Intermolecular Forces

 

The forces that hold molecules together may be called intermolecular forces. Here all such forces are electrostatic in nature as they invariably involve dipoles, i.e., these forces arise from the attraction between negative and positive charges. Hence intermolecular forces may be defined as dipole-interactions.

It is needless to mention that these forces are much weaker in comparison to those between ions in ionic compounds and atoms in covalent molecules or crystals. these forces are discussed bellow.

Dipole- dipole interaction

Dipole-Dipole interaction

The attraction of the positive end of a molecular dipole for the negative end of another dipole called dipole-dipole interaction. Hence these interactions are also, in one sense, directional. It is inversely proportional to the third power of the distance between two dipoles (r). Hence it is effective over a short intermolecular distance although it exhibits moderate strength. Dipole dipole bond dissociation energy is of the order 8.37 KJ mol^-1. Here,

The force F ∝ 1/r³

Dipole dipole interaction may occur either head-to-tail or laterally. Dipole dipole interactions are weaker than ion-dipole interactions, but stronger than those present in non-polar molecules of comparable molecular weights. The strength of this kind of bond is reflected in the physical properties of the compounds concerned. Hydrogen chloride is a polar molecule. There are dipole-dipole interactions amongst hydrogen chloride (HCl) molecules. Argon is a nonpolar entity and of the same molecular weight as that of HCl. Owing to the dipole-dipole interaction, HCl boils at a much higher temperature than argon. This difference in their boiling point is 101 ^oC.

Polar compounds dissolve in polar solvents by dipole-dipole interaction.

Ion-dipole interaction

Ion-dipole interaction

Ions have strong but short range effect on dipoles. These are known as ion-dipole interactions. When an ion approaches a dipole, the dipole orient itself in such a way that its attractive and (the end with charge opposite to that of the ion) is directed toward the ion and the other is directed away. Thus an electrostatic force of attraction is established between the ion and the dipole; this force obeys Coulomb's law of electric force. It is found to be inversely proportional to the square of the distance between the ion and the dipole (r). Thus the force acts over a short range though it is strong. Considering orientation of the dipole, this force is directional, here,

The force F ∝ 1/r²

During hydration of proton to form hydronium ion, the solvation of ions and dissolution of ionic lattices in polar solvents are the examples of ion-dipole interactions. Stabilization of ion by the attraction of solvent molecules is called salvation and hydration is a special case of solution where water is the solvent.

Induced-dipole Interaction

If polar and nonpolar molecules are present together, then polar molecules, i.e., dipoles can induce dipoles in non-polar molecules. The net result is the bonding between the polar and nonpolar molecules, i.e., bonding between the dipoles and the induced dipoles. This is called dipole- induced dipole Bond. This force varies inversely as the sixth power of the distance (r) between the dipole and the induced dipole. It is effective over and extremely short range and very weak in nature. Nonpolar molecules dissolve in polar solvents by dipole-induced dipole interaction.

The force, F ∝ 1/r⁶

A charged particle, i.e., and ion can induced dipole in a nonpolar molecule when they come close to each other. This occurs because the ion distorts the electron cloud of the molecule. This is called an ion-induced dipole and the result is the bonding between and the ion and the induced dipole which is known as ion-induced dipole bond. This type of force varies as the fourth power of the distance between the ion and the induced-dipole. It is a weak force and acts over a very short range. This is stronger than dipole-induced dipole force.

Instantaneous dipole-induced dipole interaction or London force

London force

A non polar molecule is electrically neutral. Of course, owing to the motion of the electrons within such a molecule, the centre of negative charge density due to electrons does not always coincide with the centre of positive charge density of the protons. This phenomenon disturbs the electrical symmetry of the molecule and an instantaneous dipole is formed. This instantaneous dipole, induces dipole in another adjacent molecule. Thus the instantaneous dipole and in the induced dipole remain together by an attractive force or bonding, known as instantaneous dipole-induced dipole interaction. It is true that the momentary dipoles and induced dipoles are changing constantly to the neutral state; still the attractive force so generated is able to bind the molecules together and is known as London force.

Some of the authors refer to van der Waals forces as London forces. When nonpolar molecules come very close to each other the London forces prevails on between the surfaces of the concerned molecules. London force is directly proportional to the molecular volume. As the force originates from the motion of the electrons within molecules, the strength of the force also varies directly as the number of polarizable electrons in the molecule. Again, as the molecular volume and the number of electrons in a molecule increase, the molecular weight increases. Thus London force increases rapidly with the increasing molecular weight. It is found that London force is inversely proportional to the 7th power of the distance (r) between the two dipoles so formed. Here,

The force, F ∝ 1/r⁷

 So London force is extremely short ranged in action. But if the molecules happened to come to each other below an optimum distance, a repulsive force develops between the outer electrons of the molecules concerned. On the other hand, London force is found to be maximum when the molecules are separated from each other by optimum distance. In fact, half of the distance between the nuclei of two non-bonded atoms of the same element over which van der Waals force (i.e., London force) is maximum, is called van der Waals radius. The van der Waals radius is always greater than the atomic radius of the element.

London force is the weakest of all kinds of intermolecular forces the bond dissociation energy is only the order 4 KJ/mole.