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Intermolecular Forces

Skills to develop

Intermolecular Forces

Forces binding atoms in a molecule are due to chemical bonding. The energy required to break a bond is called the bond-energy. For example the average bond-energy for O-H bonds in water is 463 kJ/mol. On average, 463 kJ is required to break 6.023x1023 O-H bonds.

The forces holding molecules together are generally called intermolecular forces. The energy required to break molecules apart is much smaller than a typical bond-energy, but intermolecular forces play important roles in determining the properties of a substance. Intermolecular forces are particularly important in how molecules interact and form biological organisms or even life.

Classifying Intermolecular Forces

In general, intermolecular forces can be divided into several categories. The four prominent types are:
  1. Strong ionic attraction
    Recall lattice energy and its relations to properties of solids. The more ionic, the higher the lattice energy. Examine the following list and see if you can explain the observed values by way of ionic attraction:
    LiF, 1036; LiI, 737; KF, 821; MgF2, 2957 kJ/mol.
  2. Intermediate dipole-dipole forces
    Substances whose molecules have dipole moment have higher melting points or boiling points compared to those of similar molecular mass, but their molecules have no dipole moment.
  3. London dispersion forces or van der Waal's force These forces always operate in any substance. The force rises from induced dipole and the interaction is weaker than the dipole-dipole interaction.
  4. Hydrogen bond
    Certain types of substances form hydrogen bonds. Hydrogen bonds affect properties of substances (mp, bp, solubility)
  5. Covalent bonding
    Covalent is really intramolecular force rather than intermolecular force. It is mentioned here, because some solids are formed due to covalent bonding. For example, in diamond, silicon, quartz etc., all the atoms in the entire crystal are linked together by covalent bonding. These solids are hard, brittle, and have high melting points. Covalent bonding holds atoms tighter than ionic attraction.
  6. Metallic bonding
    Forces between atoms in metallic solids belong to another category. Valence electrons in metals are rampant. They are not restricted to certain atoms or bonds. Rather they run freely in the entire solid, providing good conductivity for heat and electric energy. This behaviour of electrons gives special properties such as ductility and mechanical strength to metals.

The division into types is for convenience in their discussion. Of course all types can be present simultaneously for many substances.

See if you can answer the following questions.

If you are looking for specific information, your study will be efficient. Some answers can be found in the DIALOGUE. Consider carefully the purpose of each question, and figure out what there is to be learned in it.

Confidence Building Questions

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