2.2 Liquids  (Page 2/2)

Summary

• Intermolecular forces are the forces that act between stable molecules.
• The type of intermolecular force in a substance, will depend on the nature of the molecules .
• Polar molecules have an unequal distribution of charge, meaning that one part of the molecule is slightly positive and the other part is slightly negative. Non-polar molecules have an equal distribution of charge.
• There are three types of Van der Waal's forces . These are dipole-dipole, ion-dipole and London forces (momentary dipole).
• Dipole-dipole forces exist between two polar molecules , for example between two molecules of hydrogen chloride.
• Ion-dipole forces exist between ions and dipole molecules . The ion is attracted to the part of the molecule that has an opposite charge to its own. An example of this is when an ionic solid such as sodium chloride dissolves in water.
• Momentary dipole forces occur between two non-polar molecules , where at some point there is an uequal distribution of charge in the molecule. For example, there are London forces between two molecules of carbon dioxide.
• Hydrogen bonds occur between hydrogen atoms and other atoms that have a high electronegativity such as oxygen, nitrogen and fluorine. The hydrogen atom in one molecule will be attracted to the nitrogen atom in another molecule, for example. There are hydrogen bonds between water molecules and between ammonia molecules.
• Intermolecular forces affect the properties of substances. For example, the stronger the intermolecular forces, the higher the melting point of that substance, and the more likely that substance is to exist as a solid or liquid. Its boiling point will also be higher.
• In liquids , properties such as surface tension , capillarity and evaporation are the result of intermolecular forces.

Summary exercise

1. Give one word or term for each of the following descriptions:
   1. The tendency of an atom in a molecule to attract bonding electrons.
   2. A molecule that has an unequal distribution of charge.
   3. A charged atom.
2. For each of the following questions, choose the one correct answer from the list provided.
   1. The following table gives the melting points of various hydrides:

      Hydride	Melting point ( ${}^0$ C)
      HI	-50
      NH ${}_3$	-78
      H ${}_2$ S	-83
      CH ${}_4$	-184

      In which of these hydrides does hydrogen bonding occur?
      1. HI only
      2. NH ${}_3$ only
      3. HI and NH ${}_3$ only
      4. HI, NH ${}_3$ and H ${}_2$ S
      (IEB Paper 2, 2003)
   2. Refer to the list of substances below: HCl, Cl ${}_2$ , H ${}_2$ O, NH ${}_3$ , N ${}_2$ , HF Select the true statement from the list below:
      1. NH ${}_3$ is a non-polar molecule
      2. The melting point of NH ${}_3$ will be higher than for Cl ${}_2$
      3. Ion-dipole forces exist between molecules of HF
      4. At room temperature N ${}_2$ is usually a liquid
3. The respective boiling points for four chemical substances are given below: Hydrogen sulphide -60 ${}^0$ C Ammonia -33 ${}^0$ C Hydrogen fluoride 20 ${}^0$ C Water 100 ${}^0$ C
   1. Which one of the substances exhibits the strongest forces of attraction between its molecules in the liquid state?
   2. Give the name of the force responsible for the relatively high boiling points of ammonia and water and explain how this force originates.
   3. The shapes of the molecules of hydrogen sulfide and water are similar, yet their boiling points differ. Explain.
   (IEB Paper 2, 2002)