Have you ever wondered why some compounds boil at relatively low temperatures while some boil at high temperatures? Why some melt at low temperatures while some melt at high temperatures? Why some exist as a gas at room temperature while some as liquid or solid? To be able to answer those questions we must understand the concept of intermolecular forces.
Intermolecular forces are forces that bind molecules together in a compound. Different intermolecular forces vary in their strength but they are generally weaker than intramolecular chemical bonds (ionic, covalent or metallic bonds). Thus, when a compound changes from one phase to another, its molecules remain intent. Although intermolecular forces are much weaker than the major chemical bonds, their existence cannot be overlooked because they are related largely to the physical properties of a compound, such as its boiling point, melting point, e.t.c. For a liquid to boil or a solid to melt, it must absorb energy that can overcome its intermolecular forces. Thus, a liquid or solid having stronger intermolecular forces tends to boil or melt at high temperatures. Also, the difference in the phases of matter is how tightly its molecules are held together which is a function of its intermolecular forces.
TYPES OF INTERMOLECULAR FORCES.
There are three types of intermolecular forces, all these types of intermolecular forces are electrostatic in nature i.e involving attraction between positive and negative species.
When atoms of different electronegativity bond together to form a molecule, the more electronegative one tends to pull the electrons in the bonds towards itself, making it acquire a partial negative charge and the other to acquire a partial positive charge. Thus, creating a permanent dipole moment, such molecules are arranged in such a way that the partially positive end of one molecule is attracted to the partially negative end of the neighboring molecule. This type of attraction is called dipole-dipole attraction. An example of this force can be observed between the molecules of HCl where the partially negative Cl atom of one molecule is electrostatically attracted to the partially H of another molecule.
If intermolecular forces are said to be electrostatic in nature, does that infer that neutral and nonpolar compounds do not have intermolecular forces? If yes, how then do we account for the liquefaction of nonpolar gases?
In 1930, Fritz London observed that the motion of electrons in an atom or molecule can create a momentary dipole moment. Since electrons around the nucleus are not static but are in continuous motion, there comes a time when all the elections are clouded on one side of the nucleus. At that instant, the atom or molecule has an instantaneous dipole moment, this momentary dipole moment induces an instantaneous dipole moment on the neighboring molecule causing them to be attracted to each other, this type of attraction is known as dispersion forces.
Dispersion forces exist between nonpolar hydrocarbons.
Hydrogen bonding is the attraction between the partially positive hydrogen-bonded to a strongly electronegative atom like N, O or F and the lone pair of electrons on its neighboring electronegative atom. The presence of the hydrogen bonding in NH3 H2O and HF is the reason for their unusual high boiling points compare to other covalent hydrides.
A hydrogen bond is sub-divided into
Intermolecular hydrogen bond i.e between two molecules e.g HF, H2O e.t.c
Intramolecular hydrogen bond i.e within the same molecule e.g o-nitrophenol.
Author. Akowe Nasir Aija