Intermolecular Forces are attractive forces existing between covalent molecules and noble gases. They are relatively weak attractive forces when compared with electrovalent 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 Waal’s 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 dipole 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.
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 are 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 point
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:
|Hydrides||Chemical Formula||Boiling Point|
From the above table, the boiling points of H2O and HF are much higher than that of H2S and HCl. This is because of the presence of hydrogen bonding in these molecules.