Write notes on Intermolecular forces

Explanation

The type of attraction that exists among the atoms in a covalent molecule is known as " covalent bond". The attraction forces that bind molecules together in a covalent substances are called intermolecular forces or van der Waal forces. These forces are of different types like lon-Dipole, Dipole-Dipole,Dipole-Induced Dilpole and induced Dipole-induced Dipole forces.
Lon-Dipole Forces: These forces are mainly present in aqueous solutions of ionic substances.
Ex: NaCl in water solution
Water is a polar molecule and in it 'H' atom possess partial+ve charge and 'O' atoms possess partial- ve charge. when ionic compounds like NaCl dissolve in water, they dissociate into component ions like `NA^+` and `Cl^-`. Now, the water molecules orient in the presence of ions in such a way that the +ve end of the dipole is near a cation. The magnitude of interaction energy depends on the change on the ion (Z). the strength of the dipole `(mu)` and on the inverse square of the distance (r) between the ion and the dipole. It can be expressed mathematically as,
`E=Z mu//r^2`
Dipole-Dipole forces:
This type of forces exist between neutral polar molecules. These are due to the electrical interactions among dipoles on neighbouring molecules. These forces may be attractive (between unlike poles) or repulsive (between like poles) and depend on the orientation of the molecules. These forces are generally weak and are significant only when the molecules are in close contact. The strength of a given dipole-dipole intersection depends on the sizes of the dipole moments involved. THe more polar the molecule or the higher the dipole moment, the greater is the strength of interactions and higher is the boiling points of those substances.
Dipole-Dipole interaction energy between solid polar molecules is proportional to `1/r^3` and that between rotating molecules is proportional to `1/r^6` where 'r' is the distance between the polar molecules.
Induced dipole- Induced dipole forces (London dispersion forces):
To explain the To explain the intermolecular forces among individual atoms or non-polar molecules. London dispersion forces have been proposed. These forces result from the motion of electrons in an atom. At a given instant the electron distribution in an atom may be unsymmetrical giving the atom a shord lived dipole moment. This instantaneous dipole on an atom can effect the electron distributions in neighbouring atoms and induce temporary dipoles in these neighbours. As a result of which weak attractive forces develop.They are known as London forces are small and are in the range 1-10 `kJ//mol e`. THe exact magnitude depends on a property known as polarisability . A smaller molecular or atom is less polarisable and has smaller dispersion forces. A larger molecule or heavier atom is more polarisable and has large dispersion forces. These forces are always attractive and are inversely proportional to the sixth power of the distance between the two interacting particles `(r^6)`.
Dipole-Induced Dipole forces:
These forces operate between polar molecules with permanent dipole moments and the molecules with no permanent dipole moment. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming the electron cloud. The interacting range is proportional to `1/r^2` where 'r' is the distance between the molecules. The magnitude of induced dipole moment also depends on the magnitude of the dipole moment of permanent dipole and polarisability of neutral molecules.