6.4 Hydrogen Bonding

Hydrogen Bonds
1. type of permanent dipole-dipole interaction found between molecules containing:
  1. an electronegative atom with alone pair of electrons, for example, oxygen, nitrogen or fluorine.
  2. a hydrogen atom attached to an electronegative atom, for example, H-O, H-N or H-F
Anomalous Properties of Water
1. The solid form, Ice, is less dense than the liquid form, water.
  1. 1) Hydrogen bonds hold water molecules apart in an open lattice structure
  2. 2) The water molecules in ice are further apart than water
  3. 3) Solid ice is less dense than liquid water and floats
  4. With two lone pairs on the oxygen atom and two hydrogen atoms, each water molecule can form four hydrogen bonds.
    1. The hydrogen bonds extend outwards, holding water molecules slightly apart and forming an open tetrahedral lattice full of holes.
      1. The bond angle about the hydrogen atom involved in the hydrogen bond is close to 180 degrees.
2. Water has a relatively high melting point and boiling point
  1. As with all molecules, water has London Forces between molecules.
  2. Hydrogen bonds are extra forces, over and above the London forces
    1. An appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher melting and boiling points than would be expected from just London Forces
      1. When water melts, the lattice breaks and the ridged arrangement of Hydrogen bonds is broken. When water boils, the hydrogen bonds break completely

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