Intermolecular Forces

What are Intermolecular Forces?

Intermolecular Forces are attractive forces existing between covalent molecules and noble gases. They are relatively weak attractive forces when compared with electrovalentAn electrovalent or ionic bond is formed when a metal atom transfers one or more electrons to a non-metal atom. Ionic or electrovalent compounds are those compounds which are formed by... More and covalent molecules.

Types of Intermolecular Forces:

1. Vander Waals forces
2. Dipole – (Dipolar) forces
3. Hydrogen bonds

Vander Waals Forces:

These are very weak intermolecular forces operating between covalent molecules. They exist in solid, liquid, and gaseous states.

Vander Waals forces are responsible for the low melting and boiling points of covalent compounds. Vander Waals force exists in the following organic compounds: naphthalene, graphite, dichloromethane, etc.

Dipole – Dipole (dipolar) Forces:

These are forces of attraction existing between polar molecules (unsymmetrical molecules) i.e. molecules having permanent dipoles e.g. HCl.

The partial separation of charges in a polar covalent bond is called a dipole. When electrons are shared between atoms of different elements, the more electronegative atom becomes partially negative ( ^–) and the other becomes partially positive ( ⁺). Molecules having permanent dipoles e.g. HCl, line up such that the pole of one molecule attracts the negative pole of another. 

Dipole-dipole attractions are much stronger than Vander Waal’s force.

Hydrogen Bonding:

This is the force of attraction between polar covalent molecules such as water (H₂O), Hydrogen Fluoride (HF), and compounds containing polar groups like alkanols, alkanoic acids, etc.

When a highly electronegative atom such as fluorine, oxygen, or nitrogen is covalently bonded to hydrogen, a dipole develops, causing the hydrogen to be partially positive ( ⁺) while the other atom will be partially negative ( ^–). This type of intermolecular attraction is known as hydrogen bonding.

Compounds that exhibit hydrogen bonding are: HF, H₂O, NH₃, CH₃COOH, C₂H₅OH.

In the following examples, hydrogen bonds are represented with dotted lines.

(a) Hydrogen bond between H-F molecules

(b) Hydrogen bond between water molecules

A hydrogen bond is responsible for the following properties in compounds that exhibit hydrogen bonding.

1. High boiling and melting points.
2. Solubility/

The hydrogen bond, like the other types of bonds, is used to explain some observable characteristics of certain chemical compounds.

For example, consider the boiling points of the following hydrides.

Boiling points of Some Hydrides:

From the above table, the boiling points of H₂O and HF are much higher than those of H₂S and HCl. This is because of the presence of hydrogen bonding in these molecules.