CH2 - Bonding (2)

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  • CH2 - Bonding (2)
    • The Influence of Hydrogen Bonding on Water Solubility
      • For a compound to be water soluble it must be able to disrupt the hydrogen bonds between water molecules.
        • It has to be able to form attractions with water molecules that are at least s strong as the hydrogen bonds between the water molecules.
      • Ionic compounds are water soluble.
        • They dissolve in water because the positive ions are attracted to the (delta -) negative O of the polar water molecules.
          • And the positive ions are attracted to the  (delta +) positive H of the polar water molecules.
      • Other substances that are soluble in water includes those that can form hydrogen bonds themselves. E.G. Ethanol.
      • Why are hydrocarbons insoluble (immiscible) in water?
        • They are non-polar molecules.
        • They are unable to disrupt hydrogen bonds.
          • They have(e.g. octane) ID - ID bonds that are very weak compared to hydrogen bonds.
      • What happens to the solubility of alcohols as the carbon chain gets longer?
        • Solubility decreases.
          • This is because the carbon chain is non-polar.
    • Shapes of Molecules
      • The number of electron pairs surrounding the central atom determines the shape of the molecule.
      • Electron pairs repel each other in order to become as far apart as possible so that repulsion is minimised.
      • Repulsion decreases in this order:
        • Lone pair - Lone pair
          • Lone pair - bonding pair
            • Bonding pair - Bonding pair
      • The Valence Shell Electron Pair Repulsion Theory allows us to predict the shapes of simple molecules and ions.
      • Summary
        • 1. Find the group number of the central atom. This is the number of electrons it contributes.
        • 2. Count the atoms that surround the central atom - they each contribute one electron.
        • 3. Subtract one electron for each positive charge / add one electron for each negative charge.
        • 4. Divide by 2 to give the number of electron pairs that surround the central atom.
        • 5. Take lone pairs of electrons into account..
          • These cause bond angles to become smaller as bonds get pushed together.
        • 6.. There is greatest repulsion between two lone pairs than two bonding pairs.

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