Intermolecular Forces

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  • 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


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