Intermolecular Forces

What are Intermolecular Forces?

 Intermolecular forces, often abbreviated to IMF, are the attractive and repulsive forces that arise between the molecules of a substance. These forces mediate the interactions between individual molecules of a substance.

Intermolecular forces are responsible for most of the physical and chemical properties of matter.

Forces also exist between the molecules themselves and these are collectively referred to as intermolecular forces. Intermolecular forces are mainly responsible for the physical characteristics of the substance. Intermolecular forces are responsible for the condensed states of matter. The particles making up solids and liquids are held together by intermolecular forces and these forces affect a number of the physical properties of mater in these two states.

Types of Intermolecular Forces

An intermolecular force is an attractive force that arises between the positive components (or protons) of one molecule and the negative components (or electrons) of another molecule. Various physical and chemical properties of a substance are dependent on this force. The boiling point of a substance is proportional to the strength of its intermolecular forces – the stronger the intermolecular forces, the higher the boiling point.

By comparing the boiling points of different substances, we can compare the strengths of their intermolecular forces. This is because the heat absorbed by the substance at its boiling point is used to break these intermolecular forces and to convert the liquid into vapour.

Types of Intermolecular Forces

The intermolecular forces depend on the following interactions:

1. Dipole-Dipole Interactions

Dipole-dipole interactions are attractive forces among polar molecules. Polar molecules have permanent dipoles that are formed due to differences in the electronegativities of the atoms that are associated with a covalent bond. The partially positive portion of one molecule is attracted to the partially negative portion of another molecule.

Example: Dipole-dipole interactions occur in HCl molecules. Chlorine is comparatively more electronegative than hydrogen and it, therefore, acquires a partial negative charge (whereas hydrogen acquires a partial positive charge). The dipole-dipole interaction then takes place between the HCl molecules.

2. Ion-Dipole Interactions

These interactions are similar to dipole-dipole interactions except for the fact that they arise between ions and polar molecules. Example: When NaCl is mixed with water in a beaker, the polar H2O molecules are attracted to the sodium and chloride ions in the beaker. The strength of this interaction depends on:

  • The magnitude of the dipole moment
  • Size of the polar molecule
  • The size and charge of an ion

3.  Ion Induced Dipole Interactions

In this type of interaction, a non-polar molecule is polarized by an ion placed near it. The non-polar molecules, upon obtaining a charge, behave as induced dipoles. This interaction between an ion and an induced dipole is known as ion-induced dipole interaction.

4. Dipole Induced Dipole Interaction

These interactions are similar to ion-induced dipole interactions. However, the differentiating factor is that non-polar molecules are transformed into induced dipoles due to the presence of a polar molecule nearby.

5. Dispersion Forces or London Forces

It operates for a short distance and it is the weakest force. This kind of force arises due to the movement of electrons thus creating temporary positive and negative charged regions.

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