Chapter 4: Unit 16. Intermolecular Forces

Intermolecular Forces

The force of attraction that exists between molecules including polar and nonpolar molecules. These forces are responsible for solid, liquid and gas formation. Stronger the IMFs, more tightly the atoms are connected together and more condensed is the phase of the molecule. These forces are so strong that they can lead to phase changes like condensation or solidification.

1. Dipole-dipole force: Exists between any two polar molecules. Opposite poles of molecules attract each other. The attraction between positive end of one dipole and negative end of another dipole is called dipole dipole attraction. For example: two molecules of comparable molecular weight, Butane C₄H₁₀ and acetone C₂H₆O, one is polar and another is nonpolar.

London dispersion Force: Exists between two nonpolar molecules, formed due to momentary displacement of electrons and protons. Below the example of attraction between two I₂

London dispersion forces have their origin in electrostatic interactions. An instantaneous dipole is created within an atom or molecule due to partial separation of electrons from protons. This temporary dipole induces temporary dipole to adjacent molecule and that way can attract another molecule. Nonpolar atom or molecule or molecules usually don’t have any permanent dipole and therefore they are mainly gases with small molar mass but tend to be liquid or gas with increasing molar mass.

When molar mass increases, temporary separation between electrons and protons in atoms and molecules also increases, degree of London dispersion force increases. For example Ne is a monoatomic gas but Xe and Rn can develop attraction with other atoms and form molecular compounds.

3. H-bonding

When H is present in a polar molecule and attached to a very strong, small electronegative atom like N, F and O, the dipole-dipole force becomes very strong and called H-bonding. This the strongest among all intermolecular forces. The strength of H-bonding ranges from 2 to 10 Kcal/mol. Because of the strong H-bonding, these molecules require extra energy to separate from each other in liquid phase, hence very high boiling point.

Example: H₂O, NH₃, CH₃OH, HF.