forces that need to be broken to vaporise it increase'
    • Water
      • Water has anomalous properties caused by hydrogen bonding
        • High surface tension
          • This is because a lot of energy is needed to break the hydrogen bonds that hold together the network of water molecules in the surface layer
        • Relatively high melting and boiling point
          • This is because hydrogen bonds are stronger than other intermolecular forces, so more energy is needed to break them
        • Water is less dense in the solid state then the liquid state
          • This is because water has an open lattice structure; the hydrogen bonds keep all of the water molecules fixed in position. So on average, the spaces between the molecues are slightly larger than in liquid water
      • In liquid water the molecules move around in clusters containing two or more molecules held together by hydrogen bonding with these hydrogen bonds being continually broken and re-formed
        • To boil water, the molecules must be given enough energy ot overcome these hydrigen bonds and van der Waals forces between the molecules
      • In ice, the water molecules are held together by hydrogen bonds in a rigid three dimensional network
    • Other compounds containing O-H or N-H bonds can also form hydrogen bonds
      • Ammonia forms hydrogen bonds but because the nitrogen is less electronegative than oxygen, the molecule is not as strongly polarised as water, so the hydrogen bonds are not as strong

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Intermolecular forces resources »