AQA A Level Chemistry

Revision Notes

1.5.4 Effects of Forces Between Molecules

Influence of Intermolecular Forces

Properties of water

  • Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and a higher density in the liquid than the solid

High melting & boiling points

  • Water has high melting and boiling points due to the the strong intermolecular forces of hydrogen bonding between the molecules in both ice (solid H2O) and water (liquid H2O)
  • A lot of energy is therefore required to separate the water molecules and melt or boil them

 

Chemical Bonding Melting and Boiling Points of Water, downloadable AS & A Level Chemistry revision notes

Hydrogen bonds are strong intermolecular forces which are harder to break causing water to have a higher melting and boiling point than would be expected for a molecule of such a small size

  • The graph below compares the enthalpy of vaporisation (energy required to boil a substance) of different hydrides
  • The enthalpy changes increase going from H2S to H2Te due to the increased number of electrons in the Group 16 elements
  • This causes an increase in the instantaneous dipole – induced dipole forces (dispersion forces) as the molecules become larger
  • Based on this, H2O should have a much lower enthalpy change (around 17 kJ mol-1)
  • However, the enthalpy change of vaporisation is almost 3 times larger which is caused by the hydrogen bonds present in water but not in the other hydrides

Chemical Bonding Trends Boiling Points Hydrides, downloadable AS & A Level Chemistry revision notes

The high enthalpy change of evaporation of water suggests that instantaneous dipole-induced dipole forces are not the only forces present in the molecule – there are also strong hydrogen bonds, which cause the high boiling point

High surface tension

  • Water has a high surface tension
  • Surface tension is the ability of a liquid surface to resist any external forces (i.e. to stay unaffected by forces acting on the surface)
  • The water molecules at the surface of liquid are bonded to other water molecules through hydrogen bonds
  • These molecules pull downwards the surface molecules causing the surface of them to become compressed and more tightly together at the surface
  • This increases water’s surface tension

Chemical Bonding Surface Tension Water, downloadable AS & A Level Chemistry revision notes

The surface molecules are pulled downwards due to the hydrogen bonds with other molecules, whereas the inner water molecules are pulled in all directions

Density

  • Solids are denser than their liquids as the particles in solids are more closely packed together than in their liquid state
  • The water molecules are packed into an open lattice
  • This way of packing the molecules and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form
  • Therefore, ice has a lower density than liquid water by about 9%

Chemical Bonding Density Water, downloadable AS & A Level Chemistry revision notes

The ‘more open’ structure of molecules in ice causes it to have a lower density than liquid water

Exam Tip

Ice floats on water because of ice’s lower density.

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